Latest / Boring Science / Enceladus Is Not What You Think_ The Truth Behind Saturn_s Icy Moon

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Enceladus Is Not What You Think_ The Truth Behind Saturn_s Icy Moon · Boring Science

• 0:00
  Speaker 1: There is a moon in our solar system where the
• 0:02
  Speaker 1: ground is actively melting, where volcanoes erupt not with rock
• 0:08
  Speaker 1: but with liquid sulfur, and where the surface is being
• 0:11
  Speaker 1: completely replaced so fast that craters formed millions of years
• 0:16
  Speaker 1: ago no longer exist. There is another one hiding an
• 0:21
  Speaker 1: ocean beneath its ice, a liquid ocean that has been
• 0:25
  Speaker 1: sitting there longer than complex life has existed on Earth.
• 0:30
  Speaker 1: One moon orbits its planet in the wrong direction entirely,
• 0:35
  Speaker 1: moving against everything around it, a sign that it does
• 0:39
  Speaker 1: not belong where it is, and never did. Another has
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  Speaker 1: cliffs that drop 12 miles straight down. At that moon's gravity,
• 0:49
  Speaker 1: a single misstep off the edge means 11 minutes of
• 0:52
  Speaker 1: falling before you hit anything. These are not distant hypotheticals.
• 0:58
  Speaker 1: These are real places, photographed, measured, mapped. Some of them
• 1:04
  Speaker 1: have been visited by spacecraft. Some have barely been looked at.
• 1:09
  Speaker 1: All of them are stranger than the word moon suggests.
• 1:14
  Speaker 1: When most people hear that word, they picture the pale
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  Speaker 1: disk overhead, the thing that pulls the tides, the thing
• 1:22
  Speaker 1: that has been there every night their whole lives. That
• 1:26
  Speaker 1: image barely scratches what is out there. Across this solar system,
• 1:32
  Speaker 1: orbiting Jupiter and Saturn and Neptune and Uranus and Mars
• 1:37
  Speaker 1: and even Pluto, there are worlds that have weather systems,
• 1:41
  Speaker 1: active geology, subsurface oceans, organic chemistry, geysers erupting miles into
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  Speaker 1: airless skies. Worlds with mountains taller than anything on Earth,
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  Speaker 1: worlds with seas made not of water but of liquid methane,
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  Speaker 1: worlds that should be frozen and dead and are not.
• 2:06
  Speaker 1: The moons of our solar system are among the most
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  Speaker 1: extreme environments ever discovered, and most of them are unknown
• 2:14
  Speaker 1: to most people alive. We begin with the one that
• 2:18
  Speaker 1: is on fire. If you find comfort in these gentle
• 2:22
  Speaker 1: explorations of space and science, I'd be grateful if you'd
• 2:26
  Speaker 1: consider subscribing and giving this video a like. In March
• 2:36
  Speaker 1: of 1979, three days after Voyager 1 had already passed
• 2:41
  Speaker 1: its closest point to Jupiter, a navigation engineer named Linda
• 2:46
  Speaker 1: Morabito was processing images of Io when she noticed something
• 2:51
  Speaker 1: wrong with one of them. She was not looking for volcanoes.
• 2:56
  Speaker 1: Nobody was. The task in front of her was routine
• 3:00
  Speaker 1: navigation work. checking star positions in the background of the
• 3:05
  Speaker 1: image to confirm the spacecraft's location. But there was something
• 3:09
  Speaker 1: in the photograph that did not belong. On the edge
• 3:12
  Speaker 1: of Io's disk, rising above the surface, was a faint
• 3:16
  Speaker 1: umbrella-shaped structure. It looked like a cloud. Io does not
• 3:21
  Speaker 1: have clouds. Morabito stared at it. She ran through alternative explanations.
• 3:28
  Speaker 1: Another moon. An image artifact. A smear in the processing.
• 3:33
  Speaker 1: none of those held up. What she was looking at,
• 3:37
  Speaker 1: extending nearly 200 miles above the surface of this small moon,
• 3:42
  Speaker 1: was the plume of an active volcanic eruption, the first
• 3:46
  Speaker 1: ever detected on any world other than Earth. Io had
• 3:51
  Speaker 1: been assumed to be a cold, dead, heavily cratered rock.
• 3:56
  Speaker 1: The image on her screen said otherwise. What made the
• 3:59
  Speaker 1: discovery stranger still was that someone had predicted it. Eight
• 4:05
  Speaker 1: days before Voyager 1's closest approach, a paper was published
• 4:09
  Speaker 1: by a team of researchers, including Stanton Peale, suggesting that
• 4:14
  Speaker 1: Io might be volcanically active. The physics were compelling. Jupiter
• 4:20
  Speaker 1: is the largest planet in the solar system. Its gravity
• 4:23
  Speaker 1: is enormous. Io orbits close, and as it does, it
• 4:28
  Speaker 1: is being pulled on not just by Jupiter, but by
• 4:31
  Speaker 1: Europa and Ganymede, two other large moons farther out. Their
• 4:36
  Speaker 1: gravitational tugs push and pull on Io's orbit, keeping it
• 4:41
  Speaker 1: slightly elliptical rather than perfectly circular. Because the orbit is
• 4:46
  Speaker 1: not a perfect circle, Io's distance from Jupiter changes as
• 4:51
  Speaker 1: it goes around. Jupiter's gravity is stronger when Io is closer,
• 4:57
  Speaker 1: weaker when it is farther. The result is that the
• 5:01
  Speaker 1: moon gets squeezed and stretched continuously as it orbits. Not gently. Relentlessly.
• 5:10
  Speaker 1: Think about what happens when you bend a metal paperclip
• 5:13
  Speaker 1: back and forth. Do it 20 times fast and the
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  Speaker 1: metal at the bend gets warm. Do it long enough
• 5:21
  Speaker 1: and it snaps. Io has been doing the equivalent of
• 5:25
  Speaker 1: this for billions of years. and the heat produced by
• 5:28
  Speaker 1: all that internal flexing has nowhere to go except up.
• 5:34
  Speaker 1: It erupts. The prediction and the discovery arrived within eight
• 5:39
  Speaker 1: days of each other. Peale and his co-authors heard about
• 5:43
  Speaker 1: Morabito's image just as their paper was being published. The
• 5:48
  Speaker 1: timing was extraordinary. The surface Voyager revealed, and that subsequent
• 5:54
  Speaker 1: spacecraft have mapped in increasing detail, is unlike anything else
• 5:59
  Speaker 1: in the solar system. There are more than 400 active
• 6:03
  Speaker 1: volcanoes on Io. Not dormant volcanoes, not ancient calderas. Active ones,
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  Speaker 1: erupting now, reshaping the surface continuously. The ground in some
• 6:17
  Speaker 1: regions is liquid lava. In others, it is planes of
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  Speaker 1: sulfur that shift color depending on temperature, running from white
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  Speaker 1: to pale yellow to deep orange, to red, to black,
• 6:30
  Speaker 1: as you move from cooler deposits to hotter ones. If
• 6:34
  Speaker 1: you could stand on Io and look at the horizon
• 6:37
  Speaker 1: in any direction, you would see something erupting. Mountains rise
• 6:42
  Speaker 1: above those plains, some taller than Everest, some significantly taller.
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  Speaker 1: The tallest known peak on Io reaches roughly 11 miles
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  Speaker 1: above the surrounding terrain. On Earth, Mountains that large are
• 6:58
  Speaker 1: built by tectonic plates grinding together. Io has no tectonic plates.
• 7:05
  Speaker 1: Its mountains form through a different mechanism entirely. The surface
• 7:09
  Speaker 1: of Io is being constantly buried under fresh volcanic material.
• 7:14
  Speaker 1: Lava flows, sulfur deposits, ash, ejecta from plumes, all of
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  Speaker 1: it piles up. As new material accumulates on top, the
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  Speaker 1: older material beneath it gets pushed down. That constant downward
• 7:30
  Speaker 1: pressure on buried rock eventually forces some of it back
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  Speaker 1: up through fractures, and where it pushes up, mountains form.
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  Speaker 1: Not from plates colliding, but from the sheer weight of
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  Speaker 1: continuous deposition pressing down for millions of years. The mountains
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  Speaker 1: are a byproduct of the volcano rate. the more violently
• 7:55
  Speaker 1: a world resurfaces itself, the more it builds its own
• 7:59
  Speaker 1: highlands by compression. There are no craters on Io, not
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  Speaker 1: because it has never been struck by meteorites, but because
• 8:08
  Speaker 1: any crater that forms is buried or flooded or eroded
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  Speaker 1: away by volcanic activity before it has time to be preserved.
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  Speaker 1: The entire surface has been replaced, probably many times over,
• 8:23
  Speaker 1: within the last few million years. That is an almost
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  Speaker 1: incomprehensibly fast geological pace. The Moon has craters that have
• 8:33
  Speaker 1: sat unchanged for four billion years. Io cannot hold a
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  Speaker 1: crater for even a fraction of that time. Loki Patera
• 8:41
  Speaker 1: sits near the equator. It is a lava lake and
• 8:45
  Speaker 1: it is enormous. The surface area of Loki Patera is
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  Speaker 1: larger than Lake Ontario. This is not a metaphor for scale.
• 8:54
  Speaker 1: This is a comparison of actual area. The lake is
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  Speaker 1: covered by a crust of cooled and solidifying lava, but
• 9:02
  Speaker 1: periodically that crust overturns. Fresh, hot magma from beneath breaks through,
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  Speaker 1: the cooler crust sinks, and a wave of bright new
• 9:13
  Speaker 1: lava spreads across the surface of the lake. This overturn
• 9:17
  Speaker 1: happens roughly every one to two years, And it is
• 9:21
  Speaker 1: regular enough that astronomers on Earth can predict when it
• 9:24
  Speaker 1: will occur and watch it happen through ground-based telescopes. A
• 9:30
  Speaker 1: lava lake on a moon of Jupiter, 400 million miles away,
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  Speaker 1: running on a schedule precise enough to observe from here.
• 9:40
  Speaker 1: Loki Patera has been doing this, as far as researchers
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  Speaker 1: can tell, for as long as anyone has been watching.
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  Speaker 1: The plumes are what make Io's volcanism visible across enormous distances.
• 9:55
  Speaker 1: Some of Io's volcanic eruptions send material not just into
• 10:00
  Speaker 1: the air, but out into space. In an environment with
• 10:04
  Speaker 1: no real atmosphere to slow them down, volcanic plumes on
• 10:08
  Speaker 1: Io can reach heights of 300 miles. They expand into
• 10:13
  Speaker 1: the vacuum above the surface and then the material falls
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  Speaker 1: back down, spreading outward in a ring. From orbit, these
• 10:22
  Speaker 1: ring-shaped deposits are visible on the surface, sometimes hundreds of
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  Speaker 1: miles across, centered on the vent that produced them. They
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  Speaker 1: are not rings of rock. They are rings of sulfur
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  Speaker 1: dioxide frost, bright white against the orange and yellow planes.
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  Speaker 1: Some of the larger plume fields have been visibly reshaped
• 10:45
  Speaker 1: between one spacecraft's visit and the next. Not all of
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  Speaker 1: what Io erupts comes back down. A fraction of the
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  Speaker 1: material escapes Io's weak gravity entirely and enters orbit around Jupiter.
• 11:00
  Speaker 1: Over time, this volcanic output has built up a donut-shaped
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  Speaker 1: cloud of ionized gas called the Io plasma torus. It
• 11:09
  Speaker 1: wraps around Jupiter along the path of Io's orbit, glowing
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  Speaker 1: faintly in ultraviolet light. a permanent feature of the Jovian system.
• 11:20
  Speaker 1: Io is physically losing mass to Jupiter. Every eruption contributes
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  Speaker 1: a small fraction of material to this torus. The Moon
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  Speaker 1: is being consumed, slowly, by the gravity that makes it
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  Speaker 1: so violent in the first place. Jupiter's radiation belts are
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  Speaker 1: intense at Io's distance. The charged particles trapped in Jupiter's
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  Speaker 1: magnetic field bombard Io's surface constantly, breaking down sulfur compounds,
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  Speaker 1: sputtering material off into space, adding to the plasma torus.
• 11:57
  Speaker 1: Io is embedded in one of the most hostile radiation
• 12:00
  Speaker 1: environments in the solar system. Any spacecraft that attempted to
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  Speaker 1: orbit Io for an extended period would have its electronics
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  Speaker 1: degraded within weeks. Galileo, the spacecraft that spent eight years
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  Speaker 1: studying the Jupiter system, was deliberately sent into Jupiter's atmosphere
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  Speaker 1: at the end of its mission to avoid contaminating the moons.
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  Speaker 1: During its close flybys of Io, the radiation began affecting
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  Speaker 1: its instruments in measurable ways. Even brief visits cost something.
• 12:35
  Speaker 1: What the Galileo flybys confirmed was what Voyager had only glimpsed,
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  Speaker 1: The volcanic activity on Io is not localized to a
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  Speaker 1: few regions. It is global. Heat is escaping from the
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  Speaker 1: interior at a rate far higher than any other rocky
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  Speaker 1: body in the solar system. Earth has volcanoes because its
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  Speaker 1: interior is still warm from formation and from the decay
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  Speaker 1: of radioactive elements. Io's heat comes primarily from tidal flexing.
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  Speaker 1: an external mechanical source that will continue as long as
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  Speaker 1: Europa and Ganymede hold their current orbital positions, which means
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  Speaker 1: as long as Jupiter exists. The engine is not running down.
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  Speaker 1: There is a version of a moon you probably carry
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  Speaker 1: in your head, pale, cold, cratered, silent. That image comes
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  Speaker 1: from our own moon, which is geologically quiet and has
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  Speaker 1: been for billions of years. Io dismantles that image entirely.
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  Speaker 1: It is a moon the size of Earth's moon, in
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  Speaker 1: orbit around a gas giant, and it is the most
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  Speaker 1: geologically violent body in the entire solar system. Not the
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  Speaker 1: most active planet. Not the most active rocky world. The
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  Speaker 1: most active body we have found anywhere, full stop. Its
• 14:02
  Speaker 1: surface is being continuously rebuilt. Its mountains are growing and collapsing.
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  Speaker 1: Its lava lakes are turning over on schedules you could
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  Speaker 1: put in a calendar, and it is actively contributing material
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  Speaker 1: to the space around its planet in quantities large enough
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  Speaker 1: to form a structure visible from Earth. Somewhere on Io
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  Speaker 1: right now, a volcano is erupting, a lava lake is overturning,
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  Speaker 1: and sulfur is raining back down onto a surface that
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  Speaker 1: will look nothing like this in another million years. About 400,000
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  Speaker 1: miles from Jupiter, smaller than our own Moon, and wrapped
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  Speaker 1: in a shell of water ice, sits what many planetary
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  Speaker 1: scientists consider the most important object in the Solar System
• 14:52
  Speaker 1: outside of Earth itself. Europa looks nothing like Io. Where
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  Speaker 1: Io is orange and yellow and constantly tearing itself apart,
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  Speaker 1: Europa is pale and quiet and almost featureless from a distance. Smooth,
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  Speaker 1: almost glassy, with a surface that reflects sunlight so cleanly
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  Speaker 1: it shines like polished bone against the black of space.
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  Speaker 1: But that smoothness is not a sign of calm. It
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  Speaker 1: is a sign of something working underneath. A surface this
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  Speaker 1: smooth means craters are not surviving. Impact scars that would
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  Speaker 1: persist for billions of years on a dead world are
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  Speaker 1: being erased, slowly and continuously. as the crust above whatever
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  Speaker 1: is below shifts and refreezes and shifts again. Europa's surface
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  Speaker 1: is young. Geologically speaking, it is practically new. And in
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  Speaker 1: planetary science, a young surface almost always means something is
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  Speaker 1: still active beneath it. The cracks tell you more. Stretch
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  Speaker 1: across Europa's face in every direction, reddish brown and sharp,
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  Speaker 1: are long fractures called lineae. Some of them run for
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  Speaker 1: thousands of miles. They cross each other, bend, fade, then resume.
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  Speaker 1: The colour comes from something mixed into the ice along
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  Speaker 1: those lines, something that is not pure water. The leading
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  Speaker 1: explanation is that these fractures are places where the ice
• 16:32
  Speaker 1: has split open, ocean water has welled up into the
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  Speaker 1: and radiation from Jupiter has cooked whatever salts and minerals
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  Speaker 1: were dissolved in that water into reddish organic compounds. If
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  Speaker 1: that is right, then the staining you can see across
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  Speaker 1: Europa's surface from orbit is essentially sea salt, processed and darkened,
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  Speaker 1: marking every place where the ocean below has touched the
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  Speaker 1: outside world. The same force that is destroying Io is
• 17:06
  Speaker 1: at work here, but quieter. Jupiter's gravity is immense, and
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  Speaker 1: Europa sits close enough to feel its pull unevenly across
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  Speaker 1: its own body. Europa and Ganymede and Io are locked
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  Speaker 1: in a gravitational rhythm with each other, each tugging slightly
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  Speaker 1: as they orbit, and the result is that Europa's shape
• 17:28
  Speaker 1: flexes on every orbit. Not dramatically, not enough to crack
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  Speaker 1: the crust wide open the way Io is cracked. Just enough.
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  Speaker 1: Just enough heat to keep water liquid far below the
• 17:42
  Speaker 1: surface in an environment that, by any reasonable estimate, should
• 17:47
  Speaker 1: be frozen solid to the core. This is tidal heating
• 17:51
  Speaker 1: doing something different than it does at Io. At Io,
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  Speaker 1: it is turning the interior into a furnace. At Europa,
• 18:01
  Speaker 1: it is keeping a light on. Barely warm enough. precisely enough.
• 18:08
  Speaker 1: The ocean beneath the ice has not frozen because Jupiter
• 18:12
  Speaker 1: will not let it freeze. The evidence for the ocean
• 18:15
  Speaker 1: is not theory. It is measurement. When the Galileo spacecraft
• 18:21
  Speaker 1: flew past Europa in the late 1990s, it carried a magnetometer.
• 18:26
  Speaker 1: Jupiter produces one of the most powerful magnetic fields in
• 18:30
  Speaker 1: the solar system, and that field washes over everything in
• 18:34
  Speaker 1: orbit around it. When Galileo measured the way Europa responded
• 18:39
  Speaker 1: to Jupiter's magnetic field, the readings were not what you
• 18:43
  Speaker 1: would expect from a ball of rock and ice. They
• 18:46
  Speaker 1: were what you expect from a conductor. Something inside Europa
• 18:51
  Speaker 1: was responding to the changing magnetic field the way a
• 18:55
  Speaker 1: salty liquid responds. The induced magnetic field that Galileo detected
• 19:01
  Speaker 1: matches almost exactly what you would predict if Europa had
• 19:05
  Speaker 1: a global layer of electrically conductive fluid just below its surface.
• 19:11
  Speaker 1: Saltwater is an excellent conductor. Ice is not. The ocean
• 19:16
  Speaker 1: is not a hypothesis built on wishful thinking. It is
• 19:20
  Speaker 1: the explanation that fits the magnetic data. What nobody agrees
• 19:25
  Speaker 1: on is how thick the ice shell above that ocean
• 19:29
  Speaker 1: actually is. The debate has been running for decades. Some
• 19:34
  Speaker 1: researchers look at the surface features and argue for a
• 19:38
  Speaker 1: shell no more than 10 miles deep, possibly thinner in places.
• 19:43
  Speaker 1: Others look at thermal models and the way certain surface
• 19:46
  Speaker 1: structures appear to form and argue for something closer. to
• 19:50
  Speaker 1: 60 miles. The difference is not academic. A 10 mile
• 19:56
  Speaker 1: ice shell is a very different world from a 60
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  Speaker 1: mile one. A thinner shell is more likely to be
• 20:03
  Speaker 1: fractured all the way through in places. more likely to
• 20:07
  Speaker 1: allow material to cycle between the ocean and the surface,
• 20:12
  Speaker 1: more likely to produce the kind of exchange that could
• 20:15
  Speaker 1: carry chemical energy from the ocean up toward the ice
• 20:19
  Speaker 1: and back. A 60-mile shell is closer to a sealed vault.
• 20:24
  Speaker 1: The ocean inside is still there, still liquid, but it
• 20:28
  Speaker 1: is far more isolated. Whether life needs that exchange or
• 20:32
  Speaker 1: not is a separate question. but the thickness of the
• 20:36
  Speaker 1: ice matters enormously to how we think about what might
• 20:40
  Speaker 1: be happening inside. The ocean itself by best current estimates
• 20:45
  Speaker 1: holds roughly twice the volume of all Earth's oceans combined.
• 20:50
  Speaker 1: Twice the water compressed beneath miles of ice sitting above
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  Speaker 1: a rocky ocean floor that has never seen sunlight. It
• 20:59
  Speaker 1: is dark down there in a way that is hard
• 21:02
  Speaker 1: to describe. Cold above where the ice meets the water,
• 21:08
  Speaker 1: but not necessarily cold at the bottom. If Jupiter's tidal
• 21:12
  Speaker 1: forces are flexing the ocean floor the same way they
• 21:16
  Speaker 1: flex the crust above, there is heat being generated at depth.
