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Moons: The Solar System’s Secret Worlds · Star Trails: From Backyard Astronomy to Cosmic Wonder

0:07
Howdy Star Gazers and welcome to this episode
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of Star Trails. My name is Drew and I'll be your
0:13
guide to the night sky for the week of March
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the 22nd to the 28th. This week we're continuing
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our exploration of our own cosmic neighborhood
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by taking a look at some of the most unique moons
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in the solar system. and which ones of those
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might have the right conditions to develop life.
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Later in the show, we'll take a look at this
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week's sky and pick up the next two chapters
0:37
of our book club selection, Night Watch. Whether
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you're tuning in from the backyard or the balcony,
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I'm glad you're here. So grab a comfortable spot
0:47
under the night sky and let's get started. I
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need to start this week's show by first thanking
0:53
our listeners. I received some interesting feedback
0:57
concerning last week's Pluto episode. Everything
1:01
from a wonderful poem about Pluto sent in by
1:04
a listener, to a handful of folks who mentioned
1:06
Pluto as being their favorite planet. Some folks
1:10
just enjoyed the story. At this point, I think
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it's safe to say Pluto is as much of a cultural
1:16
object as it is an astronomical one. And we're
1:19
not done with Pluto yet. More on that later in
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this show. And another listener who's currently
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vacationing in New Zealand sent me a slew of
1:29
night sky images from Down Under, including one
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of my favorite objects, the Large Magellanic
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Cloud, plus a very bright image of the Milky
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Way's core, and an upside down rendition of Orion.
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It's kind of weird seeing Orion standing on his
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head. As always, if you want to contact me, just
1:51
use the form over at StarTrails .Show. With that,
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let's get into our main topic this week. When
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most of us hear the word moon, we think of a
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very specific place. A gray, cratered world hanging
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quietly in our night sky. Airless, silent, unchanging.
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And for most of human history, that was our template.
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If you'd asked someone what a moon was, they
2:19
would have described something very much like
2:21
our own. And in many ways, our moon is extraordinary.
2:27
It likely formed when a Mars -sized world slammed
2:31
into early Earth, sending debris into orbit that
2:35
eventually coalesced into the moon. It's unusually
2:39
large for a rocky planet. so large that the Earth
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-Moon system almost behaves like a binary pair.
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Its gravity stabilizes Earth's tilt, helping
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keep our climate relatively steady over long
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periods of time. And, of course, it's the only
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world beyond Earth where human beings have ever
3:00
set foot. But for all its importance, our Moon
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also gave us the wrong idea. Because when we
3:08
finally began exploring the outer solar system,
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We discovered that moons are not all like ours,
3:15
not even close. When the Voyager spacecraft flew
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past Jupiter and Saturn and sent back images,
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that changed everything. Suddenly, moons weren't
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just dead rocks orbiting distant planets. They
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were worlds, some of them violent, some of them
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dynamic, and some of them are possibly habitable.
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A moon is simply a natural satellite, anything
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that orbits a planet or a dwarf planet, but maybe
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that's a little too straightforward. Some moons
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are bigger than planets. For example, Ganymede
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and Titan are both larger than the planet Mercury.
3:56
Some systems behave like binary worlds, think
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Pluto and Charon. Some moons may have formed
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like planets, just in orbit around another world.
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while others were wanderers captured by gravity.
