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Supercharged Neutrinos, Exploding Black Holes & Lunar Mysteries Uncovered | Q&A · Space Nuts: Astronomy Insights & Cosmic Discoveries

0:01
Andrew Dunkley: Hi there. Thanks for joining us on a Q and A
0:03
edition of Space Nuts. Andrew Dunkley here,
0:05
your host. Great to have your company. Coming
0:08
up, we've got a few questions. Nick is going
0:10
to ask about supercharged neutrinos.
0:13
Andrea is making a return appearance.
0:16
She's got a couple of questions about the
0:17
dark side of the moon and shallow craters.
0:19
And Eli is asking about elements
0:22
and the speed of objects. And if we've got
0:25
time, we'll chuck another question into the
0:27
mix as well. All coming up on this edition of
0:30
Space Nuts. Seconds. Guidance is
0:33
internal. 10, 9.
0:36
Ignition sequence start.
0:37
Jonti Horner: Space nuts.
0:38
Andrew Dunkley: 5, 4, 3, 2. 1. 2, 3, 4,
0:41
5, 5, 4, 3, 2, 1. Space
0:44
nuts. Astronauts report it feels good.
0:47
And with Fred away, Jonti can play.
0:50
It's professor, uh, Jonti
0:52
Horner, professor of Astrophysics at the
0:55
University of Southern Queensland. Jonti,
0:57
hello again. Good afternoon.
0:58
Jonti Horner: How are you going to.
0:59
Andrew Dunkley: I am well. Great to see you. I think we
1:02
should just go straight into it and, uh,
1:05
hit you with our first question.
1:07
It's a topic I'm not overly familiar
1:10
with, but, uh, this one comes from Nick.
1:12
Uh, I just read that a supercharged neutrino
1:15
was detected by the Kilometer Cube
1:18
Neutrino Telescope, and a theory was
1:21
put forward that it came from an exploding
1:23
black hole. Please explain how a black hole
1:26
can explode. Love the show, Nick. Thank you,
1:29
Nick, we love that you love the show.
1:32
Thank you for sending in a question. Um,
1:35
exploding black holes. Um,
1:38
I seem to remember Fred might have written a
1:40
book about something like that once. Um, but
1:42
anyway, um, do they explode or do they
1:45
merge or do they collapse? They eventually
1:48
disappear.
1:48
Jonti Horner: I know that now. You know straight up,
1:51
I'm not a cosmologist or a cosmetologist,
1:53
which I always used to joke about
1:55
cosmologists being cosmetologists. And it
1:57
turns out a cosmetologist is a real thing. So
1:59
never mind. Um, that's, uh,
2:02
further from my area of expertise. So any
2:04
answer I give, take with a larger grain of
2:07
salt, you know, as is always the way you
2:09
know, you. The further you go from your
2:10
expertise, the more out of date your
2:12
knowledge is. My knowledge on
2:15
exploding or rather evaporating black
2:17
holes goes back to basically when I was an
2:20
undergrad and I was doing lots of courses in
2:22
lots of different things. And this goes back
2:25
to some of the work that made Stephen
2:27
Hawking so world renowned. Now, obviously,
2:30
for a lot of people, Stephen Hawking became a
2:32
global name with the publication of A Brief
2:34
History of Time, which did a very good job of
2:36
explaining very complicated things in A way
2:38
that people could at least feel like they had
2:41
a grasp of. Um, I remember reading it as a
2:43
kid, and it made my head hurt. But it, in a
2:45
good way, I could actually follow it. It was
2:47
well explained. One of the things that
2:49
Stephen Hawking did fairly early in his
2:51
career, I think in, like, 1974 or something,
2:53
was do some very theoretical work on, um,
2:56
black holes, where he postulated that black
2:58
holes could lose we through a process called
3:01
Hawking radiation. And the idea is
3:04
that black holes can effectively be
3:07
considered to have a temperature and to
3:10
radiate energy and therefore mass away
3:13
into space. And the smaller the black hole,
3:15
the hotter it is, so the quicker it would
3:17
radiate. And this is all backed up by
3:19
ridiculously complex physics and mathematics
3:21
that is way beyond my level of full
3:23
understanding. But part of the idea behind it
3:25
is what we think of as the empty vacuum of
3:28
space is actually not a true vacuum, but is
3:30
instead constantly populated by pairs
3:33
of matter and antimatter particles that
3:35
spontaneously create and then collide with
3:37
each other and disappear again. And if these,
3:40
if such an event happens near the event
3:42
horizon of a black hole, one of the particles
3:44
falls into the black hole, the other escapes,
3:45
and it's seen to lose mass, lose energy, and,
3:48
um, radiation going along with that. Now,
3:50
the bigger the black hole, the colder it
3:53
would be. So the slower it radiates anyway,
3:56
but also the bigger it is, the more
3:58
effectively it can feed from its environment.