• 21:21
  Speaker 1: Heat at the floor of an ocean sitting above rock
• 21:25
  Speaker 1: is something we understand from Earth. On Earth, hydrothermal vents
• 21:30
  Speaker 1: on the deep sea floor release superheated water laced with
• 21:35
  Speaker 1: minerals into the surrounding ocean. The organisms that live around
• 21:40
  Speaker 1: those vents have no connection to sunlight whatsoever. They run
• 21:45
  Speaker 1: entirely on chemical energy, bacteria, tube worms, crabs, shrimp, fish,
• 21:51
  Speaker 1: entire ecosystems built in complete darkness around the heat and
• 21:55
  Speaker 1: chemistry of the rock below. If Europa's ocean floor is
• 21:59
  Speaker 1: being warmed from below, The conditions for something like that
• 22:03
  Speaker 1: may exist in a place that has never seen a
• 22:05
  Speaker 1: photon from the sun. The chemistry matters as much as
• 22:09
  Speaker 1: the heat. On Earth, hydrothermal vents work because water and
• 22:14
  Speaker 1: rock react. Hot water moving through rock dissolves minerals, strips hydrogen,
• 22:21
  Speaker 1: carries energy that certain microorganisms can metabolize. The reaction that
• 22:27
  Speaker 1: drives that chemistry is not exotic. It requires liquid water, rock,
• 22:33
  Speaker 1: and heat. Europa appears to have all three. The ocean
• 22:39
  Speaker 1: is in contact with a rocky floor. The tidal heating
• 22:42
  Speaker 1: provides warmth at depth, and the water has been there
• 22:46
  Speaker 1: long enough to have dissolved whatever the rock has to offer.
• 22:51
  Speaker 1: The reddish material in the lineae suggests the ocean is
• 22:54
  Speaker 1: not pure. It carries things. Whatever those things are, They
• 23:00
  Speaker 1: have been cycling in that water for a very long time.
• 23:04
  Speaker 1: The Europa Clipper mission launched in 2024 and is currently
• 23:09
  Speaker 1: crossing the solar system toward the Jupiter system. It is
• 23:13
  Speaker 1: not a lander. It will not drill through the ice.
• 23:18
  Speaker 1: What it will do, through dozens of close flybys, is
• 23:22
  Speaker 1: map the surface in extraordinary detail. measure the magnetic field
• 23:27
  Speaker 1: with far more precision than Galileo could manage, search for
• 23:32
  Speaker 1: active plumes like the ones seen at Enceladus, and attempt
• 23:36
  Speaker 1: to determine how thick the ice shell actually is. If
• 23:41
  Speaker 1: it finds plumes, the implications are significant. Plumes from Enceladus
• 23:47
  Speaker 1: have already told us what that moon's ocean contains. sampled
• 23:52
  Speaker 1: by a spacecraft flying through them without needing to land
• 23:56
  Speaker 1: at all. If Europa is also venting its ocean into
• 24:00
  Speaker 1: space in any form, even intermittently, Clipper will be positioned
• 24:05
  Speaker 1: to taste it. The mission arrives in the early 2030s.
• 24:10
  Speaker 1: The data it returns will rewrite what we know about
• 24:14
  Speaker 1: this moon regardless of what it finds. The age of
• 24:18
  Speaker 1: the ocean is its own kind of fact. Europa has
• 24:22
  Speaker 1: probably had liquid water beneath its surface for billions of years.
• 24:28
  Speaker 1: Not hundreds of millions. Billions. The tidal mechanism that heats
• 24:34
  Speaker 1: it now was in place shortly after Jupiter's moon system
• 24:38
  Speaker 1: formed and stabilized into its current orbital resonance. That ocean
• 24:43
  Speaker 1: has been sitting in contact with rock, cycling whatever chemistry
• 24:48
  Speaker 1: the water and the floor produce between them, for longer
• 24:51
  Speaker 1: than the Cambrian explosion is from now. Longer than complex
• 24:56
  Speaker 1: multicellular life has existed on Earth. Whatever processes are running
• 25:01
  Speaker 1: in that water have had enormous time to run. Whether
• 25:06
  Speaker 1: that time has produced anything is the question we cannot
• 25:10
  Speaker 1: answer from here. Whatever is down there has had longer
• 25:14
  Speaker 1: to develop than complex life has been on Earth, and
• 25:18
  Speaker 1: we have never sent anything to look If Ganymede were
• 25:22
  Speaker 1: orbiting the Sun instead of Jupiter, there would be no
• 25:25
  Speaker 1: debate about what to call it. It would be a planet,
• 25:29
  Speaker 1: larger than Mercury, larger than Pluto, larger than every dwarf
• 25:35
  Speaker 1: planet we have catalogued in the outer solar system. The
• 25:39
  Speaker 1: only reason it carries the word moon in its name
• 25:42
  Speaker 1: is that Jupiter got to it first. At 3,270 miles across,
• 25:50
  Speaker 1: Ganymede is the largest moon in the entire solar system,
• 25:54
  Speaker 1: and it is not close. Mercury is a full planet,
• 25:58
  Speaker 1: with its own entry in textbooks and its own place
• 26:02
  Speaker 1: in the classical order of the solar system. Ganymede is wider.
• 26:08
  Speaker 1: This matters not just as a piece of trivia, but
• 26:11
  Speaker 1: as a framing device, because the assumptions that come with
• 26:15
  Speaker 1: the word moon– small, cold, geologically dead do not apply here.
• 26:23
  Speaker 1: Ganymede has a rocky iron core. Surrounding that core is
• 26:28
  Speaker 1: a deep mantle of water ice, potentially layered with pockets
• 26:32
  Speaker 1: of liquid water sandwiched between different phases of high pressure ice.
• 26:38
  Speaker 1: Above that is an outer crust of mixed rock and ice.
• 26:43
  Speaker 1: The interior of Ganymede is more complex than the interior
• 26:47
  Speaker 1: of many objects we call planets. it is not a
• 26:50
  Speaker 1: simple frozen ball it is a structured differentiated world with
• 26:56
  Speaker 1: distinct layers each behaving differently under the pressure and temperature
• 27:01
  Speaker 1: conditions specific to its depth the ice mantle is worth
• 27:05
  Speaker 1: pausing on water behaves strangely under extreme pressure it does
• 27:10
  Speaker 1: not simply remain solid or liquid it transitions through multiple
• 27:16
  Speaker 1: crystalline phases each with different densities and properties. Researchers believe
• 27:22
  Speaker 1: that inside Ganymede, layers of liquid water may be trapped
• 27:27
  Speaker 1: between layers of different ice phases, stacked one on top
• 27:31
  Speaker 1: of another like a geological layer cake. The total volume
• 27:36
  Speaker 1: of water in this system could be enormous. Estimates suggest
• 27:41
  Speaker 1: Ganymede holds more liquid water than exists on the entire
• 27:46
  Speaker 1: surface of This is not confirmed, but the internal structure
• 27:52
  Speaker 1: models are consistent enough that it is taken seriously by
• 27:56
  Speaker 1: planetary scientists. Then there is the magnetic field. This is
• 28:01
  Speaker 1: where Ganymede stops being surprising and starts being genuinely strange.
• 28:07
  Speaker 1: Ganymede is the only moon in the solar system known
• 28:11
  Speaker 1: to generate its own global magnetic field. Not a residual
• 28:15
  Speaker 1: magnetism frozen into old rock. A living, active magnetic field
• 28:21
  Speaker 1: produced by something churning inside the moon right now. To
• 28:26
  Speaker 1: generate a magnetic field this way, you need a core
• 28:30
  Speaker 1: that is at least partially molten, electrically conductive, and actively moving.
• 28:36
  Speaker 1: Ganymede apparently has all three. The dynamo running inside this
• 28:41
  Speaker 1: moon is producing a magnetosphere, a region of space shaped
• 28:46
  Speaker 1: by Ganymede's own magnetic influence, sitting inside Jupiter's far larger magnetosphere.
• 28:53
  Speaker 1: The two fields interact. Where Ganymede's field meets Jupiter's, the
• 28:59
  Speaker 1: charged particles trapped in Juppie, ITER's magnetosphere get funneled down
• 29:04
  Speaker 1: along Ganymede's magnetic field lines and slam into the moon's
• 29:08
  Speaker 1: polar regions. The result is auroras, visible auroras. The Hubble
• 29:13
  Speaker 1: Space Telescope has observed them directly, glowing bands of ultraviolet
• 29:19
  Speaker 1: light near Ganymede's poles, a moon with its own auroras.
• 29:24
  Speaker 1: That is not a description that fits easily into the
• 29:27
  Speaker 1: category of small airless rock. The surface of Ganymede is
• 29:32
  Speaker 1: divided into two distinct types of terrain, and the contrast
• 29:36
  Speaker 1: between them is sharp enough to be obvious in photographs
• 29:39
  Speaker 1: taken from millions of miles away. The darker regions are ancient.
• 29:45
  Speaker 1: They are heavily cratered, scarred by billions of years of impacts,
• 29:50
  Speaker 1: and largely unchanged since the early solar system. The lighter
• 29:55
  Speaker 1: regions are something else entirely. They are grooved. long parallel
• 30:01
  Speaker 1: ridges and troughs cut across them in sweeping bands. The
• 30:06
  Speaker 1: craters in these zones are fewer, meaning the surface is younger,
• 30:11
  Speaker 1: meaning something reshaped this terrain after the heavy bombardment period ended.
• 30:17
  Speaker 1: Whatever happened was large enough in scale to affect roughly
• 30:21
  Speaker 1: a third of Ganymede's surface. It involved stretching, faulting, possibly
• 30:27
  Speaker 1: the upwelling of warm material from below And the honest
• 30:31
  Speaker 1: answer is that nobody has a complete explanation for why
• 30:35
  Speaker 1: it happened, when exactly it happened, or what drove it.
• 30:41
  Speaker 1: The grooved terrain of Ganymede is one of the more
• 30:44
  Speaker 1: significant unsolved problems in outer solar system geology. In June
• 30:50
  Speaker 1: of 2021, NASA's Juno spacecraft swung close to Ganymede, the
• 30:56
  Speaker 1: closest any spacecraft had come in more than two decades.
• 31:01
  Speaker 1: Juno was designed primarily to study Jupiter, but its instruments
• 31:06
  Speaker 1: were pointed at Ganymede for the flyby. The data it
• 31:10
  Speaker 1: collected on the magnetosphere was detailed in ways earlier missions
• 31:15
  Speaker 1: had not been able to achieve. Juno found that Ganymede's
• 31:19
  Speaker 1: magnetic field is more structured than simple models suggested, with
• 31:25
  Speaker 1: complexity in how it connects to and interacts with Jupiter's field.
• 31:31
  Speaker 1: The plasma environment around the Moon, the mix of charged
• 31:35
  Speaker 1: particles trapped in the combined magnetic system, turned out to
• 31:40
  Speaker 1: be richer and more dynamic than expected. Juno also captured
• 31:46
  Speaker 1: surface imagery sharper than anything available since the Galileo mission's
• 31:51
  Speaker 1: close passes in the late 1990s. The images showed both
• 31:57
  Speaker 1: terrain types in high resolution, and added texture to what
• 32:01
  Speaker 1: researchers already knew, without resolving the deeper questions about what
• 32:06
  Speaker 1: caused the grooved regions. A spacecraft is now on its
• 32:10
  Speaker 1: way to go much further. JUICE, the Jupiter Icy Moons Explorer,
• 32:16
  Speaker 1: built by the European Space Agency, launched in 2023. It
• 32:22
  Speaker 1: is heading to the Jupiter system with Ganymede as its
• 32:25
  Speaker 1: primary long-term target. The plan is not a flyby. JUICE
• 32:30
  Speaker 1: will eventually enter orbit around Ganymede, becoming the first spacecraft
• 32:35
  Speaker 1: ever to orbit a moon other than our own. From orbit,
• 32:40
  Speaker 1: it will study the magnetic field in fine detail, map
• 32:44
  Speaker 1: the surface comprehensively, probe the interior using radar and gravity measurements,
• 32:50
  Speaker 1: and work toward a clearer picture of whether that layered
• 32:54
  Speaker 1: internal ocean is real and how extensive it is. The
• 32:59
  Speaker 1: largest moon in the solar system generates its own magnetic field,
• 33:05
  Speaker 1: possibly has a layered internal ocean, and we still do
• 33:09
  Speaker 1: not know what happened to reshape half its surface billions
• 33:13
  Speaker 1: of years ago. Callisto looks like a world that gave up.
• 33:19
  Speaker 1: Where Io is erupting constantly and Europa's surface is being
• 33:23
  Speaker 1: cracked and reformed by an active ocean below, Callisto just
• 33:28
  Speaker 1: sits there. frozen, battered, unchanged. Every impact it has absorbed
• 33:35
  Speaker 1: over four billion years is still exactly where it landed.
• 33:40
  Speaker 1: There are no volcanoes resurfacing the crust, no tidal forces
• 33:45
  Speaker 1: flexing the interior, no geological process of any kind smoothing
• 33:50
  Speaker 1: things over. What you see on Callisto is what the
• 33:54
  Speaker 1: early solar system did to it, preserved in full, like
• 33:59
  Speaker 1: a photograph that was never disturbed. Scientists have a name
• 34:04
  Speaker 1: for surfaces like this. They call it a saturated crust,
• 34:09
  Speaker 1: meaning there are so many craters that new ones are
• 34:12
  Speaker 1: simply forming on top of old ones. You cannot pack
• 34:15
  Speaker 1: more impacts into it. The surface is full. This makes
• 34:20
  Speaker 1: Callisto genuinely useful in a way that Io and Europa
• 34:24
  Speaker 1: are not, because nothing has been erased The Moon carries
• 34:28
  Speaker 1: a direct record of the period astronomers call the Late
• 34:32
  Speaker 1: Heavy Bombardment, a phase roughly 4 billion years ago when
• 34:38
  Speaker 1: the inner solar system was being hammered by an enormous
• 34:41
  Speaker 1: flux of asteroids and comets. That record has been wiped
• 34:46
  Speaker 1: clean on most worlds. Earth's craters are mostly gone. eroded
• 34:52
  Speaker 1: by weather and plate tectonics. Io's surface is being replaced
• 34:57
  Speaker 1: so fast that nothing older than a million years survives.
• 35:01
  Speaker 1: Europa's ice has been recycled. Callisto kept everything. If you
• 35:07
  Speaker 1: want to understand what the early solar system was throwing around,
• 35:11
  Speaker 1: Callisto is one of the only places left where you
• 35:14
  Speaker 1: can still read that history directly off the ground. The
• 35:18
  Speaker 1: most dramatic feature on that ancient surface is a structure
• 35:23
  Speaker 1: called Valhalla. It sits near the center of Callisto's Jupiter-facing hemisphere,
• 35:28
  Speaker 1: and from above it looks like a target. There is
• 35:32
  Speaker 1: a bright central plane roughly 360 miles across, then ring
• 35:38
  Speaker 1: after ring after ring spreading outward from it, each one
• 35:42
  Speaker 1: a concentric arc of ridges and scarps. reaching nearly 2,000
• 35:47
  Speaker 1: miles from the center before they fade out. The collision
• 35:52
  Speaker 1: that made Valhalla was so powerful that it sent waves
• 35:56
  Speaker 1: rippling outward through the crust in every direction. Those waves
• 36:01
  Speaker 1: simply froze in place before they could dissipate. What remains
• 36:06
  Speaker 1: is a permanent record of that single moment of impact,
• 36:10
  Speaker 1: locked into the surface for billions of years. nothing on
• 36:14
  Speaker 1: Callisto has been active enough to erase it. Valhalla is
• 36:19
  Speaker 1: still there, more or less exactly as it was left.
• 36:23
  Speaker 1: Across the rest of the surface, something subtler is happening.
• 36:27
  Speaker 1: In the lower lying areas between craters, the bright ice
• 36:31
  Speaker 1: that makes up much of Callisto's crust has been slowly disappearing.
• 36:36
  Speaker 1: Ice exposed to space at Callisto's distance from the Sun
• 36:40
  Speaker 1: will sublimate over long enough timescales converting directly from solid
• 36:46
  Speaker 1: to vapor without melting. As it does, whatever non-ice material
• 36:51
  Speaker 1: was mixed in with it gets left behind, accumulating on
• 36:56
  Speaker 1: the surface in a thin, dark layer. This material is
• 37:00
  Speaker 1: probably a mix of rocky minerals and organic compounds. It
• 37:05
  Speaker 1: tends to collect in the low spots while the higher
• 37:08
  Speaker 1: ground stays brighter. The result is a surface that gets
• 37:12
  Speaker 1: progressively darker and more muted with age. The bright crater
• 37:17
  Speaker 1: rims standing out sharply against terrain that has had billions
• 37:22
  Speaker 1: of years to accumulate this dark residue. None of this
• 37:26
  Speaker 1: suggests anything is happening inside Callisto. The surface has no
• 37:31
  Speaker 1: features that would point to internal heat, no young terrain,
• 37:36
  Speaker 1: no sign of any tectonic or volcanic process. By every
• 37:41
  Speaker 1: visual measure, this is a dead world, which is why
• 37:45
  Speaker 1: the readings from the Galileo spacecraft's magnetometer were unexpected. When
• 37:51
  Speaker 1: Galileo made several close passes of Callisto in the late 1990s,
• 37:57
  Speaker 1: its instruments detected a response in Callisto's magnetic environment that
• 38:02
  Speaker 1: was difficult to explain without invoking a conducting layer somewhere
• 38:07
  Speaker 1: beneath the surface. The pattern was similar to what Galileo
• 38:12
  Speaker 1: found at Europa, where a salty subsurface ocean was generating
• 38:17
  Speaker 1: a detectable electromagnetic signature. Callisto showed something weaker and less clear,
• 38:24
  Speaker 1: but it was there. The most straightforward explanation is a
• 38:29
  Speaker 1: subsurface ocean of liquid saltwater, buried deeper than Europa's and
• 38:34
  Speaker 1: far more uncertain, but possibly present. For a world that
• 38:39
  Speaker 1: looks completely inert on the outside, this was a genuinely
• 38:43
  Speaker 1: strange result. If there is an ocean inside Callisto, it
• 38:48
  Speaker 1: would have to be maintained by something. The most likely
• 38:52
  Speaker 1: candidate is a small amount of residual radioactive decay in
• 38:56
  Speaker 1: the rocky material throughout the interior. There is no evidence
• 39:00
  Speaker 1: of tidal heating the way Europa or Io experience it.
• 39:05
  Speaker 1: Callisto sits farther from Jupiter and is not caught in
• 39:09
  Speaker 1: the orbital resonance that drives the inner moons. Whatever heat
• 39:14
  Speaker 1: Callisto generates is coming from within the rock itself, slowly,
• 39:20
  Speaker 1: without drama, the fading warmth of ancient radioactive elements. Just enough, possibly,
• 39:28
  Speaker 1: to keep a thin layer of briny water liquid somewhere
• 39:31
  Speaker 1: in the deep interior. That same distance from Jupiter that
• 39:36
  Speaker 1: makes Callisto geologically quiet also makes it, in a practical sense,
• 39:43
  Speaker 1: one of the more hospitable places in the Jupiter system
• 39:47
  Speaker 1: for anything we might want to send there. Closer in,
• 39:51
  Speaker 1: near Io and Europa, Jupiter's radiation belts are intense enough
• 39:56
  Speaker 1: to threaten both electronics and human biology. Callisto sits outside
• 40:02
  Speaker 1: the worst of it. Several studies have looked at Callisto
• 40:06
  Speaker 1: as a possible base for crewed operations in the Jupiter system,
• 40:11
  Speaker 1: a staging point from which robotic vehicles could be sent
• 40:15
  Speaker 1: deeper in to explore the more scientifically compelling but far
• 40:20
  Speaker 1: more dangerous moons. A dead world used as a doorstep
• 40:24
  Speaker 1: to the ones that are not. The most cratered moon
• 40:27
  Speaker 1: in the solar system, the one that shows no signs
• 40:31
  Speaker 1: of life on its surface at all, may be hiding
• 40:34
  Speaker 1: an ocean underneath. The atmosphere on Titan is thicker than Earth's,
• 40:39
  Speaker 1: not slightly thicker. At the surface, the pressure is about
• 40:44
  Speaker 1: one and a half times what you feel standing at
• 40:46
  Speaker 1: sea level on this planet. If you were somehow standing
• 40:50
  Speaker 1: on Titan without a space suit, the pressure alone would
• 40:54
  Speaker 1: not kill you immediately. The cold would. The surface temperature
• 41:00
  Speaker 1: sits around negative 290 degrees Fahrenheit. But the atmosphere itself,
• 41:07
  Speaker 1: that dense orange shroud wrapping the entire moon, is breathable
• 41:11
  Speaker 1: in one specific sense. It is mostly nitrogen. the same
• 41:17
  Speaker 1: gas that makes up 78% of Earth's air. You would suffocate,
• 41:23
  Speaker 1: but you would not be crushed. That is a strange
• 41:26
  Speaker 1: thing to say about a moon of Saturn. Titan is
• 41:30
  Speaker 1: Saturn's largest moon and the second largest moon in the
• 41:33
  Speaker 1: entire solar system. It is bigger than Mercury. It is
• 41:38
  Speaker 1: the only moon anywhere in the solar system with a thick,
• 41:42
  Speaker 1: substantial atmosphere. And that atmosphere does something remarkable. It drives weather.
• 41:49
  Speaker 1: Real weather. Not the kind of weather scientists describe cautiously
• 41:54
  Speaker 1: with words like possible or inferred. Actual clouds. Actual rain.