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Rings and moons are related. Inside a certain
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distance, known as the Roche limit, material
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can't clump together into a moon, so it forms
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rings instead. In fact, if an object goes beyond
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that limit, it's likely going to break apart
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because of the planet's gravity. Saturn is the
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king of the rings, of course, but Jupiter, Uranus,
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and Neptune all have wispy ring systems. And
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there are shepherd moons around these systems
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that gravitationally keep the ring systems in
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check. Outside the Roche limit, rings can eventually
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coalesce into moons. So in a sense, rings are
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moons that never formed, and nearby moons are
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rings that finish the job. There's a great diversity
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in moon types just in our solar system. Take
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Io, one of Jupiter's moons. It's the most volcanically
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active world in the entire solar system. Its
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surface is constantly being reshaped by eruptions,
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lava flows, sulfur plumes, and landscapes that
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look more like a fever dream than a place in
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space. And the reason is gravity. Jupiter's immense
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pull stretches and squeezes Io over and over
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again. generating heat inside the moon. That
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process is called tidal heating. That same process
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leads us to something even more remarkable. Because
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if tidal forces can generate heat, they can melt
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ice. And if they can melt ice, they can create
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oceans. Hidden ones. Europa, another of Jupiter's
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moons, looks like a frozen world. Its surface
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is smooth, bright, and criss -crossed with long
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fractures that resemble sea ice on Earth. But
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beneath that icy shell, scientists believe there's
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a global ocean, possibly deeper than all of Earth's
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oceans combined. We haven't seen that ocean directly.
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Instead, we've detected it through a kind of
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cosmic detective work. The surface geology suggests
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movement. Magnetic measurements from the Galileo
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spacecraft indicate a conductive, likely salty,
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layer beneath the ice. Models of tidal heating
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show there's enough energy to keep water liquid,
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so all signs point to an ocean in the dark. And
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Europa isn't alone. At Saturn, a tiny moon called
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Enceladus shocked scientists when the Cassini
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spacecraft discovered geysers erupting from its
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south pole, jets of water vapor and ice blasting
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into space. Cassini flew through those plumes
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and detected salts, organic molecules, even hydrogen,
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evidence that this water has been in contact
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with rock deep below the surface. In other words,
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Enceladus has an ocean, and it's leaking into
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space. Then there's Titan. Titan is something
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else entirely. It's larger than the planet Mercury
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and has a thick nitrogen atmosphere, denser than
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Earth's in some ways, and a surface shaped by
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weather, clouds, rain, rivers, and lakes. But
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the liquid isn't water. It's methane and ethane.
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Titan is a world where it rains hydrocarbons
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and rivers carve through landscapes of ice as
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hard as rock. And we've been there. In 2005,
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the Huygens probe parachuted through Titan's
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orange sky and landed on its surface, the most
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distant landing humanity has ever achieved. It
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sent back images of an eerie landscape, rounded
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stones of water ice scattered across what looked
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like a dried riverbed. And beneath Titan's crust,
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there may be yet another hidden ocean. Back at
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Jupiter, Ganymede is the biggest moon in the
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solar system. If it and Titan were orbiting the
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Sun instead of gas giants, we'd be calling them
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planets. Ganymede has its own magnetic field,
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and it's the only moon we know of that does.
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Beneath its surface may lie multiple layers of
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ocean, stacked between sheets of ice like a planetary
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layer cake. Callisto, also a Jovian moon, is
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heavily cratered and ancient. It too may also
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hide a deep ocean beneath its battered exterior.
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And farther out orbiting Neptune is Triton, a
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strange icy world that likely didn't form around
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Neptune at all. It was probably captured from
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the Kuiper Belt. It orbits backward against the
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rotation of the planet. And Voyager 2 observed
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nitrogen geysers erupting from its surface. Even
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at the edge of the solar system, these moons
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are active. And then there are the quieter, lesser
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-known worlds. Mimas, a small moon of Saturn,
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looks like the Death Star from Star Wars. It's
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dominated by a massive impact crater. But recent
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evidence suggests that it may have a, guess what,
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a hidden ocean beneath its icy shell. Iepidus
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is split dramatically in two, one hemisphere
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bright as snow, the other dark as coal, creating
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one of the starkest contrasts in the solar system.
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Miranda, a moon of Uranus, looks like it was
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shattered and reassembled with cliffs and terrain
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so bizarre that it defies easy explanation. And
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out at Pluto, Charon, or is it pronounced Cairon,
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is so large relative to Pluto that the two orbit
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a point in space between them. The New Horizons
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flyby in 2015 revealed vast canyons and signs
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of ancient geological activity. In addition to
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Charon, Pluto has four irregular tiny moons,
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making it a complex six -body system. They don't
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behave like our moon or even like most of the
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large moons of Jupiter and Saturn. Instead of
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being tidally locked, some of them tumble unpredictably.