• 42:01
  Speaker 1: Actual rivers and lakes and seas sitting on the surface,
• 42:04
  Speaker 1: mapped and measured by spacecraft. The catch is that none
• 42:09
  Speaker 1: of it involves water. The liquid on Titan is methane.
• 42:14
  Speaker 1: liquid methane, and its chemical cousin ethane, filling lake basins
• 42:19
  Speaker 1: and carving river channels, and raining out of orange clouds
• 42:23
  Speaker 1: onto a frozen landscape that, from the right angle, looks
• 42:27
  Speaker 1: almost familiar. In January of 2005, a probe called Huygens
• 42:33
  Speaker 1: descended through Titan's atmosphere and became the most distant object
• 42:37
  Speaker 1: humanity has ever landed on. It took two and a
• 42:41
  Speaker 1: half hours to fall through the haze, It transmitted data
• 42:45
  Speaker 1: the entire way down. When the images came back, planetary
• 42:50
  Speaker 1: scientists went quiet for a moment. The landscape looked wrong
• 42:55
  Speaker 1: in a specific way. It looked like somewhere you might recognize.
• 43:01
  Speaker 1: Rounded pebbles covered the surface near the landing site, worn
• 43:05
  Speaker 1: smooth by flowing liquid exactly the way river rocks are
• 43:09
  Speaker 1: worn smooth on Earth. Except these pebbles were made of
• 43:13
  Speaker 1: water ice, hard as granite at Titan's temperatures. Channels were
• 43:19
  Speaker 1: carved into the terrain above. Flat plains stretched out toward
• 43:24
  Speaker 1: the horizon, pale and etched with patterns that looked like
• 43:28
  Speaker 1: dried riverbeds. The Huygens probe had landed in the middle
• 43:32
  Speaker 1: of something that resembled a floodplain. No water had put
• 43:36
  Speaker 1: it there. Methane had. The methane cycle on Titan mirrors
• 43:42
  Speaker 1: the water cycle on Earth almost step for step. Liquid
• 43:46
  Speaker 1: methane on the surface evaporates into the atmosphere. It rises, cools,
• 43:53
  Speaker 1: and condenses into clouds. Those clouds produce rain that falls
• 43:58
  Speaker 1: back to the surface, collects in low-lying basins, carves channels
• 44:04
  Speaker 1: through ice and rock, and eventually drains toward the poles
• 44:08
  Speaker 1: where the largest bodies of liquid on the moon accumulate.
• 44:13
  Speaker 1: Then it evaporates again. The cycle repeats. It has been
• 44:19
  Speaker 1: repeating for an enormously long time. Earth's water cycle built
• 44:24
  Speaker 1: canyons and deltas and oceans over billions of years. Titan's
• 44:29
  Speaker 1: methane cycle has been doing the same thing, On a
• 44:33
  Speaker 1: moon where the average surface temperature would instantly freeze any
• 44:37
  Speaker 1: water solid, the seas near Titan's North Pole are not small.
• 44:43
  Speaker 1: Ligeia Mare covers an area roughly the size of Lake
• 44:47
  Speaker 1: Superior and Lake Michigan combined. Kraken Mare is larger than
• 44:52
  Speaker 1: the Caspian Sea, the largest landlocked body of water on Earth.
• 44:57
  Speaker 1: and it stretches across hundreds of thousands of square miles
• 45:01
  Speaker 1: of Titan's northern hemisphere. These are not puddles. These are
• 45:07
  Speaker 1: oceans of liquid methane and ethane sitting quietly beneath an
• 45:11
  Speaker 1: orange sky, calm enough that radar measurements from the Cassini
• 45:16
  Speaker 1: spacecraft detected almost no surface waves at all. Titan's seas
• 45:22
  Speaker 1: are still in a way that Earth's oceans never are.
• 45:26
  Speaker 1: The composition of these liquids is not pure methane. Ethane
• 45:30
  Speaker 1: comprises a significant fraction, with trace amounts of other organic
• 45:34
  Speaker 1: compounds mixed throughout. The depths of these seas remain unmeasured,
• 45:40
  Speaker 1: though some estimates suggest they may rival Earth's oceans in places,
• 45:45
  Speaker 1: creating enormous reservoirs of liquid hydrocarbons. The density of methane
• 45:50
  Speaker 1: and ethane is lower than water, a fact with strange
• 45:54
  Speaker 1: implications for buoyancy and for any future underwater exploration of
• 45:59
  Speaker 1: these worlds. Cassini spent 13 years in the Saturn system
• 46:04
  Speaker 1: and mapped Titan repeatedly through its opaque haze using radar.
• 46:10
  Speaker 1: The picture it built up is of a world with
• 46:12
  Speaker 1: distinct geography. Dune fields near the equator, river networks at mid-latitudes,
• 46:20
  Speaker 1: seas and lakes clustered toward the poles. The North Pole
• 46:25
  Speaker 1: has the most liquid. The South Pole has some. The
• 46:29
  Speaker 1: equatorial region is mostly dry, dominated by enormous fields of
• 46:34
  Speaker 1: dark dunes, made not of sand, but of organic particles
• 46:39
  Speaker 1: that have rained out of the atmosphere over millions of years.
• 46:44
  Speaker 1: Each region looks and behaves differently. Titan has climate zones
• 46:49
  Speaker 1: the way Earth does. Cassini's observations revealed not just static
• 46:55
  Speaker 1: bodies of liquid, but evidence of seasonal variation in Titan's
• 47:00
  Speaker 1: methane cycle, with surface features suggesting that methane flows across
• 47:06
  Speaker 1: as the landscape as seasons change. The southern seas, though
• 47:11
  Speaker 1: smaller than those at the North Pole, still represent enormous
• 47:16
  Speaker 1: bodies of liquid comparable to the largest lakes on Earth.
• 47:20
  Speaker 1: Here is where the chemistry becomes strange. Methane in an
• 47:24
  Speaker 1: atmosphere does not last. Ultraviolet radiation from the sun breaks
• 47:30
  Speaker 1: methane molecules apart relatively quickly on geological timescales. If nothing
• 47:36
  Speaker 1: were replacing the methane in Titan's atmosphere, it would be gone.
• 47:41
  Speaker 1: The atmosphere would collapse. The seas would drain. The weather
• 47:45
  Speaker 1: would stop. But none of that has happened. Titan's atmosphere
• 47:51
  Speaker 1: is full of methane right now, which means something is
• 47:54
  Speaker 1: actively replenishing it. The leading hypothesis is that Titan's interior
• 48:00
  Speaker 1: is the source. Methane venting from the interior, possibly through cryovolcanism,
• 48:07
  Speaker 1: meaning eruptions not of molten rock, but of volatile compounds
• 48:11
  Speaker 1: from a warmer interior into the frigid surface environment. The
• 48:16
  Speaker 1: mechanism is not confirmed. no one has caught Titan actively venting.
• 48:23
  Speaker 1: But the alternative is that this atmospheric methane has somehow
• 48:28
  Speaker 1: persisted for billions of years without replenishment, which the chemistry
• 48:33
  Speaker 1: does not support. The source of Titan's replenishing methane could
• 48:38
  Speaker 1: be anything from internal venting to chemical reactions occurring in
• 48:42
  Speaker 1: the subsurface ocean. Cryovolcanism on Titan, if it is indeed occurring,
• 48:49
  Speaker 1: represents a fundamentally different mechanism of planetary replenishment than the
• 48:54
  Speaker 1: volcanism familiar from volcanic worlds like Earth or Venus. Above
• 48:59
  Speaker 1: the surface, above the clouds, the atmosphere does something else entirely.
• 49:05
  Speaker 1: Nitrogen and methane in the upper atmosphere are struck by
• 49:09
  Speaker 1: ultraviolet light and charged particles streaming out from the Sun.
• 49:15
  Speaker 1: The energy breaks these molecules apart and drives them to
• 49:19
  Speaker 1: recombine into more complex structures. The products are called tholins,
• 49:25
  Speaker 1: a broad class of reddish-brown organic molecules that form a
• 49:29
  Speaker 1: thick photochemical smog wrapping the entire Moon. This smog is
• 49:35
  Speaker 1: what gives Titan its color. From space, It looks like
• 49:40
  Speaker 1: a fuzzy orange ball, completely featureless, no surface detail visible
• 49:45
  Speaker 1: in ordinary light. The haze is global and permanent. It
• 49:50
  Speaker 1: filters out most of the sunlight before it reaches the surface,
• 49:55
  Speaker 1: leaving the ground perpetually dim, lit roughly the way Earth
• 49:59
  Speaker 1: looks just after sunset. Tholins are not just atmospheric pollution,
• 50:05
  Speaker 1: they are chemically significant. Some of the organic molecules forming
• 50:10
  Speaker 1: in Titan's upper atmosphere are the same classes of compounds
• 50:14
  Speaker 1: found in the prebiotic chemistry of early Earth, the building
• 50:18
  Speaker 1: blocks that, under the right conditions, can lead to amino
• 50:22
  Speaker 1: acids and more complex biological molecules. On Titan, they rain
• 50:27
  Speaker 1: out of the sky and accumulate in dune fields that
• 50:30
  Speaker 1: stretch for thousands of miles. The equatorial dunes are made
• 50:34
  Speaker 1: substantially of this material, dark and organic, piling up in
• 50:40
  Speaker 1: ridges that Cassini's radar tracked across enormous distances. Titan's surface
• 50:46
  Speaker 1: is being continuously coated in the products of atmospheric chemistry
• 50:51
  Speaker 1: that resembles something from a laboratory study of life's origins.
• 50:56
  Speaker 1: The organic smog raining onto the surface has accumulated over
• 51:01
  Speaker 1: millions of years. Some estimates suggest the dune fields contain
• 51:06
  Speaker 1: billions of tons of these organic materials. Wind speeds on
• 51:11
  Speaker 1: Titan are generally low compared to Earth, creating a relatively
• 51:16
  Speaker 1: placid environment for these chemical processes to unfold. The chemical
• 51:22
  Speaker 1: reactions occurring between organic molecules and methane on Titan's surface
• 51:27
  Speaker 1: create environments unlike anything found elsewhere in the solar system.
• 51:32
  Speaker 1: Whether any of this chemistry goes further is unknown. Titan
• 51:37
  Speaker 1: is far too cold for liquid water, which most biochemistry
• 51:42
  Speaker 1: as we understand it requires. But some researchers have proposed
• 51:46
  Speaker 1: that entirely different forms of chemistry could operate in liquid methane,
• 51:52
  Speaker 1: using it the way life on Earth uses water. This
• 51:56
  Speaker 1: is speculative. Nobody has found evidence for it. But Titan
• 52:01
  Speaker 1: is the only place in the solar system, other than Earth,
• 52:05
  Speaker 1: where stable liquids pool on a surface, where rain falls
• 52:09
  Speaker 1: and rivers run, and where complex organic chemistry is actively
• 52:15
  Speaker 1: happening on a planetary scale. That combination makes it worth
• 52:20
  Speaker 1: looking at very carefully. The Dragonfly mission is going to
• 52:25
  Speaker 1: do exactly that. Dragonfly is a rotorcraft lander. Essentially, a
• 52:31
  Speaker 1: large drone, nuclear-powered, designed to fly through Titan's thick atmosphere
• 52:36
  Speaker 1: from site to site, rather than rolling slowly across the
• 52:40
  Speaker 1: surface the way a rover does. It is scheduled to
• 52:44
  Speaker 1: arrive in the 2030s. The dense atmosphere that makes Titan
• 52:49
  Speaker 1: so strange also makes flight relatively easy compared to somewhere
• 52:54
  Speaker 1: like Mars, where the air is so thin that rotorcraft struggle.
• 53:00
  Speaker 1: On Titan, you can fly with much less effort. Dragonfly
• 53:05
  Speaker 1: will land near the equatorial dune fields first, sample the
• 53:10
  Speaker 1: organic material sitting there, and then fly to other sites,
• 53:15
  Speaker 1: potentially including the rim of a crater where water ice
• 53:19
  Speaker 1: and organic molecules may have mixed in liquid water during
• 53:23
  Speaker 1: the impact's heat, creating conditions that briefly resembled early Earth's chemistry.
• 53:29
  Speaker 1: Each landing site is a different laboratory. Each one will
• 53:33
  Speaker 1: give a different chemical snapshot of what Titan's surface is
• 53:36
  Speaker 1: made of and what reactions are happening there. Somewhere below
• 53:40
  Speaker 1: those orange clouds, methane rain is falling into seas the
• 53:45
  Speaker 1: size of terrestrial oceans. And in the 2030s, a spacecraft
• 53:50
  Speaker 1: shaped like a dragonfly is going to land there and
• 53:54
  Speaker 1: start looking around. Enceladus is 300 miles across. That is
• 54:01
  Speaker 1: smaller than the state of Arizona. It is smaller than
• 54:04
  Speaker 1: most mid-sized countries. By the standards of the outer solar system,
• 54:10
  Speaker 1: it is barely a footnote. Objects this small run out
• 54:15
  Speaker 1: of internal heat quickly. The physics are straightforward. Small bodies
• 54:20
  Speaker 1: have more surface area relative to their volume. They radiate
• 54:25
  Speaker 1: heat into space faster, and within a billion years or so,
• 54:29
  Speaker 1: they cool into inert frozen rock. Enceladus has had four
• 54:34
  Speaker 1: and a half billion years to do exactly that. There
• 54:38
  Speaker 1: should be nothing left. No heat, no geology, no activity
• 54:43
  Speaker 1: of any kind. Just a cold, cratered, silent ball of
• 54:48
  Speaker 1: ice drifting around Saturn. And yet. In 2005, the Cassini
• 54:55
  Speaker 1: spacecraft swung around Saturn's southern hemisphere, and aimed its instruments
• 54:59
  Speaker 1: at Enceladus. What it found stopped the mission team cold.
• 55:05
  Speaker 1: Erupting from the south pole of this tiny, supposedly dead
• 55:09
  Speaker 1: moon were enormous geysers. Not small leaks. Not trace emissions.
• 55:16
  Speaker 1: Vast curtains of water vapor and ice particles blasting hundreds
• 55:21
  Speaker 1: of miles into space. catching the light of the distant
• 55:25
  Speaker 1: sun in brilliant plumes that towered over the surface. Cassini
• 55:30
  Speaker 1: had not been sent to Enceladus expecting anything like this.
• 55:35
  Speaker 1: Nobody had. The discovery rewrote the mission's priorities almost overnight.
• 55:41
  Speaker 1: The geysers were not just spectacular, they were consequential. Saturn's E-ring,
• 55:48
  Speaker 1: the broad, diffuse ring that sits beyond the main ring system,
• 55:52
  Speaker 1: had always been a slight puzzle. It is younger than
• 55:56
  Speaker 1: the others, more tenuous, made of particles too small and
• 56:01
  Speaker 1: too loosely bound to have survived billions of years in space.
• 56:06
  Speaker 1: It needed a source. Cassini found it. Enceladus sits almost
• 56:12
  Speaker 1: exactly at the centre of the E-ring, and its geysers
• 56:15
  Speaker 1: are continuously injecting fresh material into it. This tiny moon
• 56:20
  Speaker 1: is actively building one of Saturn's rings. Every second, hundreds
• 56:25
  Speaker 1: of kilograms of ice and vapor are being launched off
• 56:29
  Speaker 1: the surface and into the surrounding space. Some of it
• 56:33
  Speaker 1: falls back. A great deal of it does not. What
• 56:37
  Speaker 1: this meant was profound. A small moon was not merely leaking.
• 56:43
  Speaker 1: it was actively sculpting its planetary environment. The E-ring would
• 56:47
  Speaker 1: not exist without Enceladus. The ring's composition, its distribution, its
• 56:53
  Speaker 1: gradual dissipation over time, all of it traces back to
• 56:58
  Speaker 1: this single source. The implications rippled through planetary science. If
• 57:04
  Speaker 1: a tiny moon could be geologically active enough to feed
• 57:08
  Speaker 1: an entire ring system, What else might be happening in
• 57:12
  Speaker 1: the outer solar system that conventional models had dismissed as impossible?
• 57:17
  Speaker 1: The source of the eruptions narrows to four specific fractures
• 57:21
  Speaker 1: near the South Pole, running roughly parallel to each other.
• 57:26
  Speaker 1: They have a name now, the Tiger Stripes. Each fracture
• 57:31
  Speaker 1: is about 85 miles long. They are separated by roughly
• 57:35
  Speaker 1: 25 miles of terrain, evenly spaced, running almost perfectly parallel.
• 57:44
  Speaker 1: The tiger stripes are warmer than the surrounding surface by
• 57:47
  Speaker 1: a measurable margin. In the frigid environment around Saturn, where
• 57:53
  Speaker 1: temperatures drop to around minus 300 degrees Fahrenheit, even a
• 57:58
  Speaker 1: modest temperature difference stands out clearly to an infrared instrument.
• 58:04
  Speaker 1: The stripes are warm, and from those warm fractures, the
• 58:08
  Speaker 1: plumes rise. The precision of their geometry is itself notable.
• 58:14
  Speaker 1: Four parallel lines, evenly spaced, running for dozens of miles
• 58:19
  Speaker 1: without deviation. Such perfect symmetry suggests deep structural control. These
• 58:27
  Speaker 1: fractures are not random cracks in the surface. They are
• 58:31
  Speaker 1: expressions of something fundamental about Enceladus' interior. The cracks appear
• 58:37
  Speaker 1: to be ancient features, possibly formed early in the moon's history,
• 58:42
  Speaker 1: that have been reactivated by ongoing interior processes. They represent
• 58:48
  Speaker 1: weak points in the ice shell, where internal pressure finds
• 58:52
  Speaker 1: the path of least resistance to escape. The heat is
• 58:55
  Speaker 1: the part that does not make sense. Enceladus is too
• 59:00
  Speaker 1: small and too far from Saturn to generate the amount
• 59:04
  Speaker 1: of energy it is putting out through tidal heating alone.
• 59:08
  Speaker 1: tidal heating work It is the same mechanism squeezing Io
• 59:14
  Speaker 1: into a volcanic hellscape, the same gentle version warming Europa's ocean.
• 59:21
  Speaker 1: But the amount of heat tidal forcing should produce in
• 59:24
  Speaker 1: a body this small, at Enceladus' distance from Saturn, falls
• 59:30
  Speaker 1: short of what is actually measured at the South Pole
• 59:33
  Speaker 1: by a significant margin. The numbers do not close. Something
• 59:38
  Speaker 1: inside Enceladus is generating more energy than the standard calculations allow.
• 59:45
  Speaker 1: The exact mechanism is not settled. Some researchers believe the
• 59:51
  Speaker 1: tidal heating is concentrated specifically in the South Polar region.
• 59:56
  Speaker 1: in ways that are more efficient than a uniform model
• 59:59
  Speaker 1: would suggest. Others think the moon's interior has properties that
• 1:00:05
  Speaker 1: amplify the effect. Whatever the answer, Enceladus is hotter than
• 1:00:10
  Speaker 1: it has any right to be, and the heat is
• 1:00:13
  Speaker 1: escaping through those four parallel cracks. What the plumes are
• 1:00:17
  Speaker 1: made of tells the more important story. Cassini did not
• 1:00:22
  Speaker 1: just photograph the geysers, it flew through them. directly through
• 1:00:27
  Speaker 1: the plume material with instruments open and collecting data water
• 1:00:32
  Speaker 1: vapor was expected ice particles were expected what Cassini also
• 1:00:38
  Speaker 1: found was sodium chloride table salt carbon dioxide molecular hydrogen
• 1:00:45
  Speaker 1: silica particles of a very specific size range and complex
• 1:00:50
  Speaker 1: organic molecules the kind assembled from carbon hydrogen nitrogen and
• 1:00:57
  Speaker 1: oxygen in combinations that suggest active chemistry. Each of these
• 1:01:02
  Speaker 1: components is a message. Salt in the plume means the
• 1:01:06
  Speaker 1: water it came from has been sitting in contact with
• 1:01:10
  Speaker 1: a rocky seafloor. Ocean water picks up salts by dissolving
• 1:01:15
  Speaker 1: minerals from rock over time. Earth's oceans are salty for
• 1:01:20
  Speaker 1: exactly this reason. The presence of sodium chloride in Enceladus'
• 1:01:25
  Speaker 1: plumes means there is an ocean, and that ocean has
• 1:01:29
  Speaker 1: a rocky bottom it has been interacting with for a
• 1:01:33
  Speaker 1: long time. That alone would be remarkable. But the silica
• 1:01:38
  Speaker 1: particles go further. Silica particles of a specific size, nanometer
• 1:01:44
  Speaker 1: scale grains, form only under a narrow set of conditions.
• 1:01:50
  Speaker 1: Hot water, above roughly 90 degrees Celsius, flowing through silicate rock,
• 1:01:56
  Speaker 1: precipitate silica grains in that size range. Below that temperature,
• 1:02:01
  Speaker 1: the chemistry does not produce them. Above a certain temperature,
• 1:02:06
  Speaker 1: the grains grow larger. The particles Cassini found in the
• 1:02:10
  Speaker 1: plume of Enceladus are the right size. That means hydrothermal vents.
• 1:02:17
  Speaker 1: Somewhere on the floor of Enceladus' ocean, hot water is
• 1:02:21
  Speaker 1: flowing through rock the same way it does at Earth's
• 1:02:24
  Speaker 1: mid-ocean ridges. Molecular hydrogen fits the same picture. Hot water
• 1:02:30
  Speaker 1: reacting with certain minerals in a process called serpentinization releases
• 1:02:35
  Speaker 1: molecular hydrogen as a byproduct. Cassini detected it. The ocean
• 1:02:41
  Speaker 1: floor of Enceladus appears to be actively reacting with hot water.