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Their rotations are influenced by the constantly
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shifting gravitational pull of both Pluto and
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Charon. It's a dynamically messy system. We've
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talked about some of the most well -known moons
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in this episode, but here are a couple of really
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weird ones you might not know about. Hyperion
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orbits Saturn and resembles a sponge or a piece
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of coral drifting through space. Its surface
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is covered in deep, sharp -edged craters giving
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it a porous appearance. But the truly strange
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part is its motion, and this is a trait it shares
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with some of Pluto's moons. Hyperion isn't tidally
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locked, and it tumbles around Saturn in an unpredictable
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way. If you were standing on Hyperion, which
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would be complicated, Saturn wouldn't rise and
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set in a regular way. It would drift unpredictably
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across the sky. Another excellent oddball is
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Phoebe, also a moon of Saturn. I'm beginning
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to think Saturn is probably the most fun place
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to be in the solar system. It has everything.
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Rings, weird moons, and more. Anyway, Phoebe
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orbits Saturn backward, much like Triton does
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around Neptune. So it didn't form with Saturn,
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and like Triton, it's possibly a relic from the
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Kuiper Belt. It's dark, irregular, and ancient.
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A fossil from the early solar system just hanging
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out in Saturn's orbit. Before we drift away from
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Saturn, I also have to mention Janus and Epimetheus.
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These two moons share nearly the same orbit,
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and every few years when they get near each other,
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they effectively swap places. These are called
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co -orbital moons. And what I mean by swapping
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places is this. One moon orbits closer to the
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planet than the other. When they approach one
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another, a gravitational dance takes place between
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the two, and one assumes the closer inner orbit,
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trading places with the other, now pushed to
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the outer track. And they do this repeatedly.
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I didn't forget about Mars. It has Phobos and
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Deimos, two little rocky potatoes that were likely
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captured by the Martian gravity. In other words,
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they didn't form alongside Mars the way our moon
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formed alongside Earth. Our moon is the only
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large moon of the inner solar system, which makes
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us quite an outlier. The outer planets are moon
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machines. Jupiter now has more than 90 known
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moons, Saturn more than 140, and Uranus and Neptune
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both have dozens. Astronomers think the difference
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in numbers between the outer and inner planets
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is similar to why the outer planets are so much
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larger. The inner solar system was hotter when
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it formed. Close to the sun, lighter materials
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like water, ice, and gases couldn't condense
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easily. that left mostly rock and metal. Out
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beyond the frost line, ices could form, and ice
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is abundant. That gave the outer planets far
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more raw material to build not just planets,
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but entire systems of moons. Also, since the
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outer planets are all enormous, their gravity
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is strong enough to capture passing objects and
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hold on to large numbers of moons. None of that
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explains why Pluto, smaller than even our moon,
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somehow has five moons of its own. Well, the
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moons of Pluto and Earth may actually share an
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origin story. Earth's moon is likely the result
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of a giant impact, which is a rare and chaotic
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event. Pluto's moons may have also formed from
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such an impact. A collision early in Pluto's
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history would have thrown debris into orbit,
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which later coalesced into Charon and the smaller
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moons. Mercury and Venus may have once had moons,
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but over billions of years, gravitational interactions
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with the sun could have destabilized those systems,
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causing moons to either crash into the planet
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or escape. So the outer solar system ends up
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looking like kind of a miniature solar system
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factory. with planets surrounded by their own
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collections of worlds, while the inner solar
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system is comparatively sparse. I remember reading
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in Carl Sagan's Cosmos years ago that there could
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be multiple places to sustain life in our solar
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system, not just Earth and Mars. Of course, Sagan
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was including many of the moons in the outer
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solar system that had been recently discovered.