• 1:02:46
  Speaker 1: On Earth, deep ocean hydrothermal vents support entire ecosystems with
• 1:02:52
  Speaker 1: no sunlight whatsoever. Bacteria tube worms, shrimp, crabs, communities of
• 1:02:59
  Speaker 1: life powered by chemical energy rather than solar energy. The
• 1:03:04
  Speaker 1: vent fluids carry hydrogen and sulfur compounds that microbes can metabolize.
• 1:03:09
  Speaker 1: Enceladus appears to have the same basic setup, warm salty water,
• 1:03:15
  Speaker 1: a rocky floor, hydrothermal activity, chemical energy in the form
• 1:03:20
  Speaker 1: of molecular hydrogen. Whatever life needs from a hydrothermal environment,
• 1:03:26
  Speaker 1: Enceladus seems to have it. These earth-based vent ecosystems thrive
• 1:03:32
  Speaker 1: in complete darkness, thousands of meters below the ocean surface,
• 1:03:37
  Speaker 1: where sunlight never penetrates. They prove that life does not
• 1:03:42
  Speaker 1: require sunlight as an energy source. The bacteria and other
• 1:03:47
  Speaker 1: microorganisms that cluster around hydrothermal chimneys extract chemical energy directly
• 1:03:55
  Speaker 1: from the minerals dissolved in the vent water. This chemosynthetic
• 1:04:00
  Speaker 1: metabolism is fundamentally different from photosynthesis, but it is no
• 1:04:06
  Speaker 1: less effective at sustaining complex communities. The ocean of Enceladus,
• 1:04:11
  Speaker 1: if it harbors life, would operate on the same principle. Chemistry,
• 1:04:17
  Speaker 1: rather than sunlight as the foundation of its food web,
• 1:04:21
  Speaker 1: The organic molecules complicate the picture further. Some of what
• 1:04:26
  Speaker 1: Cassini detected in the plume was relatively simple, methane, carbon dioxide.
• 1:04:33
  Speaker 1: But some of the molecules were heavier and more complex,
• 1:04:37
  Speaker 1: with structures that do not form easily through random processes.
• 1:04:42
  Speaker 1: Complex organics in a warm, salty, chemically active ocean in
• 1:04:47
  Speaker 1: contact with a hydrothermal floor The researchers who analyzed that
• 1:04:52
  Speaker 1: data were careful about their language. They did not claim life,
• 1:04:59
  Speaker 1: but they noted that the chemistry being detected inside that
• 1:05:02
  Speaker 1: plume was the kind of chemistry that, on Earth, tends
• 1:05:07
  Speaker 1: to have biology behind it. Consider what Cassini actually did.
• 1:05:13
  Speaker 1: It flew a spacecraft through the erupting output of an
• 1:05:16
  Speaker 1: alien ocean. No drilling. No landings. no years of surface
• 1:05:22
  Speaker 1: operations in a brutal environment. The ocean of Enceladus is
• 1:05:27
  Speaker 1: sending itself into space, and all Cassini had to do
• 1:05:32
  Speaker 1: was be in the right place. The mission team flew
• 1:05:35
  Speaker 1: through the plume multiple times, each pass collecting more data,
• 1:05:41
  Speaker 1: refining the picture. It is one of the least complicated
• 1:05:45
  Speaker 1: pathways to ocean chemistry sampling. that anyone has ever proposed
• 1:05:50
  Speaker 1: for another world. And it happened not by design, but
• 1:05:54
  Speaker 1: because Enceladus itself made it possible. The ocean this plume
• 1:06:00
  Speaker 1: is drawing from sits beneath the south polar ice shell,
• 1:06:04
  Speaker 1: possibly extending globally beneath the entire surface. The ice shell
• 1:06:10
  Speaker 1: itself is thinner near the south pole, which is part
• 1:06:14
  Speaker 1: of why the venting happens there. Beneath that ice liquid water.
• 1:06:19
  Speaker 1: Estimates suggest the ocean contains a substantial volume, not enormous
• 1:06:24
  Speaker 1: by Earth standards, but real, persistent, chemically active. It has
• 1:06:31
  Speaker 1: probably been liquid for hundreds of millions of years, possibly longer.
• 1:06:36
  Speaker 1: The tidal heating that maintains it has been running since
• 1:06:40
  Speaker 1: Enceladus settled into its current orbital configuration. The implications of
• 1:06:47
  Speaker 1: such stability are significant. An ocean that has remained liquid
• 1:06:52
  Speaker 1: for such an extended period provides the kind of persistent
• 1:06:56
  Speaker 1: environment where complex chemistry might develop and potentially where life,
• 1:07:02
  Speaker 1: if it exists, could evolve and adapt over deep time.
• 1:07:08
  Speaker 1: There is a small moon. It has been orbiting Saturn
• 1:07:11
  Speaker 1: since before complex life appeared on Earth. Inside it, an
• 1:07:16
  Speaker 1: ocean sits in contact with a rocky floor. Hot water
• 1:07:20
  Speaker 1: moves through that rock. Salt dissolves in. Hydrogen releases. Complex
• 1:07:27
  Speaker 1: chemistry proceeds. And then the ocean erupts through cracks in
• 1:07:31
  Speaker 1: the ice and sprays itself into space. A moon smaller
• 1:07:36
  Speaker 1: than most US states is erupting its ocean into space,
• 1:07:41
  Speaker 1: and we already flew through it. Giovanni Cassini discovered Iapetus
• 1:07:48
  Speaker 1: in 1671 and was immediately confused by it. He was
• 1:07:53
  Speaker 1: working at the Paris Observatory, scanning Saturn through one of
• 1:07:57
  Speaker 1: the best telescopes available at the time, and he found
• 1:08:01
  Speaker 1: a small point of light orbiting the planet at a
• 1:08:04
  Speaker 1: considerable distance. He noted it. He tracked it. Then, as
• 1:08:10
  Speaker 1: Iapetus moved around to the other side of Saturn, It disappeared,
• 1:08:15
  Speaker 1: not faded, not dimmed slightly. Gone. He could not find
• 1:08:21
  Speaker 1: it at all. For years, Cassini struggled to explain what
• 1:08:26
  Speaker 1: he was seeing. He eventually concluded that one side of
• 1:08:29
  Speaker 1: this moon must be bright and the other side must
• 1:08:32
  Speaker 1: be dark, and that the dark side was so dark
• 1:08:36
  Speaker 1: the telescope could simply not pick it up. This was
• 1:08:40
  Speaker 1: a reasonable conclusion. It was also correct. What he could
• 1:08:45
  Speaker 1: not have known was just how extreme the difference actually was.
• 1:08:51
  Speaker 1: Iapetus is tidally locked to Saturn, the same way our
• 1:08:55
  Speaker 1: own Moon is locked to Earth. It always shows the
• 1:08:59
  Speaker 1: same face in the direction of its orbit. The hemisphere
• 1:09:03
  Speaker 1: that faces forward as Iapetus moves is called the leading hemisphere,
• 1:09:09
  Speaker 1: and it is covered in material so dark it reflects
• 1:09:13
  Speaker 1: only about 3 or 4% of the light that hits it.
• 1:09:18
  Speaker 1: Coal is slightly more reflective. The trailing hemisphere, by contrast,
• 1:09:24
  Speaker 1: is bright white ice, reflecting close to 60% of incoming sunlight.
• 1:09:30
  Speaker 1: These two surfaces sit right next to each other on
• 1:09:32
  Speaker 1: the same moon, separated by gradual transition zones that still
• 1:09:37
  Speaker 1: cover enormous distances. From a spacecraft looking down, the overall
• 1:09:43
  Speaker 1: pattern looks almost exactly like a yin-yang symbol, painted across
• 1:09:48
  Speaker 1: a world roughly 900 miles wide. The first explanation that
• 1:09:54
  Speaker 1: seemed intuitive was that Iapetus was sweeping up dark material
• 1:09:59
  Speaker 1: as it moved through space, the way a car windshield
• 1:10:03
  Speaker 1: collects insects. The leading hemisphere faces into the direction of travel,
• 1:10:09
  Speaker 1: so anything drifting in its path would accumulate there. This
• 1:10:14
  Speaker 1: turned out to be partly right. The dark material is real,
• 1:10:20
  Speaker 1: it is external, and it does come from elsewhere in
• 1:10:24
  Speaker 1: the Saturn system. Phoebe, a large captured moon orbiting Saturn
• 1:10:29
  Speaker 1: in the opposite direction far out from the planet, sheds
• 1:10:32
  Speaker 1: material constantly as micrometeorite impacts knock dust off its surface.
• 1:10:39
  Speaker 1: That debris drifts inward. Iapetus, moving in the opposite orbital direction,
• 1:10:45
  Speaker 1: collects it on its leading face. Other outer irregular moons
• 1:10:50
  Speaker 1: contribute material in the same way. The coating that builds
• 1:10:55
  Speaker 1: up over time is a reddish-brown organic crust. But that
• 1:11:00
  Speaker 1: explanation alone does not get you to the extreme contrast
• 1:11:04
  Speaker 1: Cassini observed in 1671. There are other moons that collect
• 1:11:10
  Speaker 1: dark material on their leading hemispheres. None of them look
• 1:11:15
  Speaker 1: like this. something is amplifying the difference. The current model
• 1:11:21
  Speaker 1: is called thermal segregation, and it works like a slow
• 1:11:25
  Speaker 1: runaway process. Once dark material lands on the leading hemisphere,
• 1:11:30
  Speaker 1: it absorbs more heat from the sun than the surrounding
• 1:11:33
  Speaker 1: bright ice does. This warms the surface just slightly. At
• 1:11:39
  Speaker 1: the temperatures present on Iapetus, That small temperature difference is
• 1:11:43
  Speaker 1: enough to cause nearby ice to sublimate, converting directly from
• 1:11:48
  Speaker 1: solid to vapor. That water vapor drifts and redeposits somewhere cooler,
• 1:11:55
  Speaker 1: which means it moves toward the bright trailing hemisphere and
• 1:11:59
  Speaker 1: refreezes there. Over time, the dark regions lose their ice,
• 1:12:05
  Speaker 1: become darker still, absorb even more heat, drive off even
• 1:12:11
  Speaker 1: more ice. The bright regions gain ice, become more reflective,
• 1:12:17
  Speaker 1: stay cooler, hold onto their ice. Each cycle amplifies what
• 1:12:22
  Speaker 1: was already there. Run this process for hundreds of millions
• 1:12:26
  Speaker 1: of years and the result is one of the starkest
• 1:12:29
  Speaker 1: surface contrasts on any object in the solar system. What
• 1:12:34
  Speaker 1: makes Iapetus genuinely strange is that neither the dark coating
• 1:12:39
  Speaker 1: nor the thermal feedback alone produces this pattern. You need both.
• 1:12:45
  Speaker 1: The organic material from Phoebe seeds the dark regions in
• 1:12:49
  Speaker 1: the right place, and then thermal segregation takes over and
• 1:12:53
  Speaker 1: pushes both hemispheres toward their extremes. Take away either factor,
• 1:12:59
  Speaker 1: and the contrast collapses. With both, you get a moon
• 1:13:04
  Speaker 1: that a 17th century astronomer could watch vanish through his telescope,
• 1:13:10
  Speaker 1: None of that explains the ridge. Running almost exactly along
• 1:13:14
  Speaker 1: the equator of Iapetus is a mountain range, not a
• 1:13:18
  Speaker 1: small one. The ridge extends for roughly 800 miles and
• 1:13:23
  Speaker 1: rises in places to about 12 miles above the surrounding terrain.
• 1:13:28
  Speaker 1: That is taller than anything on Earth. Olympus Mons on Mars,
• 1:13:33
  Speaker 1: which is the largest known volcano in the solar system,
• 1:13:37
  Speaker 1: reaches about 16 miles. The equatorial ridge on Iapetus is
• 1:13:42
  Speaker 1: in that category of height, sitting on a moon less
• 1:13:46
  Speaker 1: than a thousand miles across. In some sections, it forms
• 1:13:50
  Speaker 1: a single sharp spine. In others, it breaks up into
• 1:13:54
  Speaker 1: a chain of isolated peaks. From orbit, it looks almost artificial,
• 1:14:00
  Speaker 1: a seam running around the middle of the world like
• 1:14:03
  Speaker 1: a mould line on a plastic ball. Nobody has a
• 1:14:08
  Speaker 1: satisfying explanation for it. One hypothesis is that Iapetus once
• 1:14:13
  Speaker 1: had a ring of its own, material orbiting close to
• 1:14:17
  Speaker 1: the moon that eventually lost energy and collapsed inward, settling
• 1:14:21
  Speaker 1: along the equator and piling up over time. The equatorial
• 1:14:26
  Speaker 1: alignment would support this. The problem is that the physics
• 1:14:32
  Speaker 1: of how that material would have accumulated into mountains, rather
• 1:14:36
  Speaker 1: than simply coating the surface evenly, is not well understood.
• 1:14:41
  Speaker 1: Another hypothesis involves internal tectonics early in the moon's history,
• 1:14:47
  Speaker 1: some kind of equatorial stress or compression as the body
• 1:14:51
  Speaker 1: cooled and contracted. This also struggles under detailed modeling. The
• 1:14:57
  Speaker 1: equatorial ridge remains one of the more genuinely unresolved features
• 1:15:02
  Speaker 1: in the solar system, not just poorly understood but lacking
• 1:15:07
  Speaker 1: even a dominant working theory that survives close examination. Cassini,
• 1:15:13
  Speaker 1: the spacecraft this time rather than the astronomer, flew past
• 1:15:18
  Speaker 1: Iapetus in 2007 at relatively close range and sent back
• 1:15:23
  Speaker 1: detailed images of both the dark terrain and the ridge.
• 1:15:29
  Speaker 1: The images are remarkable. They did not resolve the mystery
• 1:15:33
  Speaker 1: of the ridge, but they confirmed it was real sharp,
• 1:15:37
  Speaker 1: and stranger up close than it appeared from a distance.
• 1:15:41
  Speaker 1: Four centuries after its discovery, Iapetus still has a mountain
• 1:15:46
  Speaker 1: range running around its equator that nobody can fully explain,
• 1:15:50
  Speaker 1: sitting on a world that looks like it was painted
• 1:15:53
  Speaker 1: in two colours by a force we still have not identified.
• 1:15:58
  Speaker 1: Voyager 2 passed Miranda on January 24th, 1986. and the
• 1:16:05
  Speaker 1: images it sent back made planetary scientists genuinely uncertain about
• 1:16:11
  Speaker 1: what they were looking at. Miranda is one of Uranus's
• 1:16:16
  Speaker 1: inner moons, small, only about 300 miles across, and sitting
• 1:16:22
  Speaker 1: nearly 1.8 billion miles from the Sun at the farthest
• 1:16:27
  Speaker 1: point in Voyager 2's long journey through the outer solar system.
• 1:16:32
  Speaker 1: The spacecraft had already passed Jupiter and Saturn. It was
• 1:16:37
  Speaker 1: running on a trajectory that would take it past Uranus
• 1:16:41
  Speaker 1: and then swing it south and out of the solar
• 1:16:44
  Speaker 1: system entirely. Miranda was almost an afterthought on that itinerary.
• 1:16:51
  Speaker 1: The encounter lasted hours. The images it produced have been
• 1:16:55
  Speaker 1: debated for nearly four decades. What those images showed was
• 1:17:00
  Speaker 1: a surface that looked wrong. Not wrong in the way
• 1:17:04
  Speaker 1: of a world that had simply been hit many times
• 1:17:07
  Speaker 1: or slowly eroded. Wrong in a deeper sense. The terrain
• 1:17:13
  Speaker 1: on Miranda did not fit together. Ancient, heavily cratered plains
• 1:17:18
  Speaker 1: sat directly next to regions of sharp grooves and ridges
• 1:17:22
  Speaker 1: that looked completely unlike anything beside them. The boundaries between
• 1:17:28
  Speaker 1: these zones were abrupt. There was no gradual transitions. One
• 1:17:33
  Speaker 1: type of terrain simply ended and another began, as if
• 1:17:38
  Speaker 1: pieces of completely different worlds had been pressed together at
• 1:17:42
  Speaker 1: the seams. Scientists had a name for those grooved and
• 1:17:47
  Speaker 1: ridged regions. They called them Korone. Three Korone have been
• 1:17:52
  Speaker 1: identified and named on Miranda, Arden, Elsinore and Inverness. Each
• 1:17:59
  Speaker 1: one is roughly oval in shape marked by parallel ridges
• 1:18:03
  Speaker 1: and grooves running across its interior. They look almost like
• 1:18:08
  Speaker 1: racetrack ovals traced into the surface, surrounded by entirely different terrain.
• 1:18:15
  Speaker 1: Nothing quite like them exists anywhere else in the solar system.
• 1:18:20
  Speaker 1: The leading explanation now is that they formed through localised
• 1:18:25
  Speaker 1: upwelling of warm material from Miranda's interior. bubbling up from
• 1:18:30
  Speaker 1: below and deforming the surface in distinct regions, rather than
• 1:18:35
  Speaker 1: across the whole moon. The driving force behind that heating
• 1:18:38
  Speaker 1: was probably ancient tidal stress, back when Miranda's orbit was
• 1:18:43
  Speaker 1: more elliptical, and the gravitational pull from Uranus was flexing
• 1:18:48
  Speaker 1: the interior unevenly. The orbit has since settled, the interior
• 1:18:53
  Speaker 1: has since cooled. What the corony looked like when they
• 1:18:57
  Speaker 1: were actively forming, nobody can say with certainty, because we
• 1:19:02
  Speaker 1: have never seen another one in progress. The cliffs are
• 1:19:06
  Speaker 1: what stop most people. Verona Rupes is a scarp on
• 1:19:11
  Speaker 1: Miranda's surface that drops roughly 12 miles straight down. 12 miles.
• 1:19:18
  Speaker 1: That makes it one of the deepest known cliffs in
• 1:19:21
  Speaker 1: the solar system, possibly the deepest. On Earth, the tallest
• 1:19:26
  Speaker 1: sea cliffs reach barely half a mile. The walls of
• 1:19:30
  Speaker 1: the Grand Canyon are about a mile deep. Verona Rupes
• 1:19:35
  Speaker 1: would make those features look like curb edges. And because
• 1:19:39
  Speaker 1: Miranda is so small, its surface gravity is extremely low,
• 1:19:44
  Speaker 1: only a tiny fraction of what you feel standing on Earth.
• 1:19:48
  Speaker 1: If you stepped off the edge of Verona Rupes, you
• 1:19:52
  Speaker 1: would not plummet the way you would on a larger world.
• 1:19:56
  Speaker 1: You would fall slowly, in a long, quiet arc. The
• 1:20:01
  Speaker 1: descent would take roughly 12 minutes before the bottom arrived.
• 1:20:06
  Speaker 1: The impact velocity when it finally came, would be survivable
• 1:20:10
  Speaker 1: in theory, though the 12 miles of vertical rock face
• 1:20:14
  Speaker 1: is another problem entirely. The early explanation for Miranda's chaotic
• 1:20:19
  Speaker 1: terrain was dramatic. Some researchers proposed that the moon had
• 1:20:24
  Speaker 1: been hit hard enough at some point in its history
• 1:20:27
  Speaker 1: to shatter it completely, and that the pieces had slowly
• 1:20:31
  Speaker 1: pulled themselves back together under gravity but in the wrong order.
• 1:20:36
  Speaker 1: a jigsaw puzzle reassembled without the picture on the box.
• 1:20:41
  Speaker 1: The idea had a certain appeal. It explained the patchwork quality,
• 1:20:47
  Speaker 1: the mismatched terrain, the sense that nothing was where it
• 1:20:51
  Speaker 1: was supposed to be. But the physics did not hold
• 1:20:55
  Speaker 1: up well under closer examination. Reassembling a shattered moon from
• 1:21:01
  Speaker 1: scattered fragments would not produce the kind of clean, defined
• 1:21:05
  Speaker 1: boundaries visible between Miranda's terrain types. It would produce something
• 1:21:11
  Speaker 1: far messier. The shattering theory has largely been set aside,
• 1:21:16
  Speaker 1: though not entirely without advocates. The upwelling model replaced it.
• 1:21:21
  Speaker 1: Under this explanation, Miranda's interior was never uniform. When tidal
• 1:21:27
  Speaker 1: heating was strongest, Certain regions of the interior became warm
• 1:21:32
  Speaker 1: enough for material to push upward and deform the crust
• 1:21:35
  Speaker 1: from below. The heating was patchy, uneven, concentrated in specific
• 1:21:42
  Speaker 1: locations rather than spread across the whole body. Those locations
• 1:21:47
  Speaker 1: became the coronet. The regions that were never disturbed by
• 1:21:51
  Speaker 1: upwelling kept their ancient cratered surfaces intact. The result? was
• 1:21:57
  Speaker 1: a moon that looked assembled from mismatched parts. Not because
• 1:22:02
  Speaker 1: it had been broken apart, but because different parts of
• 1:22:06
  Speaker 1: it had experienced completely different geological histories at different times.
• 1:22:12
  Speaker 1: What makes Miranda particularly difficult to study is that Voyager
• 1:22:16
  Speaker 1: 2's flyby in January of 1986 remains the only close
• 1:22:21
  Speaker 1: observation humanity has ever conducted. No spacecraft has returned. The
• 1:22:28
  Speaker 1: images from that single encounter, taken at distances and angles
• 1:22:32
  Speaker 1: that left much of the surface unseen, are still the
• 1:22:36
  Speaker 1: entire dataset. Roughly half of Miranda was never photographed at
• 1:22:41
  Speaker 1: useful resolution. The southern hemisphere during the flyby was pointed
• 1:22:47
  Speaker 1: away from the sun, lost in shadow. What terrain exists there?
• 1:22:53
  Speaker 1: Whether more Korone are hiding in that darkness? Whether Verona
• 1:22:57
  Speaker 1: Rupes is the deepest feature on the surface, or just
• 1:23:01
  Speaker 1: the deepest one that happened to be in frame, none
• 1:23:05
  Speaker 1: of that is known. Miranda has been photographed exactly once,
• 1:23:11
  Speaker 1: by a spacecraft that flew past it nearly 40 years ago,
• 1:23:15
  Speaker 1: and the images are still producing arguments. Every large moon
• 1:23:20
  Speaker 1: in the solar system orbits its planet in the same
• 1:23:23
  Speaker 1: direction that planet rotates. except one. That exception is Triton,
• 1:23:30
  Speaker 1: Neptune's largest moon, and the direction it travels is not
• 1:23:34
  Speaker 1: a minor technical detail. It is the single most important
• 1:23:39
  Speaker 1: thing about it. When the solar system was forming, everything
• 1:23:44
  Speaker 1: in the disk of gas and dust around the young
• 1:23:46
  Speaker 1: sun was spinning the same way. Planets formed from that
• 1:23:51
  Speaker 1: disk They're moons formed from smaller disks of material spinning
• 1:23:57
  Speaker 1: around each planet. The result is that every large moon
• 1:24:02
  Speaker 1: that grew naturally alongside its planet orbits in the same
• 1:24:06
  Speaker 1: direction as that planet's rotation. Prograde. It is not a coincidence.
• 1:24:13
  Speaker 1: It is the predictable outcome of formation from a shared
• 1:24:17
  Speaker 1: spinning disk. When a moon goes the other way, there
• 1:24:21
  Speaker 1: is only one explanation. It did not form there. Something
• 1:24:25
  Speaker 1: put it there after the fact. Triton going backward around
• 1:24:29
  Speaker 1: Neptune is the clearest possible sign that Neptune captured it
• 1:24:33
  Speaker 1: from somewhere else entirely. The leading theory is that Triton
• 1:24:39
  Speaker 1: started out as a Kuiper Belt object. The Kuiper Belt
• 1:24:43
  Speaker 1: is the vast region of icy bodies beyond Neptune's orbit,
• 1:24:48
  Speaker 1: the same population that includes Pluto, Eris, and hundreds of
• 1:24:53
  Speaker 1: thousands of smaller frozen worlds. Triton, at roughly 1700 miles across,
• 1:25:01
  Speaker 1: would have been one of the larger members of that population.
• 1:25:06
  Speaker 1: At some point in the early solar system, it passed
• 1:25:09
  Speaker 1: close enough to Neptune that the planet's gravity caught it.
• 1:25:14
  Speaker 1: The mechanics of how that capture worked are not simple.
• 1:25:19
  Speaker 1: A single object falling toward a planet and passing close
• 1:25:23
  Speaker 1: to it does not automatically get trapped. It needs to
• 1:25:27
  Speaker 1: lose energy somehow to drop into orbit rather than swinging
• 1:25:31
  Speaker 1: around and flying back out. The most likely mechanism involves
• 1:25:36
  Speaker 1: a binary companion. Triton may have been traveling through the
• 1:25:41
  Speaker 1: outer solar system with a partner body when the pair
• 1:25:44
  Speaker 1: encountered Neptune. The gravitational interaction tore the pair apart and
• 1:25:51
  Speaker 1: the energy released in that breakup was enough to bleed
• 1:25:54
  Speaker 1: off the speed Triton needed to escape. Neptune kept Triton.
• 1:26:00
  Speaker 1: The companion was flung away. Whatever the exact mechanism, the
• 1:26:05
  Speaker 1: capture would have been violent. Triton's initial orbit around Neptune
• 1:26:10
  Speaker 1: was almost certainly highly elongated, a stretched ellipse rather than
• 1:26:15
  Speaker 1: anything close to circular. An orbit like that is unstable
• 1:26:20
  Speaker 1: over geological timescales. Tidal forces from Neptune slowly worked on
• 1:26:26
  Speaker 1: that ellipse, pulling it into a more circular shape. That
• 1:26:31
  Speaker 1: circularization process takes time, and while it is happening, the
• 1:26:36
  Speaker 1: tidal stresses on the captured body are enormous. The Moon
• 1:26:41
  Speaker 1: gets squeezed and stretched with every pass around the planet.
• 1:26:45
  Speaker 1: That squeezing generates heat. For Triton, researchers estimate the tidal
• 1:26:52
  Speaker 1: heating during orbital circularization could have kept the moon's interior
• 1:26:57
  Speaker 1: liquid for somewhere between one and several billion years. A
• 1:27:02
  Speaker 1: world that was effectively a captured ball of ice and
• 1:27:05
  Speaker 1: rock from the outer solar system may have spent a
• 1:27:08
  Speaker 1: long stretch of its early history as a warm, liquid
• 1:27:12
  Speaker 1: body deep inside. That is an extraordinary thing to think about.
• 1:27:18
  Speaker 1: The most geologically chaotic phase of Triton's life happened because
• 1:27:22
  Speaker 1: it ended up somewhere it was never supposed to be.
• 1:27:25
  Speaker 1: By the time Voyager 2 flew past in 1989, the
• 1:27:30
  Speaker 1: only spacecraft ever to visit the Neptune system, Triton had
• 1:27:34
  Speaker 1: settled into its current nearly circular retrograde orbit. What the
• 1:27:39
  Speaker 1: cameras sent back was strange. The surface temperature measured around
• 1:27:44
  Speaker 1: minus 390 degrees Fahrenheit. making Triton one of the coldest
• 1:27:50
  Speaker 1: surfaces ever recorded anywhere in the solar system. Neptune sits
• 1:27:56
  Speaker 1: roughly 30 times farther from the Sun than Earth does.
• 1:28:00
  Speaker 1: The sunlight reaching Triton is about 900 times weaker than
• 1:28:05
  Speaker 1: what reaches us. At that distance, almost everything freezes. The
• 1:28:11
  Speaker 1: surface is covered in nitrogen ice and methane ice and
• 1:28:14
  Speaker 1: carbon dioxide ice. it looked like a frozen dead world
• 1:28:19
  Speaker 1: at the edge of everything. Then Voyager 2 caught something erupting,
• 1:28:24
  Speaker 1: active geysers. On a surface almost 400 degrees below zero Fahrenheit,
• 1:28:31
  Speaker 1: nitrogen was venting from below the surface and shooting up
• 1:28:35
  Speaker 1: into the thin atmosphere in dark plumes that rose several
• 1:28:39
  Speaker 1: miles and then spread downwind, leaving dark streaks across the
• 1:28:44
  Speaker 1: frozen plain below. There were at least eight active plumes
• 1:28:49
  Speaker 1: visible in the Voyager images. The mechanism behind them is
• 1:28:53
  Speaker 1: not internal heat. The Sun, even at that extreme distance,
• 1:28:58
  Speaker 1: is responsible. Triton's surface is largely transparent nitrogen ice. Sunlight
• 1:29:05
  Speaker 1: passes through that ice layer and gets absorbed by the
• 1:29:08
  Speaker 1: darker material just beneath it. The heat builds up, the
• 1:29:13
  Speaker 1: nitrogen below converts to gas, pressure builds, and eventually it
• 1:29:18
  Speaker 1: punches through the surface and erupts. The plumes are solar-powered
• 1:29:24
  Speaker 1: geysers on the coldest surface we have ever measured. The
• 1:29:28
  Speaker 1: dark streaks they leave can extend for miles downwind across
• 1:29:32
  Speaker 1: the bright frozen plain. Triton's surface is also young, geologically speaking.
• 1:29:40
  Speaker 1: There are very few impact craters visible in the Voyager images.
• 1:29:44
  Speaker 1: On a world that has been sitting in the outer
• 1:29:47
  Speaker 1: solar system for billions of years, that absence tells you something.
• 1:29:53
  Speaker 1: Whatever craters formed early in Triton's history have been erased.
• 1:29:58
  Speaker 1: The surface has been resurfaced repeatedly through some combination of
• 1:30:03
  Speaker 1: internal heat left over from the capture process and ongoing activity.
• 1:30:09
  Speaker 1: The exact mechanisms are not well understood, and the data
• 1:30:13
  Speaker 1: is limited. Voyager 2 gave us most of what we
• 1:30:18
  Speaker 1: know about Triton in a single flyby that lasted a
• 1:30:22
  Speaker 1: matter of hours. Part of what makes the Voyager images
• 1:30:26
  Speaker 1: so difficult to interpret is the terrain itself. A large
• 1:30:31
  Speaker 1: portion of Triton's surface is covered in a feature that
• 1:30:35
  Speaker 1: has no equivalent anywhere else in the solar system. Planetary
• 1:30:40
  Speaker 1: geologists called it cantaloupe terrain because it looks like the
• 1:30:45
  Speaker 1: skin of a cantaloupe melon. A repeating pattern of rounded
• 1:30:50
  Speaker 1: dimples and ridges, roughly uniform in size, covering the surface
• 1:30:55
  Speaker 1: in a pattern that seems almost too regular to be random.
• 1:31:00
  Speaker 1: The leading explanation involves a process called diaporism, where blobs
• 1:31:06
  Speaker 1: of warmer, less dense material rise through denser material above them,
• 1:31:12
  Speaker 1: punching upward like bubbles and deforming the surface into those
• 1:31:17
  Speaker 1: rounded shapes. It would be driven by the heat left
• 1:31:22
  Speaker 1: over from Triton's violent orbital history. But that is still
• 1:31:27
  Speaker 1: a hypothesis. The cantaloupe terrain on Triton has been photographed once,
• 1:31:32
  Speaker 1: by one spacecraft, three and a half decades ago, and
• 1:31:36
  Speaker 1: the argument over what made it continues. There is a
• 1:31:40
  Speaker 1: last chapter written into Triton's orbital path, and it is
• 1:31:45
  Speaker 1: a dramatic one. Retrograde orbits around planets are not stable
• 1:31:50
  Speaker 1: over extremely long timescales. Tidal interactions between Triton and Neptune
• 1:31:56
  Speaker 1: are slowly sapping energy from Triton's orbit. Every year, imperceptibly,
• 1:32:02
  Speaker 1: Triton is getting slightly closer to Neptune. The process is gradual,
• 1:32:08
  Speaker 1: but 3.6 billion years from now, Triton will have spiraled
• 1:32:13
  Speaker 1: inward to what is called the Roche limit. The distance
• 1:32:17
  Speaker 1: at which Neptune's tidal forces become stronger than Triton's own
• 1:32:21
  Speaker 1: gravity holds itself together. At that point, the moon will
• 1:32:27
  Speaker 1: come apart. The material will spread into a ring around Neptune.
• 1:32:32
  Speaker 1: A ring built from the debris of a world that
• 1:32:34
  Speaker 1: was never supposed to be there in the first place.
• 1:32:37
  Speaker 1: A captured wanderer from the outer solar system finally returned
• 1:32:42
  Speaker 1: to pieces. Triton does not belong where it is. Its
• 1:32:46
  Speaker 1: orbit is slowly failing, and in a few billion years
• 1:32:50
  Speaker 1: it will break apart and become the most spectacular ring
• 1:32:54
  Speaker 1: system in the outer solar system. Hyperion looks like someone
• 1:32:59
  Speaker 1: took a moon and punched holes in it. Not small holes. Deep,
• 1:33:05
  Speaker 1: steep-walled craters punched into an already irregular, lumpy surface. separated
• 1:33:11
  Speaker 1: by thin sharp ridges that look nothing like the rounded
• 1:33:14
  Speaker 1: crater rims you see on the Moon or Mars. The
• 1:33:18
  Speaker 1: overall shape is not even close to spherical. Hyperion stretches
• 1:33:23
  Speaker 1: roughly 170 miles at its longest dimension and considerably less
• 1:33:28
  Speaker 1: in the other directions. It is a battered, asymmetrical chunk
• 1:33:33
  Speaker 1: of something and it looks wrong in a way that
• 1:33:36
  Speaker 1: is hard to pin down until you realize what you
• 1:33:39
  Speaker 1: are actually looking at. The texture is the first thing
• 1:33:43
  Speaker 1: that registers. Scientists who studied Cassini's close images of Hyperion
• 1:33:48
  Speaker 1: described it as sponge-like, and that is not an exaggeration.
• 1:33:54
  Speaker 1: The craters do not blend into each other the way
• 1:33:56
  Speaker 1: they do on heavily bombarded worlds like Callisto, where overlapping
• 1:34:01
  Speaker 1: impacts eventually smooth things out into a kind of rough
• 1:34:05
  Speaker 1: but continuous terrain. On Hyperion, each crater stays sharply defined.
• 1:34:12
  Speaker 1: The walls stay steep. The floors stay deep. This happens
• 1:34:16
  Speaker 1: because of what Hyperion is made of, or more precisely,
• 1:34:20
  Speaker 1: what it is not made of. Its density is unusually low,
• 1:34:26
  Speaker 1: somewhere around 40% empty space. The interior is porous, probably
• 1:34:32
  Speaker 1: riddled with voids. When something hits Hyperion, The material does
• 1:34:37
  Speaker 1: not splash outward across the surface the way it would
• 1:34:40
  Speaker 1: on a denser world. Instead, the impact compresses and excavates stri,
• 1:34:47
  Speaker 1: ite down. The debris that would normally scatter and fill
• 1:34:51
  Speaker 1: nearby terrain gets absorbed into the porous structure. The result
• 1:34:56
  Speaker 1: is a surface covered in holes that stay exactly where
• 1:34:59
  Speaker 1: they formed, sharp and isolated, never filling in. The floors
• 1:35:04
  Speaker 1: of those craters are dark and reddish. The same kind
• 1:35:08
  Speaker 1: of organic material shows up on moons and small bodies
• 1:35:11
  Speaker 1: throughout the outer solar system, the product of ultraviolet light
• 1:35:16
  Speaker 1: and charged particles processing simple carbon-bearing compounds over billions of years.
• 1:35:23
  Speaker 1: On Hyperion, these dark deposits settle into the lowest points
• 1:35:27
  Speaker 1: of the terrain, coating the crater floors while the higher
• 1:35:32
  Speaker 1: ridges stay slightly brighter. The contrast gives the surface an
• 1:35:37
  Speaker 1: almost hand-painted quality. Dark pits separated by lighter walls, the
• 1:35:43
  Speaker 1: sponge pattern made even more visible. But the shape and
• 1:35:48
  Speaker 1: the texture are not the strangest thing about Hyperion. The
• 1:35:52
  Speaker 1: strangest thing is what it is doing as it moves.
• 1:35:56
  Speaker 1: Most moons in the solar system are tidally locked. Gravity
• 1:36:01
  Speaker 1: from the planet they orbit gradually slows their rotation and
• 1:36:04
  Speaker 1: fixes them, so the same face always points toward the planet.
• 1:36:09
  Speaker 1: Our moon does this. Titan does this. Virtually every moon
• 1:36:15
  Speaker 1: of any significant size does this, given enough time. The
• 1:36:20
  Speaker 1: process is slow but inexorable, and for most moons, billions
• 1:36:25
  Speaker 1: of years has been more than enough time to complete it.
• 1:36:28
  Speaker 1: Hyperion never locked, Its rotation is genuinely chaotic, in the
• 1:36:34
  Speaker 1: precise mathematical sense of the word. Chaotic motion is not
• 1:36:39
  Speaker 1: just complicated or irregular. It means the system is so
• 1:36:43
  Speaker 1: sensitive to its initial conditions that even in theory, even
• 1:36:48
  Speaker 1: with perfect knowledge of where Hyperion is and how fast
• 1:36:52
  Speaker 1: it is spinning right now, you cannot calculate how it
• 1:36:56
  Speaker 1: will be oriented weeks from now. Small uncertainties in the
• 1:37:01
  Speaker 1: current state grow exponentially. The future becomes unpredictable, not because
• 1:37:07
  Speaker 1: we lack information, but because the physics itself amplifies any
• 1:37:12
  Speaker 1: uncertainty without limit. The culprit is Titan. Hyperion orbits Saturn
• 1:37:19
  Speaker 1: in a resonance with Titan, completing three orbits for every
• 1:37:23
  Speaker 1: four that Titan completes. This keeps the two moons in
• 1:37:27
  Speaker 1: a repeating gravitational relationship and Titan is large enough to
• 1:37:31
  Speaker 1: give Hyperion a significant tug at regular intervals. Saturn is
• 1:37:37
  Speaker 1: pulling on Hyperion as well. The combination of these two
• 1:37:41
  Speaker 1: gravitational influences, acting on a body that is already asymmetrical
• 1:37:47
  Speaker 1: and irregularly shaped, produces a torque that never settles into
• 1:37:52
  Speaker 1: a stable pattern. Hyperion tumbles. It is always tumbling, end
• 1:37:59
  Speaker 1: over end in a pattern that shifts and never repeats.
• 1:38:03
  Speaker 1: When Cassini made multiple passes of Hyperion over the years,
• 1:38:08
  Speaker 1: each flyby showed the moon oriented completely differently. This was
• 1:38:13
  Speaker 1: not a surprise by then, because astronomers had already worked
• 1:38:17
  Speaker 1: out theoretically that Hyperion's rotation should be chaotic well before
• 1:38:21
  Speaker 1: Cassini arrived at Saturn. but seeing it confirmed in the images,
• 1:38:26
  Speaker 1: a different face each time, craters that were in shadow
• 1:38:30
  Speaker 1: now in sunlight, the whole body presenting itself in a
• 1:38:34
  Speaker 1: new configuration with every encounter, is something else entirely. It
• 1:38:41
  Speaker 1: is worth pausing on what this means practically. If you
• 1:38:45
  Speaker 1: were standing on Hyperion, The direction of Saturn in the
• 1:38:49
  Speaker 1: sky would drift unpredictably. The Sun would not rise and
• 1:38:54
  Speaker 1: set on any reliable schedule. There would be no way
• 1:38:58
  Speaker 1: to construct a calendar based on where anything appears overhead,
• 1:39:03
  Speaker 1: because nothing appears in the same place twice. The orientation
• 1:39:08
  Speaker 1: of the entire world would be genuinely unknown from one
• 1:39:12
  Speaker 1: month to the next. Hyperion is almost certainly not an
• 1:39:16
  Speaker 1: original object. Something this porous, this irregular, this physically strange
• 1:39:23
  Speaker 1: did not form this way from scratch. The leading interpretation
• 1:39:28
  Speaker 1: is that it is a remnant, a fragment left behind
• 1:39:32
  Speaker 1: after a much larger body was struck hard enough to
• 1:39:35
  Speaker 1: break apart. The original object is gone. What remains is
• 1:39:41
  Speaker 1: this porous, tumbling, sponge-shaped piece of wreckage still orbiting Saturn
• 1:39:47
  Speaker 1: billions of years after whatever destroyed its parent. It is
• 1:39:52
  Speaker 1: probably the surviving fragment of a much larger body that
• 1:39:55
  Speaker 1: no longer exists, tumbling through its orbit in a way
• 1:40:00
  Speaker 1: that makes it impossible to predict how it will be
• 1:40:03
  Speaker 1: oriented on any given day. When the Voyager spacecraft sent
• 1:40:09
  Speaker 1: back the first detailed images of Mimas in 1980, scientists
• 1:40:15
  Speaker 1: noticed something immediately. The moon looked like a weapon from
• 1:40:19
  Speaker 1: a movie that had not yet been released when the
• 1:40:21
  Speaker 1: mission was planned. Star Wars came out in 1977. Voyager
• 1:40:28
  Speaker 1: launched the same year. By the time the images arrived,
• 1:40:32
  Speaker 1: everyone already knew what the Death Star looked like. And here,
• 1:40:38
  Speaker 1: orbiting Saturn, was essentially the same object, a nearly spherical
• 1:40:43
  Speaker 1: grey body with one enormous circular crater dominating an entire hemisphere,
• 1:40:50
  Speaker 1: complete with a central peak rising from the middle like
• 1:40:53
  Speaker 1: a bullseye. Nobody had designed it that way. Mimas had
• 1:40:59
  Speaker 1: been sitting there for billions of years, long before George
• 1:41:03
  Speaker 1: Lucas put pen to paper. The crater is called Herschel,
• 1:41:08
  Speaker 1: named after the astronomer William Herschel, who discovered Mimas in 1789.
• 1:41:14
  Speaker 1: It is roughly 80 miles across on a moon that
• 1:41:20
  Speaker 1: is only about 250 miles in diameter. That ratio is extraordinary.
• 1:41:26
  Speaker 1: Herschel takes up nearly a third of the moon's entire face.
• 1:41:31
  Speaker 1: The walls drop about six miles to the crater floor.
• 1:41:35
  Speaker 1: The central peak rises nearly four miles from that floor,
• 1:41:39
  Speaker 1: which puts it in roughly the same height range as
• 1:41:42
  Speaker 1: Mount Everest, measured from sea level. The impact that formed
• 1:41:47
  Speaker 1: it came close to ending Mimas entirely. Scientists who have
• 1:41:52
  Speaker 1: modelled the collision estimate that only a slightly larger impactor
• 1:41:57
  Speaker 1: would have split the moon apart. It held together, but barely.