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The possibility that some of these moons might
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be habitable, not on the surface or in sunlight,
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but beneath the ice, is a fascinating one. On
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Earth, we've discovered ecosystems thriving in
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complete darkness, deep in the ocean, powered
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not by sunlight, but by chemical energy from
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hydrothermal vents. If similar environments exist
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on Europa or Enceladus, liquid water, energy,
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chemistry, then the basic ingredients for life
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may be present. And here's the part that changes
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how we think about the universe. When we search
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for habitable worlds, we tend to look for planets
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like Earth, rocky, sitting at just the right
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distance from their star. But our own solar system
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is telling us something surprising. Moons warmed
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by gravity instead of sunlight, oceans hidden
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beneath ice, small distant worlds orbiting giant
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planets, their oceans circulating quietly in
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the dark. These moons are intriguing areas for
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future exploration. Remember, many of these moons
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have only been seen once during the Voyager flybys
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of the 1980s. There's a lot more science to learn.
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Future missions such as Europa Clipper, which
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was launched in 2024, is designed to study the
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subsurface ocean of Europa. And Dragonfly is
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expected to launch in 2028 on a mission to Titan.
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It's a robotic helicopter designed to fly on
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Titan where it will study habitability and chemistry.
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And, of course, NASA is making plans to return
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to our moon with the Artemis program, with a
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landing scheduled for 2027. We have one more
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episode about our solar system to close out March.
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Next week, we journey into the Kuiper Belt and
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beyond. After a quick break, we'll be back to
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cover this week's night sky. Stay with us. Welcome
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back. Let's step outside now and take a look
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at this week's sky. We're coming off of a new
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moon, which means this is one of the best weeks
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of the month for deep sky observing. As the week
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begins, a delicate, waxing crescent moon hangs
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low in the western sky just after sunset, growing
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a little fuller each night. By mid -week, the
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moon reaches first quarter, appearing half -lit,
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and on the evening of March 26th, it forms a
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nice pairing with Jupiter in the constellation
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Gemini. This is a great time to notice something
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subtle, earth -shine. That faint glow illuminating
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the dark portion of the crescent moon is actually
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sunlight reflected off of Earth. Now let's talk
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planets. The standout this week is Venus, blazing
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in the western sky just after sunset. It's reemerged
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as the evening star, shining brighter than anything
19:00
else in the night sky except the moon. You'll
19:03
find it low in the west, lingering for more than
19:06
an hour after sunset. If you catch it with a
19:09
small telescope you may even notice it showing
19:12
a crescent phase, much like a tiny version of
19:15
the moon. I've been seeing Venus on my evening
19:18
walks lately and its brightness is so startling
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I have a hard time looking away. It's really
19:24
one of my favorite sights in the night sky. Higher
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up dominating the evening is Jupiter. It's bright,
19:32
steady, and unmistakable. Look at it in a telescope
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and you'll see it flanked by the moons we've
19:37
been discussing, Io, Europa, Ganymede, and Callisto.
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You'll see them change position from night to
19:45
night, and sometimes they render shadows on the
19:48
Jovian surface. If you're up early, there's a
19:51
more subtle challenge waiting for you. Late in
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the week, Mercury begins to peek into the pre
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-dawn sky, very low on the horizon before sunrise.
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It's not an easy target, but if you have a clear
20:04
eastern horizon and a bit of patience, you might
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catch it briefly before the sun washes it away.
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Now this is the time of year astronomers quietly
20:13
get excited about. Spring brings what's known
20:17
as galaxy season. As the Milky Way dips below
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the horizon in the evening, we're able to look
20:23
past the plane of our galaxy into deep space.
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And that means galaxies and lots of them. In
20:31
the constellation Leo, you can find the Leo Triplet,
20:35
three galaxies interacting with each other across
20:38
millions of light years. In Ursa Major, near
20:42
the Big Dipper, look for M81 and M82, a pair
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of galaxies, one graceful and spiral, the other
20:51
ragged and explosive. And if you sweep your telescope
20:55
toward Virgo and Coma Berenices, you'll begin
20:58
to encounter entire fields of galaxies. Faint
21:02
smudges of light, each one a vast island universe.