• 1:42:03
  Speaker 1: you can see the evidence on the other side. Directly
• 1:42:07
  Speaker 1: opposite Herschel, on Mimas's far hemisphere, the terrain becomes chaotic.
• 1:42:14
  Speaker 1: The surface is broken and jumbled in a way that
• 1:42:17
  Speaker 1: matches nothing else on the moon. The leading explanation is
• 1:42:21
  Speaker 1: that when the impactor hit, the shock waves traveled through
• 1:42:25
  Speaker 1: Mimas's body and converged on the opposite side with enough
• 1:42:30
  Speaker 1: focused energy to fracture the surface from below. The same
• 1:42:35
  Speaker 1: effect has been observed on other bodies. Mercury has similar
• 1:42:40
  Speaker 1: chaotic terrain directly opposite its Caloris basin. On a small
• 1:42:46
  Speaker 1: rigid body like Mimas, those focused waves had nowhere to go.
• 1:42:52
  Speaker 1: They simply tore the surface apart on the far side
• 1:42:55
  Speaker 1: while the rest of the moon survived. For decades that
• 1:42:59
  Speaker 1: was the story of Mimas. It looked like the Death Star.
• 1:43:03
  Speaker 1: It had an almost catastrophic crater. And beyond that, it
• 1:43:08
  Speaker 1: appeared to be a fairly simple, geologically dead chunk of
• 1:43:12
  Speaker 1: ice and rock. It was cold. It was inert. And
• 1:43:17
  Speaker 1: compared to Europa or Enceladus, it seemed to have nothing
• 1:43:21
  Speaker 1: more to say. Then Cassini changed that. The Cassini spacecraft
• 1:43:26
  Speaker 1: carried an instrument that could map surface temperatures across a
• 1:43:30
  Speaker 1: body as it orbited. When researchers turned it toward Mimas
• 1:43:35
  Speaker 1: and mapped the thermal signature, they expected a straightforward result.
• 1:43:40
  Speaker 1: A cold moon warmed slightly on the sunlit side, cooling
• 1:43:44
  Speaker 1: gradually toward the terminator and the dark hemisphere. That is
• 1:43:49
  Speaker 1: how inert icy bodies behave. What the data showed instead
• 1:43:55
  Speaker 1: was something that looked, with only minor exaggeration, like the
• 1:43:59
  Speaker 1: video game character Pac-Man. a warm crescent shape on the
• 1:44:04
  Speaker 1: leading hemisphere, with the warm area cutting off sharply in
• 1:44:09
  Speaker 1: a pattern that made no physical sense for a simple
• 1:44:12
  Speaker 1: frozen world. The Cassini team published the finding in 2010.
• 1:44:19
  Speaker 1: Nobody had a clean explanation. The most widely accepted interpretation
• 1:44:24
  Speaker 1: involves surface texture. Most of Mimas appears to be covered
• 1:44:29
  Speaker 1: in an extremely fine, porous layer of ice particles, almost
• 1:44:33
  Speaker 1: like fresh powdery snow, but sparser. Material like this heats
• 1:44:39
  Speaker 1: up almost instantly when sunlight hits it and loses that
• 1:44:43
  Speaker 1: heat just as fast when the sun moves away. Certain regions,
• 1:44:49
  Speaker 1: possibly where the ice is more compacted or structured differently,
• 1:44:53
  Speaker 1: behave more like solid ice. which holds temperature longer and
• 1:44:58
  Speaker 1: creates a different thermal pattern when mapped. The boundary between
• 1:45:04
  Speaker 1: these regions, running across the face of Mimas, traces out
• 1:45:09
  Speaker 1: that Pac-Man shape. The surface is not uniform in ways
• 1:45:13
  Speaker 1: that are just beginning to be understood. That explanation is
• 1:45:17
  Speaker 1: reasonable and fits the data well enough that many researchers
• 1:45:21
  Speaker 1: accept it. But another possibility has been gaining quiet attention.
• 1:45:27
  Speaker 1: A small number of studies have proposed that Mimas may
• 1:45:31
  Speaker 1: have a subsurface liquid ocean, buried beneath an ice shell
• 1:45:35
  Speaker 1: thick enough that it leaves almost no obvious trace on
• 1:45:39
  Speaker 1: the surface. The evidence is far thinner than for Europa
• 1:45:43
  Speaker 1: or Enceladus. There are no geysers, no cracked terrain, no
• 1:45:49
  Speaker 1: magnetic field signature pointing toward a salty conductor underneath. But
• 1:45:55
  Speaker 1: the thermal anomaly is real, and some models suggest an
• 1:46:00
  Speaker 1: interior ocean could produce subtle surface effects consistent with what
• 1:46:06
  Speaker 1: Cassini observed. The debate is unresolved. What is not in
• 1:46:12
  Speaker 1: debate is the other significant thing Mimas does. Its orbital
• 1:46:17
  Speaker 1: period around Saturn is almost exactly one half of the
• 1:46:21
  Speaker 1: orbital period of the ring particles sitting at a specific
• 1:46:25
  Speaker 1: gap in Saturn's famous ring system. This two-to-one resonance means
• 1:46:30
  Speaker 1: that any particle trying to maintain an orbit in that
• 1:46:33
  Speaker 1: zone gets a gravitational nudge from Mimas at regular intervals,
• 1:46:39
  Speaker 1: always in the same direction, building up until the particle
• 1:46:43
  Speaker 1: is pushed out. The result is the Cassini Division. The
• 1:46:48
  Speaker 1: dark gap, roughly 3,000 miles wide, that runs through Saturn's
• 1:46:53
  Speaker 1: rings and is visible through even a modest telescope on Earth.
• 1:46:59
  Speaker 1: Mimas carved it out through nothing more than showing up
• 1:47:03
  Speaker 1: at the same place repeatedly over millions of years. Mimas
• 1:47:08
  Speaker 1: is the moon that looks like a movie prop, but
• 1:47:11
  Speaker 1: it is also the object creating the most prominent gap
• 1:47:14
  Speaker 1: in Saturn's rings and it may have an ocean and
• 1:47:18
  Speaker 1: frankly none of that was supposed to be true for
• 1:47:21
  Speaker 1: something this small. Asaph Hall had been searching for Martian
• 1:47:27
  Speaker 1: moons for years and had nearly given up when his
• 1:47:30
  Speaker 1: wife told him to try one more night. That was
• 1:47:34
  Speaker 1: August 1877. He went back to the telescope. Within days
• 1:47:40
  Speaker 1: he found two moons orbiting Mars, Deimos on August 12th,
• 1:47:45
  Speaker 1: Phobos on August 18th. Both small both dark, both strange.
• 1:47:53
  Speaker 1: Nobody had seen them before, despite centuries of watching Mars,
• 1:47:57
  Speaker 1: because they are extraordinarily difficult to spot against the glare
• 1:48:01
  Speaker 1: of the planet itself. Hall's wife, Angeline, had pushed him
• 1:48:06
  Speaker 1: to continue when he was ready to abandon the search entirely.
• 1:48:10
  Speaker 1: Her name deserves to be in the story. Without that conversation,
• 1:48:15
  Speaker 1: the discovery might have waited years longer. Phobos is the
• 1:48:19
  Speaker 1: inner moon. and it is not large. Roughly 17 miles
• 1:48:24
  Speaker 1: across at its widest, lumpy and irregular in shape, more
• 1:48:29
  Speaker 1: like a battered potato than anything you would draw if
• 1:48:32
  Speaker 1: someone asked you to sketch a moon. Its surface is
• 1:48:37
  Speaker 1: heavily cratered and blanketed, under a thick layer of powdery regolith,
• 1:48:43
  Speaker 1: a loose dusty coating that has built up over billions
• 1:48:46
  Speaker 1: of years of micrometeorite impacts grinding the surface down into
• 1:48:51
  Speaker 1: fine material. But the stranger thing about Phobos is not
• 1:48:56
  Speaker 1: what it looks like. It is where it is and
• 1:49:00
  Speaker 1: how fast it moves. Phobos orbits Mars at an altitude
• 1:49:05
  Speaker 1: of only about 3,700 miles. That is extraordinarily close. Our
• 1:49:12
  Speaker 1: own Moon orbits Earth from roughly 239,000 miles out. Phobos
• 1:49:19
  Speaker 1: is practically skimming Mars' upper atmosphere by comparison. And because
• 1:49:24
  Speaker 1: it orbits so close, it moves fast. One full orbit
• 1:49:29
  Speaker 1: takes just under 8 hours. Mars itself rotates once every
• 1:49:33
  Speaker 1: 24 hours. Phobos laps it. The moon completes three full
• 1:49:39
  Speaker 1: orbits while Mars turns once. The result of this is
• 1:49:44
  Speaker 1: something that exists nowhere else in the solar system. From
• 1:49:48
  Speaker 1: the surface of Mars, Phobos rises in the west and
• 1:49:52
  Speaker 1: sets in the east. Every other moon of every other
• 1:49:56
  Speaker 1: planet rises in the east. Phobos alone goes the wrong way,
• 1:50:02
  Speaker 1: not because its orbit is retrograde. but because it is
• 1:50:06
  Speaker 1: moving faster than the planet beneath it is turning. The
• 1:50:10
  Speaker 1: surface of Phobos tells its own violent story. The most
• 1:50:15
  Speaker 1: prominent feature is Stickney Crater, a six mile wide impact
• 1:50:20
  Speaker 1: scar sitting on a moon that is only 17 miles across.
• 1:50:25
  Speaker 1: That impact was catastrophic. Something hit Phobos hard enough to
• 1:50:30
  Speaker 1: excavate a crater one third the width of the entire
• 1:50:34
  Speaker 1: body without shattering it completely. The shock would have sent
• 1:50:38
  Speaker 1: fractures running through the moon in every direction. And there
• 1:50:43
  Speaker 1: are fractures. Long parallel grooves stretch across the surface of Phobos,
• 1:50:49
  Speaker 1: radiating outward from Stickney. And the obvious explanation is that
• 1:50:54
  Speaker 1: they are scars left by the collision. But some researchers
• 1:50:59
  Speaker 1: are not convinced. An alternative interpretation is that these grooves
• 1:51:05
  Speaker 1: are not damaged from the Stickney impact at all. They
• 1:51:09
  Speaker 1: may be structural failures caused by something else entirely. The
• 1:51:14
  Speaker 1: slow gravitational squeezing of Phobos as its orbit tightens. Because
• 1:51:21
  Speaker 1: Phobos' orbit is tightening. Mars is pulling Phobos inward at
• 1:51:26
  Speaker 1: a rate of about 6 feet per year. That sounds small.
• 1:51:30
  Speaker 1: Over a human lifetime, it is nothing. But across geological time,
• 1:51:36
  Speaker 1: six feet per year adds up. In roughly 50 million years,
• 1:51:41
  Speaker 1: Phobos will get close enough to Mars that one of
• 1:51:44
  Speaker 1: two things will happen. Either it will crash into the
• 1:51:48
  Speaker 1: planet's surface in an impact visible across the entire solar system,
• 1:51:55
  Speaker 1: or it will cross the Roche limit. the distance at
• 1:51:58
  Speaker 1: which tidal forces exceed the gravity holding a body together,
• 1:52:03
  Speaker 1: and it will be pulled apart, shredded into fragments. Those
• 1:52:09
  Speaker 1: fragments would spread into a ring around Mars, something Mars
• 1:52:13
  Speaker 1: has never had. A planet that currently has two moons
• 1:52:17
  Speaker 1: will have, in geological terms relatively soon, either a crater
• 1:52:23
  Speaker 1: or a ring. The grooves on Phobos If that alternative
• 1:52:27
  Speaker 1: theory is right, are the first visible signs of what
• 1:52:30
  Speaker 1: is coming. The moon is already beginning to crack under
• 1:52:34
  Speaker 1: the stress of its own approaching destruction. Deimos is a
• 1:52:39
  Speaker 1: different story entirely. Smaller than Phobos, only about nine miles across,
• 1:52:46
  Speaker 1: Deimos orbits much farther out. Its surface is smoother. Where
• 1:52:52
  Speaker 1: Phobos is rough and heavily scarred, Deimos looks almost soft.
• 1:52:57
  Speaker 1: Craters that should be sharp-edged have been filled and blurred
• 1:53:01
  Speaker 1: by fine material settling into them over time. It is
• 1:53:05
  Speaker 1: a quieter looking world, and its fate is quieter too.
• 1:53:10
  Speaker 1: Deimos orbits just outside the distance at which Mars' tidal
• 1:53:14
  Speaker 1: forces would drag it inward. Instead, those tidal forces push
• 1:53:19
  Speaker 1: it outward, very slowly. in the same way that our
• 1:53:23
  Speaker 1: moon is slowly drifting away from Earth. Deimos is not
• 1:53:27
  Speaker 1: falling toward Mars. It is leaving. The drift is far
• 1:53:32
  Speaker 1: slower than Phobos' inward spiral. Across a human lifetime it
• 1:53:37
  Speaker 1: is effectively nothing, but the direction is clear. Deimos is
• 1:53:41
  Speaker 1: moving away from Mars. Eventually, far in the future, it
• 1:53:46
  Speaker 1: may drift far enough that Mars can no longer hold it.
• 1:53:51
  Speaker 1: two moons of the same planet, one falling, one drifting,
• 1:53:57
  Speaker 1: side by side in orbit right now, both discovered on
• 1:54:01
  Speaker 1: consecutive nights by the same exhausted astronomer in 1877, both
• 1:54:06
  Speaker 1: heading in opposite directions over the long arc of time.
• 1:54:10
  Speaker 1: For most of the time since their discovery, the assumption
• 1:54:14
  Speaker 1: was that Phobos and Deimos were captured asteroids, Their small size,
• 1:54:19
  Speaker 1: dark surfaces and irregular shapes looked similar to objects in
• 1:54:24
  Speaker 1: the outer asteroid belt. The capture hypothesis made a kind
• 1:54:29
  Speaker 1: of intuitive sense. But there are problems with it. Capturing
• 1:54:35
  Speaker 1: an asteroid into a nearly circular orbit around a planet
• 1:54:39
  Speaker 1: is surprisingly difficult to do without some mechanism to bleed
• 1:54:43
  Speaker 1: off excess velocity. Captured objects tend to arrive in elongated,
• 1:54:49
  Speaker 1: tilted orbits, not the nearly circular equatorial orbits that both
• 1:54:55
  Speaker 1: Phobos and Deimos follow. The current leading hypothesis is something different.
• 1:55:01
  Speaker 1: The idea gaining support now is that Phobos and Deimos,
• 1:55:05
  Speaker 1: formed from debris thrown into orbit by a giant impact
• 1:55:10
  Speaker 1: on Mars, similar in principle to how Earth's moon is
• 1:55:14
  Speaker 1: thought to have formed from the debris of a collision
• 1:55:17
  Speaker 1: with a Mars-sized body billions of years ago. This would
• 1:55:21
  Speaker 1: explain the near circular orbits and the alignment with Mars's
• 1:55:26
  Speaker 1: equatorial plane far more naturally than any capture scenario does.
• 1:55:32
  Speaker 1: The surface composition does not perfectly match this model either,
• 1:55:36
  Speaker 1: so the debate continues. What is clear is that these
• 1:55:41
  Speaker 1: two moons are not what they appear to be and
• 1:55:44
  Speaker 1: their origin is still not settled. One of Mars's moons
• 1:55:48
  Speaker 1: is falling toward it and the other is slowly drifting
• 1:55:52
  Speaker 1: away and in 50 million years Mars will have neither.
• 1:55:57
  Speaker 1: Charon is half the size of Pluto and that one
• 1:56:01
  Speaker 1: fact changes everything about how the system works. Most moons
• 1:56:05
  Speaker 1: are small. Our own moon feels enormous to us It
• 1:56:10
  Speaker 1: fills the sky, it drives the tides, it has shaped
• 1:56:14
  Speaker 1: calendars and myths for all of recorded history. But in
• 1:56:18
  Speaker 1: terms of mass, the moon is about 1.2% of Earth.
• 1:56:23
  Speaker 1: It is a footnote sitting next to a planet. Charon
• 1:56:27
  Speaker 1: is not a footnote. Charon is roughly half Pluto's diameter
• 1:56:32
  Speaker 1: and holds about 12% of Pluto's mass. No other moon
• 1:56:36
  Speaker 1: in the solar system comes close to that ratio. not one.
• 1:56:41
  Speaker 1: When you try to find a comparison, you cannot find one,
• 1:56:45
  Speaker 1: because nothing like this system exists anywhere else we have looked.
• 1:56:50
  Speaker 1: The consequence of that mass ratio is a gravitational one,
• 1:56:54
  Speaker 1: and it is strange. When a small object orbits a
• 1:56:58
  Speaker 1: much larger one, the center of gravity of the whole system,
• 1:57:02
  Speaker 1: the barycenter, sits deep inside the larger body. Earth's barycenter
• 1:57:08
  Speaker 1: with the moon sits well within Earth's interior. The Moon
• 1:57:13
  Speaker 1: orbits around a point inside our planet. That is the
• 1:57:17
  Speaker 1: normal arrangement. Pluto and Charon do not have a normal arrangement.
• 1:57:24
  Speaker 1: Because Charon is so massive relative to Pluto, the barycenter
• 1:57:28
  Speaker 1: of their system sits out in open space between the
• 1:57:32
  Speaker 1: two bodies. Neither Pluto nor Charon orbits the other. Both
• 1:57:38
  Speaker 1: of them orbit a point that floats in the space
• 1:57:41
  Speaker 1: between them. They are locked face to face, each keeping
• 1:57:46
  Speaker 1: the same hemisphere pointed permanently toward the other, circling together
• 1:57:51
  Speaker 1: around a shared gravitatis, onal center that neither of them contains.
• 1:57:56
  Speaker 1: This is not a planet with a moon. This is
• 1:57:59
  Speaker 1: a double world. Some planetary scientists will tell you the
• 1:58:03
  Speaker 1: Pluto-Charon system is the only confirmed double dwarf planet in
• 1:58:08
  Speaker 1: the solar system. They are not wrong to call it that.
• 1:58:12
  Speaker 1: The formal definitions get complicated, but the physics is clear.
• 1:58:18
  Speaker 1: These two bodies move together the way partners move, not
• 1:58:22
  Speaker 1: the way a host moves with a satellite. Six days
• 1:58:26
  Speaker 1: and nine hours is how long one full orbit takes.
• 1:58:31
  Speaker 1: In that time, Pluto and Charon complete one full turn
• 1:58:35
  Speaker 1: around each other, locked in the same face-to-face orientation they
• 1:58:40
  Speaker 1: have maintained for billions of years. For a long time,
• 1:58:44
  Speaker 1: that was essentially all we knew. Charon was discovered in
• 1:58:49
  Speaker 1: 1978 by astronomer James Christie, who noticed that images of
• 1:58:54
  Speaker 1: Pluto showed a slight elongation that moved over time. The
• 1:59:00
  Speaker 1: elongation was Charon, too small and too close to separate
• 1:59:05
  Speaker 1: cleanly with the telescopes of the era. For decades, Charon
• 1:59:09
  Speaker 1: was a blur on the edge of Pluto in even
• 1:59:12
  Speaker 1: the best images. Hubble improved that picture somewhat, but only
• 1:59:17
  Speaker 1: enough to confirm the basic geometry. The surface of Charon
• 1:59:22
  Speaker 1: was unknown, What lay underneath that small, distant, icy exterior
• 1:59:28
  Speaker 1: was a complete mystery. Then New Horizons arrived in July 2015.
• 1:59:33
  Speaker 1: The spacecraft had been traveling for nine and a half years.
• 1:59:39
  Speaker 1: It crossed the orbit of Neptune before it reached Pluto.
• 1:59:44
  Speaker 1: When it finally made its closest approach and the data
• 1:59:47
  Speaker 1: began coming back, The surface of Charon turned out to
• 1:59:51
  Speaker 1: be nothing like what most researchers had anticipated. There were mountains.
• 1:59:57
  Speaker 1: There was evidence of geological activity far more recent than
• 2:00:02
  Speaker 1: anyone had predicted for a body this cold and this small.
• 2:00:08
  Speaker 1: And running across a significant portion of the visible surface
• 2:00:12
  Speaker 1: was a canyon system that stopped scientists cold when they
• 2:00:15
  Speaker 1: first processed the images. The canyon is enormous, roughly 1,000
• 2:00:22
  Speaker 1: miles long and up to 6 miles deep in places.
• 2:00:26
  Speaker 1: For scale, the Grand Canyon is 277 miles long and
• 2:00:32
  Speaker 1: a little over a mile deep at its deepest. Charon's
• 2:00:36
  Speaker 1: canyon system dwarfs it entirely. In proportion to the body
• 2:00:40
  Speaker 1: it sits on, there is nothing like it on Earth.
• 2:00:44
  Speaker 1: The leading explanation is that Charon once had a subsurface
• 2:00:49
  Speaker 1: ocean that froze. Water expands when it freezes. If the
• 2:00:54
  Speaker 1: interior of Charon was liquid and then solidified, the expansion
• 2:00:59
  Speaker 1: would have cracked and stretched the surface, pulling it apart
• 2:01:03
  Speaker 1: in exactly the kind of long, rifted canyon system that
• 2:01:07
  Speaker 1: New Horizons photographed. A moon that may have had an
• 2:01:11
  Speaker 1: interior ocean, now frozen, wearing the evidence of that freezing
• 2:01:17
  Speaker 1: as a scar across its surface. The North Pole is
• 2:01:22
  Speaker 1: the other thing. In New Horizons images, Charon's North Polar
• 2:01:27
  Speaker 1: region is capped in a deep reddish-brown material unlike the
• 2:01:31
  Speaker 1: rest of the surface. Scientists named it Mordor Macula. The
• 2:01:37
  Speaker 1: color and composition of this material match Tholin's complex organic
• 2:01:43
  Speaker 1: molecules built from simpler compounds when they are exposed to
• 2:01:47
  Speaker 1: ultraviolet radiation and charged particles. This happens elsewhere in the
• 2:01:53
  Speaker 1: solar system. What makes Charon's North Pole different is the
• 2:01:58
  Speaker 1: source of the raw material. Pluto has an atmosphere. It
• 2:02:03
  Speaker 1: is thin and seasonal, swelling as Pluto moves closer to
• 2:02:08
  Speaker 1: the Sun in its long elliptical orbit. and collapsing back
• 2:02:12
  Speaker 1: as it recedes. But it is real. Methane from Pluto's
• 2:02:17
  Speaker 1: atmosphere escapes into space, and some fraction of that material
• 2:02:21
  Speaker 1: travels across the gap between the two worlds and reaches Charon.
• 2:02:26
  Speaker 1: At the poles, where temperatures drop low enough during Charon's
• 2:02:30
  Speaker 1: long winter, that methane freezes onto the surface. When sunlight returns,
• 2:02:37
  Speaker 1: the methane itself may escape, but the heavier organic molecules
• 2:02:41
  Speaker 1: processed from it by radiation do not. They stay. They accumulate.
• 2:02:48
  Speaker 1: They build up over millions of years into the dark
• 2:02:51
  Speaker 1: reddish cap that New Horizons photographed. Pluto is painting its
• 2:02:57
  Speaker 1: own moon. The atmosphere of one world is leaving a
• 2:03:02
  Speaker 1: permanent mark on the surface of another. The distance between
• 2:03:07
  Speaker 1: them is only about 12,000 miles, close enough that material
• 2:03:12
  Speaker 1: genuinely crosses from one to the other on timescales that
• 2:03:16
  Speaker 1: matter geologically. New Horizons flew through the Pluto system in
• 2:03:20
  Speaker 1: a matter of hours. It was moving too fast to
• 2:03:24
  Speaker 1: slow down, and there was no second spacecraft waiting to
• 2:03:27
  Speaker 1: take over. Everything we know about Cairns Canyons, its frozen
• 2:03:33
  Speaker 1: ocean hypothesis, its strange red pole. All of it comes
• 2:03:38
  Speaker 1: from one fleeting encounter nearly a decade ago. Pluto's atmosphere
• 2:03:44
  Speaker 1: is actively painting the north pole of its moon in
• 2:03:47
  Speaker 1: organic compounds, and we have only ever seen any of
• 2:03:51
  Speaker 1: this once. The moon is 400 times smaller than the
• 2:03:55
  Speaker 1: Sun and 400 times closer to Earth than the Sun,
• 2:04:00
  Speaker 1: which means from our surface, they appear almost exactly the
• 2:04:04
  Speaker 1: same size in the sky and this is almost certainly
• 2:04:07
  Speaker 1: a coincidence. Nobody designed this. No force in orbital mechanics
• 2:04:14
  Speaker 1: requires it. The moon just happens to sit at the
• 2:04:17
  Speaker 1: right distance to fit almost perfectly over the solar disk
• 2:04:21
  Speaker 1: and that accident has shaped human history in ways most
• 2:04:25
  Speaker 1: people never think about. During a total solar eclipse When
• 2:04:30
  Speaker 1: the moon slides exactly over the sun, the outer atmosphere
• 2:04:35
  Speaker 1: of the sun, the corona, becomes visible from Earth's surface.
• 2:04:41
  Speaker 1: That thin white halo of plasma, normally invisible against the
• 2:04:46
  Speaker 1: brightness of the solar disk, suddenly appears around the edges.
• 2:04:51
  Speaker 1: Victorian astronomers use these brief windows to study it. In 1868,
• 2:04:58
  Speaker 1: During a total eclipse visible from India, astronomers detected a
• 2:05:02
  Speaker 1: spectral line in the Sun's atmosphere that did not match
• 2:05:06
  Speaker 1: any element known on Earth. They called it Helium, after Helios,
• 2:05:12
  Speaker 1: the Greek sun god. It was the first element discovered
• 2:05:16
  Speaker 1: in space before it was found on Earth. That discovery
• 2:05:20
  Speaker 1: came from the coincidence of the Moon being exactly the
• 2:05:23
  Speaker 1: right apparent size. The moon itself is strange in ways
• 2:05:28
  Speaker 1: that go beyond where it sits. Our moon is large.
• 2:05:33
  Speaker 1: Not large in absolute terms, but large relative to its
• 2:05:37
  Speaker 1: host planet in a way that is nearly unique in
• 2:05:40
  Speaker 1: the solar system. Earth's moon is about one quarter the
• 2:05:44
  Speaker 1: diameter of Earth and about 1.2% of its mass. That
• 2:05:50
  Speaker 1: ratio is enormous compared to the moons of other rocky planets.
• 2:05:55
  Speaker 1: Mars has two moons, both tiny. Mercury and Venus have none.
• 2:06:02
  Speaker 1: For a planet the size of Earth to have a
• 2:06:04
  Speaker 1: moon this substantial, something unusual had to happen. The leading
• 2:06:09
  Speaker 1: explanation is called the giant impact hypothesis. About 4.5 billion
• 2:06:15
  Speaker 1: years ago, when Earth was still young, a body roughly
• 2:06:19
  Speaker 1: the size of Mars struck the early Earth at an
• 2:06:22
  Speaker 1: oblique angle. Astronomers call this impactor Theia. The collision was
• 2:06:29
  Speaker 1: not a direct hit. It was more like a glancing blow,
• 2:06:34
  Speaker 1: energetic enough to vaporize and eject an enormous quantity of
• 2:06:38
  Speaker 1: material from Earth's outer layers into orbit. That debris, a
• 2:06:43
  Speaker 1: ring of rock and vapor circling the young Earth, gradually
• 2:06:48
  Speaker 1: clumped together under its own gravity and formed the Moon.
• 2:06:53
  Speaker 1: The whole process probably took only a few thousand years
• 2:06:57
  Speaker 1: in geological terms. Almost instant. The giant impact hypothesis explains
• 2:07:04
  Speaker 1: several things that were otherwise difficult to account for. The
• 2:07:08
  Speaker 1: Moon's composition is strikingly similar to Earth's mantle rock. Not
• 2:07:13
  Speaker 1: exactly the same, but close enough to suggest they share
• 2:07:17
  Speaker 1: a common source. The Moon also has almost no iron core. Earth,
• 2:07:23
  Speaker 1: by contrast, has a substantial iron core that generates its
• 2:07:27
  Speaker 1: magnetic field. The explanation is that by the time Thayer
• 2:07:33
  Speaker 1: hit the early Earth, the iron in both bodies had
• 2:07:37
  Speaker 1: already sunk toward their respective cores through a process called differentiation.
• 2:07:44
  Speaker 1: The collision excavated mainly the rocky outer material, leaving the
• 2:07:49
  Speaker 1: iron behind. The moon that formed from that debris was
• 2:07:54
  Speaker 1: built mostly from silicate rock with almost no iron mixed in.
• 2:07:59
  Speaker 1: The moon always shows the same face to Earth. The
• 2:08:03
  Speaker 1: far side exists and has been photographed extensively since the
• 2:08:08
  Speaker 1: Soviet Luna 3 mission in 1959. But from Earth's surface
• 2:08:13
  Speaker 1: you will never see it. This is called tidal locking
• 2:08:18
  Speaker 1: and it happened gradually. Early in the moon's history, it
• 2:08:23
  Speaker 1: rotated freely. But Earth's gravity exerted a tidal force on
• 2:08:28
  Speaker 1: the moon, creating a slight bulge on the moon's near
• 2:08:31
  Speaker 1: side that acted as a break. Over millions of years,
• 2:08:36
  Speaker 1: that friction slowed the moon's rotation until it matched its
• 2:08:40
  Speaker 1: orbital period exactly. One rotation per orbit, the same face
• 2:08:46
  Speaker 1: always facing Earth. The moon is also slowly leaving, Every
• 2:08:51
  Speaker 1: year the moon moves about 1.5 inches farther from Earth.
• 2:08:56
  Speaker 1: That sounds trivial. Over a human lifetime it amounts to
• 2:09:01
  Speaker 1: a few feet, but project it backwards and the numbers
• 2:09:05
  Speaker 1: become dramatic. Early in Earth's history the moon was much closer.
• 2:09:11
  Speaker 1: Some estimates put it at roughly 10 times closer than
• 2:09:15
  Speaker 1: it is today, though the exact figure is uncertain. At
• 2:09:20
  Speaker 1: that distance Tidal forces would have been orders of magnitude stronger.
• 2:09:25
  Speaker 1: The oceans would have surged in tides far larger than
• 2:09:29
  Speaker 1: anything we experience now. The friction from those enormous tides
• 2:09:34
  Speaker 1: was also acting on Earth's rotation, slowing it down. A
• 2:09:40
  Speaker 1: day on early Earth was somewhere around six hours. The
• 2:09:44
  Speaker 1: Moon has been gradually breaking Earth's spin for billions of years.
• 2:09:50
  Speaker 1: The 24-hour day is partly the moon's doing. That recession
• 2:09:55
  Speaker 1: is measurable today. The Apollo missions left retroreflectors on the
• 2:10:00
  Speaker 1: lunar surface. Observatories on Earth fire laser pulses at them
• 2:10:05
  Speaker 1: and time the return. The distance is known to within
• 2:10:09
  Speaker 1: a few centimeters. The moon is unambiguously drifting away, and
• 2:10:14
  Speaker 1: the measurement is direct. The strangest thing about Earth's moon, though,
• 2:10:20
  Speaker 1: may be what it does for the planet quietly and continuously,
• 2:10:25
  Speaker 1: without anyone noticing. Earth's axial tilt is currently about 23.4 degrees.
• 2:10:32
  Speaker 1: That tilt is what produces the seasons. It stays relatively
• 2:10:37
  Speaker 1: stable over long periods, varying only slightly over cycles of
• 2:10:42
  Speaker 1: tens of thousands of years. The reason it stays stable
• 2:10:47
  Speaker 1: is the moon. The Moon's gravitational presence damps out the
• 2:10:51
  Speaker 1: wobbles that would otherwise develop in Earth's tilt over millions
• 2:10:55
  Speaker 1: of years. Mars, which has no large moon, has an
• 2:11:01
  Speaker 1: axial tilt that has varied enormously over geological time, swinging
• 2:11:06
  Speaker 1: through a range that may extend from nearly zero degrees
• 2:11:10
  Speaker 1: to over 40 degrees, a planet tilting that dramatically changes
• 2:11:15
  Speaker 1: its climate in extreme ways. Ice ages deepen and shift.
• 2:11:20
  Speaker 1: Seasons become unrecognizable. The conditions for stable ecosystems over hundreds
• 2:11:27
  Speaker 1: of millions of years become much harder to maintain. Earth's
• 2:11:31
  Speaker 1: axial tilt has been held roughly in place by the
• 2:11:34
  Speaker 1: moon for long enough that complex life could develop and persist.
• 2:11:39
  Speaker 1: That stability is not guaranteed. It is a consequence of
• 2:11:44
  Speaker 1: having a large moon in the right place. Without the
• 2:11:48
  Speaker 1: Moon holding Earth's axial tilt stable, the planet's climate would
• 2:11:53
  Speaker 1: have lurched through changes severe enough that complex life might
• 2:11:57
  Speaker 1: never have gotten a foothold, and you would not be
• 2:12:00
  Speaker 1: listening to this. Inside the orbit of Io, deep within
• 2:12:04
  Speaker 1: Jupiter's radiation belts, there are four small moons that most
• 2:12:09
  Speaker 1: people have never heard of, while Io, Europa, Ganymede. Callisto
• 2:12:15
  Speaker 1: get the attention. The oceans, the volcanoes, the magnetic fields.
• 2:12:21
  Speaker 1: There is an inner tier of worlds sitting far closer
• 2:12:25
  Speaker 1: to Jupiter. In a region of space so hostile that
• 2:12:29
  Speaker 1: sending a spacecraft to orbit any of them would be
• 2:12:32
  Speaker 1: a short mission. The radiation environment near Jupiter's equatorial plane,
• 2:12:39
  Speaker 1: at the distances these moons occupy. is intense enough to
• 2:12:43
  Speaker 1: damage electronics and degrade hardware at a rate that makes
• 2:12:47
  Speaker 1: long-term observation essentially impossible. These are not places where spacecraft linger.
• 2:12:54
  Speaker 1: The four moons are Metis, Adrastea, Amalthea and Thebe, arranged
• 2:13:00
  Speaker 1: in order outward from Jupiter. Metis and Adrastea are tiny,
• 2:13:05
  Speaker 1: both under 30 miles across, orbits so close to Jupiter
• 2:13:09
  Speaker 1: that they are actually inside the planet's synchronous rotation radius,
• 2:13:14
  Speaker 1: meaning they orbit faster than Jupiter spins. Thebes sits farther out,
• 2:13:19
  Speaker 1: beyond Amalthea, irregularly shaped like the rest, cratered and dark.
• 2:13:26
  Speaker 1: But Amalthea is the largest of the group and the
• 2:13:29
  Speaker 1: one that has produced the most surprises. Amalthea is about
• 2:13:34
  Speaker 1: 168 miles long and 96 miles It is not round.
• 2:13:41
  Speaker 1: It looks something like a battered potato, elongated and lumpy,
• 2:13:46
  Speaker 1: covered in craters, with its long axis pointing permanently toward
• 2:13:51
  Speaker 1: Jupiter because tidal forces locked it in place long ago.
• 2:13:56
  Speaker 1: It orbits Jupiter in roughly 12 hours, travelling inside the
• 2:14:00
  Speaker 1: path of Io. That already sits deep inside one of
• 2:14:05
  Speaker 1: the harshest radiation environments in the solar system. Amalthea sits
• 2:14:10
  Speaker 1: closer still. When the Galileo spacecraft swung past Amalthea in
• 2:14:15
  Speaker 1: November 2002, one of its final maneuvers before its mission ended,
• 2:14:21
  Speaker 1: the data it returned contained something unexpected. Amalthea is putting
• 2:14:27
  Speaker 1: out more heat than it receives from the Sun. For
• 2:14:30
  Speaker 1: a small, geologically dead rock at that distance from the Sun,
• 2:14:35
  Speaker 1: this should not happen. The Sun is simply too far
• 2:14:39
  Speaker 1: away Amalthea is simply too small to have retained any
• 2:14:43
  Speaker 1: meaningful internal heat from its formation. Something else is warming it.
• 2:14:50
  Speaker 1: The leading explanation involves Jupiter's magnetic field. Jupiter generates the
• 2:14:55
  Speaker 1: most powerful planetary magnetic field in the solar system, and
• 2:15:00
  Speaker 1: Amalthea moves through it constantly. As a conducting body moving
• 2:15:05
  Speaker 1: through a magnetic field, electrical currents are induced inside the moon,
• 2:15:11
  Speaker 1: Those currents generate heat. Amalthea is not being heated from inside,
• 2:15:17
  Speaker 1: and it is not being heated by the sun. It
• 2:15:20
  Speaker 1: is being heated by the electromagnetic environment it is flying through.
• 2:15:26
  Speaker 1: The moon is, in a practical sense, being cooked by
• 2:15:30
  Speaker 1: Jupiter's magnetosphere from the outside in. Then there is the color.
• 2:15:36
  Speaker 1: Amalthea is one of the reddest objects in the entire
• 2:15:39
  Speaker 1: solar system. When Voyager 1 flew past in 1979 and
• 2:15:45
  Speaker 1: returned the first close images, scientists noted immediately how strikingly
• 2:15:50
  Speaker 1: red it was. The source of that color is almost
• 2:15:54
  Speaker 1: certainly Io. Io's volcanic eruptions throw enormous quantities of sulfur
• 2:15:59
  Speaker 1: compounds into space. Much of that material eventually coats the
• 2:16:04
  Speaker 1: surfaces of moons orbiting in the same region. Amalthea sits
• 2:16:09
  Speaker 1: inside Io's orbit. but sulfur ejected from Io's plumes spreads
• 2:16:14
  Speaker 1: outward and inward, settling onto nearby surfaces. Amalthea has been
• 2:16:20
  Speaker 1: accumulating this material for a very long time. Io is,
• 2:16:24
  Speaker 1: in effect, spray painting everything nearby in sulfur compounds. Amalthea
• 2:16:30
  Speaker 1: is close enough to have received a thorough coating. The
• 2:16:33
  Speaker 1: inner moons also have a structural role in the Jupiter
• 2:16:36
  Speaker 1: system that has nothing to do with their own geology.
• 2:16:40
  Speaker 1: Jupiter has a faint ring system, nothing like Saturn's, far
• 2:16:45
  Speaker 1: too dim to see with the naked eye, but real
• 2:16:48
  Speaker 1: and measurable. The source of that ring material is Amalthea, Thebe,
• 2:16:54
  Speaker 1: Metis and Adrastea. Micrometeorites strike these moons constantly, and each
• 2:17:01
  Speaker 1: impact kicks dust and debris off the surface. That material
• 2:17:06
  Speaker 1: drifts into orbit around Jupiter, and spreads into the thin
• 2:17:10
  Speaker 1: ring structures that Voyager 1 first detected in 1979. The
• 2:17:16
  Speaker 1: rings exist because the moons are there and are being
• 2:17:20
  Speaker 1: slowly eroded by impacts. Without them, there would be no rings.
• 2:17:25
  Speaker 1: What is striking about all of this is how little
• 2:17:28
  Speaker 1: we actually know. Amalthea has been photographed by Voyager 1,
• 2:17:33
  Speaker 1: Voyager 2, and Galileo. The Galileo flyby in 2002 remains
• 2:17:40
  Speaker 1: the closest encounter any spacecraft has made. After that, the
• 2:17:45
  Speaker 1: mission ended deliberately. The spacecraft was crashed into Jupiter. to
• 2:17:51
  Speaker 1: avoid any risk of contaminating Europa. No mission since has
• 2:17:56
  Speaker 1: gone back. The radiation environment makes dedicated study difficult to
• 2:18:01
  Speaker 1: justify against other priorities. The moons are small enough that
• 2:18:06
  Speaker 1: they do not generate the same scientific urgency as the
• 2:18:10
  Speaker 1: Galilean worlds. They orbit in a zone that destroys the
• 2:18:15
  Speaker 1: tools we would need to study them. Amalthea is being
• 2:18:19
  Speaker 1: heated by electromagnetic induction, spray-painted by sulfur from Io. It
• 2:18:26
  Speaker 1: sits inside a radiation zone that would destroy most space.
• 2:18:31
  Speaker 1: Kraft sent to study it closely, and it has barely
• 2:18:34
  Speaker 1: been looked at. Saturn has a ring that is invisible
• 2:18:38
  Speaker 1: to the naked eye, nearly invisible to most telescopes, and
• 2:18:43
  Speaker 1: wider than the full moon appears from Earth, and it
• 2:18:46
  Speaker 1: comes from a moon most people have never heard of.
• 2:18:50
  Speaker 1: Phoebe is about 135 miles across. It orbits Saturn at
• 2:18:56
  Speaker 1: an enormous distance, far beyond the regular moon system, in
• 2:19:01
  Speaker 1: a direction opposite to Saturn's rotation. That last detail is
• 2:19:05
  Speaker 1: the critical one. Every moon that formed alongside Saturn from
• 2:19:10
  Speaker 1: the same spinning disk of material orbits in the same
• 2:19:13
  Speaker 1: direction Saturn rotates. Phoebe goes the other way. That retrograde
• 2:19:20
  Speaker 1: orbit is not a quirk. It is a signature. It
• 2:19:24
  Speaker 1: means Phoebe did not form here. Saturn captured it from
• 2:19:28
  Speaker 1: somewhere else. The Cassini spacecraft flew past Phoebe in June
• 2:19:33
  Speaker 1: of 2004, just before entering Saturn orbit. It was the
• 2:19:38
  Speaker 1: closest look humanity had ever gotten, and the images were striking.
• 2:19:44
  Speaker 1: Phoebe is dark and heavily cratered, irregular in shape, nothing
• 2:19:49
  Speaker 1: like the smooth, icy inner moons of Saturn. It looks ancient.
• 2:19:55
  Speaker 1: The surface is covered in craters of every size. some
• 2:19:59
  Speaker 1: deep enough that the material blasted out of them exposed
• 2:20:02
  Speaker 1: what was underneath. And what was underneath the dark outer
• 2:20:06
  Speaker 1: layer was bright, icy. The dark surface of Phoebe is
• 2:20:11
  Speaker 1: a crust, a skin of organic material sitting over an
• 2:20:15
  Speaker 1: interior that is largely composed of ice. That combination told
• 2:20:21
  Speaker 1: researchers something important. The composition of Phoebe does not match
• 2:20:26
  Speaker 1: Saturn's regular moons at all, It matches the objects in
• 2:20:30
  Speaker 1: the Kuiper belt, the vast reservoir of icy bodies beyond
• 2:20:35
  Speaker 1: Neptune that includes Pluto, Eris, and thousands of other worlds.