21:07
For something a little more low -key, try tracking
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down the Coma star cluster in Coma Berenices,
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a loose, delicate scattering of stars that's
21:16
best seen with binoculars under dark skies. Or,
21:20
if you're feeling ambitious, hunt down the Ghost
21:23
of Jupiter Nebula in Hydra, a faint planetary
21:27
nebula that resembles a pale ghostly disk. And
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finally, take a moment this week to simply look
21:34
up without a plan. Find Orion setting in the
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west, slowly leaving our evening skies. Look
21:41
overhead to Gemini, with Jupiter shining among
21:44
the twin stars Castor and Pollux. And rising
21:48
in the east, you'll begin to see the constellations
21:51
of Spring, Leo, Virgo, and Boötes taking their
21:55
place. We're slowly creeping towards the end
22:04
of our book club selection, Night Watch, and
22:08
there are just a few chapters left. Chapters
22:11
8 and 9 bring us back to something familiar.
22:14
our own moon and its relationship with the Sun.
22:19
These are two very short chapters, and I'm not
22:22
sure if they offer much to experienced observers,
22:25
but since our topic this week was all about the
22:27
moons of our solar system, it seemed appropriate
22:30
to talk about them in this episode. Chapter 8
22:33
is all about observing the Moon and Sun. These
22:37
two familiar objects are easy to take for granted,
22:40
but endlessly rewarding if you spend a little
22:43
time with them at the eyepiece. The Moon, especially,
22:47
is one of the best targets for backyard astronomy.
22:50
Even a small telescope will reveal an incredible
22:53
landscape of craters, mountain ranges, and vast
22:57
lava plains known as maria. One of the best times
23:00
to observe is along the line between light and
23:03
dark, called the Terminator, where shadows stretch
23:07
across the surface and give those features depth
23:09
and texture. This chapter also includes some
23:12
nice maps of the moon's surface. Chapter 8 also
23:17
encourages something we don't talk about often,
23:20
observing the sun. Now, this comes with an important
23:24
caveat. You must use proper solar filters. But
23:28
when you do, even a modest telescope can reveal
23:31
sunspots, those dark shifting regions tied to
23:35
the sun's magnetic activity. With specialized
23:38
equipment, you can even see solar prominences,
23:42
massive arcs of plasma rising from the sun's
23:45
surface. Then in chapter 9, we move into eclipses.
23:50
These are the moments when the sun, earth, and
23:52
moon align just right to create something extraordinary.
23:57
A solar eclipse happens when the Moon passes
24:00
in front of the Sun, casting its shadow on Earth.
24:03
A lunar eclipse occurs when Earth moves between
24:06
the Sun and the Moon, and our shadow darkens
24:09
the lunar surface. Total solar eclipses, in particular,
24:14
have captivated people throughout history. In
24:17
fact, the book describes so -called eclipse cults.
24:20
These are people who travel across the globe
24:22
to stand in the narrow path of totality where
24:26
day briefly turns to night. Lunar eclipses are
24:29
more common and a bit more forgiving. You don't
24:33
need special equipment to enjoy them and they
24:35
often turn the moon a deep coppery red as Earth's
24:39
atmosphere filters sunlight into the shadow.
24:42
You can refer to the tables in this chapter to
24:45
discover when the next eclipses are happening.
24:49
We'll wrap up our discussion on the remaining
24:51
chapters of Night Watch in two weeks. We're almost
24:54
done. If you're a newer observer and you've made
24:57
it this far in the book, then your knowledge
24:59
of the night sky and how to view it has increased
25:02
exponentially since we began this journey. That's
25:11
going to do it for this week. If you found this
25:13
episode interesting, please share it with a friend
25:15
who might enjoy it. The easiest way to do that
25:18
is by sending folks to our website, StarTrails
25:22
.Show. And if you want to support the show, use
25:25
the link on the site to buy me a coffee. That
25:28
really helps. Be sure to follow Star Trails on
25:31
Blue Sky and YouTube. Links are in the show notes.
25:35
Until we meet again beneath the stars, clear
25:38
skies everyone.