• 2:20:41
  Speaker 1: Phoebe appears to be one of those objects. At some
• 2:20:45
  Speaker 1: point in the solar system's history, it passed close enough
• 2:20:49
  Speaker 1: to Saturn to be gravitationally captured and locked into orbit.
• 2:20:55
  Speaker 1: Saturn reached out across an enormous distance, and grabbed a
• 2:20:59
  Speaker 1: piece of the outer solar system. This is not a
• 2:21:02
  Speaker 1: clean or simple process. Capture requires the incoming body to
• 2:21:07
  Speaker 1: lose enough energy during the encounter to become bound. That
• 2:21:12
  Speaker 1: usually involves a close gravitational interaction, a third body, or
• 2:21:17
  Speaker 1: some combination that bleeds off the speed an object needs
• 2:21:20
  Speaker 1: to escape. Whatever the mechanism, Phoebe ended up in a wide, tilted,
• 2:21:26
  Speaker 1: retrograde orbit far from Saturn, where it has been ever since.
• 2:21:31
  Speaker 1: It never became part of the regular moon system. It
• 2:21:35
  Speaker 1: remained alien, a small frozen world from the edge of things,
• 2:21:40
  Speaker 1: orbiting a planet it was never born near. For decades,
• 2:21:45
  Speaker 1: Phoebe was considered an interesting oddity. Then, in 2009, astronomers
• 2:21:52
  Speaker 1: found something that changed how Phoebe fits into the broader
• 2:21:56
  Speaker 1: Saturn system. Using the Spitzer Space Telescope, a team detected
• 2:22:01
  Speaker 1: an enormous diffuse ring surrounding Saturn at roughly the distance
• 2:22:05
  Speaker 1: of Phoebe's orbit. It is not like Saturn's bright inner rings.
• 2:22:11
  Speaker 1: There are no sharp edges, no visible structure. It is
• 2:22:15
  Speaker 1: a vast, faint cloud of dust and particles spread across
• 2:22:20
  Speaker 1: a region so large it is almost impossible to describe
• 2:22:24
  Speaker 1: in familiar terms. If it were somehow visible to the
• 2:22:28
  Speaker 1: naked eye, it would appear in the sky as a
• 2:22:31
  Speaker 1: band twice the width of the full moon. The ring
• 2:22:35
  Speaker 1: is tilted relative to Saturn's other rings, which orbit roughly
• 2:22:40
  Speaker 1: in Saturn's equatorial plane. The Phoebe ring sits at a
• 2:22:44
  Speaker 1: different angle, matching Phoebe's own orbital inclination. That tilt, along
• 2:22:50
  Speaker 1: with its location, pointed directly at the source. Micrometeorites constantly
• 2:22:56
  Speaker 1: hit Phoebe. Each impact kicks small amounts of material off
• 2:23:01
  Speaker 1: the surface and into space. Over millions of years, this slow,
• 2:23:07
  Speaker 1: steady loss of dust has built up a ring tracing
• 2:23:10
  Speaker 1: Phoebe's own path around Saturn. That dust does not just
• 2:23:15
  Speaker 1: sit there. It drifts inward. spiralling slowly towards Saturn, under
• 2:23:21
  Speaker 1: the influence of solar radiation pressure and gravitational forces. As
• 2:23:27
  Speaker 1: it moves inward, it intersects the orbit of Iapetus, and
• 2:23:32
  Speaker 1: this is where the connection becomes something more than coincidence. Iapetus,
• 2:23:38
  Speaker 1: Saturn's two-toned moon, has one hemisphere almost entirely black and
• 2:23:43
  Speaker 1: one almost entirely white. The dark material has puzzled researchers
• 2:23:49
  Speaker 1: for centuries. The Phoebe ring explains where it came from.
• 2:23:54
  Speaker 1: Dust spiraling inward from Phoebe collects on the leading hemisphere
• 2:23:58
  Speaker 1: of Iapetus as Iapetus sweeps through its orbit. Two moons
• 2:24:04
  Speaker 1: with no obvious relationship turn out to be connected by
• 2:24:08
  Speaker 1: a ring invisible to every telescope for the first 338
• 2:24:12
  Speaker 1: years after Iapetus was discovered. Phoebe is not alone in
• 2:24:18
  Speaker 1: being a captured body. Saturn has dozens of smaller irregular
• 2:24:23
  Speaker 1: moons beyond its regular system, most of them just a
• 2:24:27
  Speaker 1: few miles across. Most of them discovered only in the
• 2:24:31
  Speaker 1: last 20 years as telescope technology improved. Jupiter has more
• 2:24:37
  Speaker 1: than 90 known moons now. and the vast majority of
• 2:24:40
  Speaker 1: them are small irregular bodies in distant, tilted, often retrograde orbits.
• 2:24:47
  Speaker 1: Uranus has irregular moons. Neptune has Triton, which is just
• 2:24:52
  Speaker 1: a very large version of the same story. The giant
• 2:24:56
  Speaker 1: planets are surrounded by swarms of captured objects, small bodies
• 2:25:01
  Speaker 1: from the outer solar system that wandered too close and
• 2:25:05
  Speaker 1: never left. Most of them have never been visited by
• 2:25:08
  Speaker 1: a spacecraft Most of them have no names that anyone
• 2:25:12
  Speaker 1: outside a specialist community would recognize. They are dark and
• 2:25:17
  Speaker 1: small and far away, and they have been orbiting their
• 2:25:21
  Speaker 1: captured paths for billions of years. Phoebe is one of
• 2:25:27
  Speaker 1: dozens of captured outer moons at each of the giant planets,
• 2:25:32
  Speaker 1: each of them a small alien body from the outer
• 2:25:36
  Speaker 1: solar system. held in permanent gravitational captivity. Look at enough
• 2:25:41
  Speaker 1: moons and a pattern emerges that the textbooks about solar
• 2:25:45
  Speaker 1: system formation did not quite predict. The standard story is
• 2:25:50
  Speaker 1: a clean one. A disk of gas and dust surrounds
• 2:25:53
  Speaker 1: a young star. Gravity pulls material together. Clumps form. Clumps collide.
• 2:26:01
  Speaker 1: Planets grow. Moons form from the leftover debris orbiting those planets.
• 2:26:07
  Speaker 1: spinning in the same direction, settled into neat circular paths,
• 2:26:12
  Speaker 1: everything calm and orderly. That is the version that fits
• 2:26:16
  Speaker 1: neatly into diagrams. Then you go look at the actual moons.
• 2:26:21
  Speaker 1: Triton orbits Neptune backward. Charon is so massive relative to
• 2:26:26
  Speaker 1: Pluto that the two objects orbit a point in empty
• 2:26:30
  Speaker 1: space between them. Miranda looks like it was broken into
• 2:26:34
  Speaker 1: pieces and put back together by someone who lost the instructions.
• 2:26:39
  Speaker 1: Iapetus is painted jet black on one side and brilliant
• 2:26:44
  Speaker 1: white on the other, and the best explanation involves a
• 2:26:48
  Speaker 1: runaway feedback loop that took hundreds of millions of years
• 2:26:52
  Speaker 1: to complete. These are not edge cases. These are some
• 2:26:58
  Speaker 1: of the most significant moons in the solar system, and
• 2:27:01
  Speaker 1: none of them fit the quiet orderly picture. What they
• 2:27:05
  Speaker 1: fit instead is chaos. Specifically, they fit the kind of
• 2:27:10
  Speaker 1: chaos that planetary scientists now believe defined the early solar
• 2:27:14
  Speaker 1: system for its first several hundred million years. Two models
• 2:27:20
  Speaker 1: try to describe this period. The Grand Tak hypothesis proposes
• 2:27:25
  Speaker 1: that Jupiter formed and then migrated inward toward the Sun,
• 2:27:30
  Speaker 1: nearly reaching the orbit of Mars before Saturn's gravity pulled
• 2:27:34
  Speaker 1: it back outward. This inward and outward migration would have
• 2:27:39
  Speaker 1: scattered enormously numbers of smaller bodies. Asteroids, comets, proto-moons, Kuiper
• 2:27:46
  Speaker 1: belt objects. All of them were flung across the solar
• 2:27:50
  Speaker 1: system during this reshuffling. Some were ejected entirely. Others were
• 2:27:56
  Speaker 1: thrown into new orbits. Some were captured. The Nice model
• 2:28:01
  Speaker 1: describes a later episode. After the planets settled into roughly
• 2:28:06
  Speaker 1: their current positions, a period of gravitational instability caused Saturn,
• 2:28:12
  Speaker 1: Uranus and Neptune to shift outward. This triggered what is
• 2:28:17
  Speaker 1: called the late heavy bombardment, a period roughly four billion
• 2:28:22
  Speaker 1: years ago when the inner solar system was pelted with impactors.
• 2:28:26
  Speaker 1: The moon's surface pockmarked with ancient craters, carries the record
• 2:28:32
  Speaker 1: of that bombardment. So does Callisto. So does every heavily
• 2:28:37
  Speaker 1: cratered body we have visited. The timing of those craters
• 2:28:42
  Speaker 1: tells us something real happened to destabilize the solar system
• 2:28:46
  Speaker 1: after the initial formation period was already complete. The irregular
• 2:28:51
  Speaker 1: moons are the clearest evidence of this violent history. Every
• 2:28:55
  Speaker 1: giant planet has them. Jupiter has more than 90 known moons,
• 2:29:01
  Speaker 1: and the vast majority are small irregular bodies orbiting at
• 2:29:05
  Speaker 1: enormous distances in tilted, elongated, often retrograde paths. Saturn has
• 2:29:13
  Speaker 1: a similar population. So do Uranus and Neptune. These moons
• 2:29:18
  Speaker 1: do not belong to the neat inner system of large
• 2:29:21
  Speaker 1: tidally locked regular moons. They move like captured prisoners. Their
• 2:29:27
  Speaker 1: orbits make no sense if they formed alongside their planets.
• 2:29:32
  Speaker 1: They make complete sense if they were scattered inward from
• 2:29:35
  Speaker 1: the outer solar system during a period of planetary migration
• 2:29:40
  Speaker 1: and then gravitationally trapped. Phoebe at Saturn, the irregular moons
• 2:29:47
  Speaker 1: clustering in distant retrograde groups at Jupiter. These are fragments
• 2:29:53
  Speaker 1: of the early Aotei. Our solar system brought to where
• 2:29:57
  Speaker 1: they are now by a history that was anything but gentle.
• 2:30:02
  Speaker 1: But the story the moons tell is not only about violence,
• 2:30:06
  Speaker 1: it is also about water. The icy moons of Jupiter
• 2:30:10
  Speaker 1: and Saturn, Europa, Ganymede, Callisto, Enceladus, Titan, have collectively forced
• 2:30:18
  Speaker 1: a rethinking of one of the most basic assumptions in
• 2:30:21
  Speaker 1: the search for life. For decades, the habitable zone meant
• 2:30:25
  Speaker 1: one thing, The narrow band around a star where liquid
• 2:30:29
  Speaker 1: water could exist on a planetary surface. Too close and
• 2:30:33
  Speaker 1: it boils away. Too far and it freezes. Earth sits
• 2:30:38
  Speaker 1: inside that band. Mars is near the edge. Jupiter is
• 2:30:43
  Speaker 1: far outside it. And yet Europa has a liquid ocean.
• 2:30:47
  Speaker 1: Enceladus has a warm salty ocean with active seafloor vents.
• 2:30:52
  Speaker 1: Titan has a chemically rich environment with organic molecules raining
• 2:30:57
  Speaker 1: out of the sky. These worlds are at the orbit
• 2:31:00
  Speaker 1: of Jupiter and Saturn. By the classical definition, they have
• 2:31:05
  Speaker 1: no business being anything other than frozen solid. The reason
• 2:31:09
  Speaker 1: they are not frozen comes down to tidal heating. When
• 2:31:13
  Speaker 1: a moon orbits a massive planet and is being pulled
• 2:31:17
  Speaker 1: by the gravity of neighbouring moons as well, The resulting
• 2:31:21
  Speaker 1: flexing generates heat from the inside. This heat has nothing
• 2:31:26
  Speaker 1: to do with the star. It does not care how
• 2:31:29
  Speaker 1: far the system is from its sun. It is mechanical energy,
• 2:31:34
  Speaker 1: converted to thermal energy through the constant squeezing and releasing
• 2:31:39
  Speaker 1: of the moon's interior. This means that any moon around
• 2:31:43
  Speaker 1: any giant planet in any solar system, at almost any
• 2:31:47
  Speaker 1: distance from any star, could potentially maintain a liquid water
• 2:31:52
  Speaker 1: ocean if the orbital geometry is right. The implications are significant.
• 2:31:58
  Speaker 1: If subsurface oceans exist around Jupiter and Saturn in our
• 2:32:03
  Speaker 1: own solar system, and there is strong evidence that several
• 2:32:07
  Speaker 1: of them do, then this is probably not unusual. Most
• 2:32:11
  Speaker 1: solar systems appear to have giant planets. Giant planets have moons,
• 2:32:17
  Speaker 1: Moons around giant planets experience tidal heating. The universe may
• 2:32:23
  Speaker 1: be full of subsurface oceans sitting beneath ice shells, warmed
• 2:32:27
  Speaker 1: from below, insulated from above, completely invisible from the outside.
• 2:32:34
  Speaker 1: The classical habitable zone turns out to describe only one
• 2:32:38
  Speaker 1: type of habitable environment. Tidal heating describes another, and it
• 2:32:44
  Speaker 1: operates across a far larger volume of space. This is
• 2:32:48
  Speaker 1: not a minor adjustment to how planetary scientists think about
• 2:32:52
  Speaker 1: life in the universe. It is a fundamental expansion. The
• 2:32:57
  Speaker 1: number of potentially habitable environments goes from a narrow ring
• 2:33:02
  Speaker 1: around each star to something that includes the entire population
• 2:33:06
  Speaker 1: of moons around giant planets everywhere. In our solar system alone,
• 2:33:13
  Speaker 1: The count of worlds with confirmed or strongly suspected liquid
• 2:33:18
  Speaker 1: water now sits at four or five, depending on how
• 2:33:22
  Speaker 1: you weigh the evidence for Callisto and Mimas. A generation ago,
• 2:33:28
  Speaker 1: that number was one. The search for life in the
• 2:33:32
  Speaker 1: solar system has quietly shifted over the past two decades,
• 2:33:37
  Speaker 1: away from planets and toward the moons orbiting them. Right now,
• 2:33:41
  Speaker 1: at this moment, Two spacecraft are traveling toward the Jupiter system,
• 2:33:46
  Speaker 1: specifically to study its icy moons. Europa Clipper launched in
• 2:33:51
  Speaker 1: October 2024. JUICE, the Jupiter Icy Moons Explorer, built by
• 2:33:58
  Speaker 1: the European Space Agency, launched in 2023. They are heading
• 2:34:04
  Speaker 1: to the same neighborhood by different routes, carrying different instruments,
• 2:34:09
  Speaker 1: asking slightly different questions. Both of them will arrive in
• 2:34:14
  Speaker 1: the early 2030s. What they find there may be the
• 2:34:19
  Speaker 1: most consequential scientific result of this century. Europa Clipper will
• 2:34:25
  Speaker 1: not orbit Europa. The radiation environment around Jupiter is brutal
• 2:34:32
  Speaker 1: enough that a spacecraft parked in orbit around Europa would
• 2:34:36
  Speaker 1: be destroyed within months. Instead, it will orbit Jupiter and
• 2:34:41
  Speaker 1: conduct nearly 50 close flybys of Europa, dipping in, collecting
• 2:34:47
  Speaker 1: data and pulling back out before the radiation accumulates to
• 2:34:51
  Speaker 1: dangerous levels. Each pass will build a more detailed picture
• 2:34:56
  Speaker 1: of what is underneath the ice. The magnetic field readings
• 2:35:01
  Speaker 1: will tell scientists how thick the ice shell actually is.
• 2:35:06
  Speaker 1: Infrared instruments will map the surface temperature looking for spots
• 2:35:11
  Speaker 1: where warm water from below might be pushing closer to
• 2:35:15
  Speaker 1: the surface. If there are active plumes on Europa, water
• 2:35:20
  Speaker 1: erupting the way Enceladus erupts, Europa Clipper will fly through
• 2:35:25
  Speaker 1: them and taste what comes out. Enceladus gave us molecular
• 2:35:30
  Speaker 1: hydrogen and complex organics through exactly this kind of encounter.
• 2:35:36
  Speaker 1: Europa could give us something more specific. Something harder to
• 2:35:40
  Speaker 1: explain without biology. Nobody is saying that is likely, but
• 2:35:45
  Speaker 1: the instruments are capable of detecting it, and that is
• 2:35:49
  Speaker 1: a new thing in the history of this search. JUICE
• 2:35:53
  Speaker 1: takes a different path. Its primary target is Ganymede. It
• 2:35:58
  Speaker 1: will eventually enter orbit around Ganymede, making it the first
• 2:36:02
  Speaker 1: spacecraft to orbit a moon other than our own. Along
• 2:36:07
  Speaker 1: the way, it will conduct multiple flybys of Callisto and Europa.
• 2:36:12
  Speaker 1: Ganymede's magnetic field, its internal ocean, the strange grooved terrain
• 2:36:18
  Speaker 1: that nobody has fully explained. Juice will spend years working
• 2:36:24
  Speaker 1: through these problems from close range. Then there is Dragonfly.
• 2:36:28
  Speaker 1: This one is different in kind, not just in scale.
• 2:36:32
  Speaker 1: Dragonfly is a rotorcraft lander, nuclear-powered, Scheduled to arrive at
• 2:36:38
  Speaker 1: Titan in the mid-2030s, it will fly, not roll across
• 2:36:44
  Speaker 1: the surface the way a rover does, covering a few
• 2:36:47
  Speaker 1: miles over several years. It will lift off, navigate through
• 2:36:52
  Speaker 1: Titan's thick atmosphere, and land at a new site, then
• 2:36:57
  Speaker 1: do it again. Over the course of its mission, it
• 2:37:02
  Speaker 1: could cover more than 100 miles of Titan's surface. sampling
• 2:37:07
  Speaker 1: the chemistry at riverbeds, dune fields and crater floors where
• 2:37:12
  Speaker 1: liquid water briefly existed after ancient impacts. The organic chemistry
• 2:37:19
  Speaker 1: building up in Titan's environment is not simple. Tholins, the
• 2:37:24
  Speaker 1: complex molecules that rain down from the upper atmosphere and
• 2:37:28
  Speaker 1: coat the surface, include some of the same compounds involved
• 2:37:32
  Speaker 1: in prebiotic chemistry on early Earth. Dragonfly will land in
• 2:37:37
  Speaker 1: the middle of it and start measuring. If the chemistry
• 2:37:41
  Speaker 1: in certain locations has moved further along the prebiotic pathway
• 2:37:46
  Speaker 1: than it should have on its own, That is something
• 2:37:49
  Speaker 1: worth paying very close attention to. Beyond these missions, there
• 2:37:54
  Speaker 1: are proposals that are further out and considerably more ambitious.
• 2:37:59
  Speaker 1: Engineers have drawn up concepts for a lander that would
• 2:38:03
  Speaker 1: set down on Europa's ice, melt its way through using
• 2:38:07
  Speaker 1: a nuclear heat source, and deploy something like a small
• 2:38:10
  Speaker 1: submarine into the ocean below. The ice is potentially miles thick.
• 2:38:16
  Speaker 1: The melting process alone could take years. Nothing like this
• 2:38:21
  Speaker 1: has ever been attempted, and it is probably decades away
• 2:38:25
  Speaker 1: from being technically and financially feasible. But the concept exists.
• 2:38:32
  Speaker 1: The engineering problems are being studied. At some point, the
• 2:38:37
  Speaker 1: question of what is in Europa's ocean will be important
• 2:38:41
  Speaker 1: enough that the answer will be worth that kind of effort.
• 2:38:45
  Speaker 1: The philosophical weight of what these missions might find is
• 2:38:50
  Speaker 1: not easy to overstate. If Europa Clipper detects something in
• 2:38:54
  Speaker 1: a plume sample that biological chemistry can explain and abiotic
• 2:38:59
  Speaker 1: chemistry cannot, or if Dragonfly finds organic structures in Titan's
• 2:39:05
  Speaker 1: sediment that look less like random synthesis and more like selection,
• 2:39:10
  Speaker 1: the implication runs in one direction. Life that arose twice
• 2:39:15
  Speaker 1: in the same solar system almost certainly arose everywhere conditions allow.
• 2:39:22
  Speaker 1: The universe contains hundreds of billions of galaxies, each with
• 2:39:27
  Speaker 1: hundreds of billions of stars, an enormous fraction of them
• 2:39:31
  Speaker 1: with moons in environments that tidal heating could warm. A
• 2:39:37
  Speaker 1: second origin of life in our solar system alone would
• 2:39:41
  Speaker 1: make the cosmos almost certainly full of it. that is
• 2:39:45
  Speaker 1: the stake sitting underneath these missions. If life exists anywhere
• 2:39:50
  Speaker 1: else in the solar system, the most likely answer is
• 2:39:54
  Speaker 1: going to come back from something orbiting a planet, not
• 2:39:58
  Speaker 1: from the planet itself, and we are going to find
• 2:40:01
  Speaker 1: out within most people's lifetimes.