4:00
Even if that's little bits of dust falling
4:02
in. Or if it's near a star, star, it can feed
4:05
off that star, get an accretion disk. So the
4:07
black holes that form in the modern universe
4:10
are formed by stars reaching the end of their
4:12
lives and are massive. They're more massive
4:15
than the sun by L1, where they're formed from
4:17
stars much more massive than the Sun. You get
4:20
massive black holes, you get intermediate
4:22
mass black holes, and you get supermassive
4:24
black holes. And they're all the big whopping
4:25
ones. And, um, the time scale, as I
4:27
understand it, m for those black holes to
4:29
decay through Hawking radiation is
4:32
ridiculously, ridiculously, ridiculously
4:34
longer than the edge of the universe. Yes.
4:36
And, um, they're probably not emitting
4:38
Hawking radiation at a level that we could
4:40
detect because they are very cold. In his
4:42
quantification of it, however, at, uh,
4:45
the birth of the universe, when the
4:47
temperature and pressure was immense after
4:48
the Big Bang, there were
4:51
theoretically a class of black holes created
4:54
called primordial black holes. So these were
4:56
black holes that were not born from the fiery
4:59
Death of a star, but were instead born
5:02
out of the Big Bang and the pressures and the
5:04
temperatures. And um, these could be black
5:06
holes down to the mass of a thumbnail or down
5:09
really, really tiny ones. Planet mass black
5:11
holes.
5:12
Andrew Dunkley: Yep.
5:12
Jonti Horner: The smaller you are as a black hole, the more
5:14
quickly you radiate things away, so the
5:16
shorter your lifetime. And so you have this
5:19
idea that these primordial black holes
5:21
evaporated over time and effectively
5:24
none of them will survive to the current day.
5:27
Those evaporating black holes would
5:30
evaporate over time and give off radiation
5:32
that we have never yet detected. But a black
5:35
hole coming to the end of its life will
5:36
evaporate faster and m faster. There's a
5:38
quote on an article I found recently which
5:40
may be tied to this, um, article entitled
5:43
An Exploding Black Hole Could Reveal the
5:45
Foundations of the Universe, published from
5:47
September last year, talking about
5:50
the predictions that as our technology gets
5:52
better in the coming years, we may be able to
5:55
detect this Hawking radiation in an event
5:57
where the black hole reaches its critical
5:59
phase and evaporates entirely within the next
6:02
few years. So not quite the neutrino
6:04
discovery that we were talking about in the
6:05
question, but a related thing. And there's a
6:08
quote here from Andrea Tham, who
6:10
Associate Professor Andrea Tham, I, I do hate
6:13
it when articles don't give people's well
6:15
earned titles until later in the sentence or
6:17
don't give them at all. Um, which is another
6:20
ont. I could go on, that's separate, but it
6:22
particularly affects my, um,
6:25
early career colleagues, um, affects
6:27
colleagues from non traditional backgrounds
6:29
and stuff. And it's a very
6:32
diminutizing thing, diminishing thing. It
6:35
lowers their expertise. Anyway, this is a
6:37
quote from Associate Professor Andrea Tham
6:39
from University of Massachusetts Amherst,
6:41
says as primordial black holes evaporate,
6:44
they become ever lighter, uh, so hotter.
6:47
They therefore emit even more radiation. It's
6:49
a runaway process until they explode.
6:52
Um, it's that Hawking radiation that our
6:54
telescopes can detect.
6:55
Andrew Dunkley: Yeah.
6:56
Jonti Horner: So what's happening is you've got these
6:57
primordial black holes that are really itty
6:59
bitty diddy ones that are therefore
7:02
evaporating quicker than they can gain mass.
7:04
They'll be on this critical threshold. And so
7:06
you get this runaway death where the more
7:08
massive ones live longer before they get
7:11
small enough to finally evaporate and then
7:13
explode. And so I would guess
7:17
that the observation of this super
7:19
neutrino that has been linked potentially to
7:22
an exploding black hole is not two black
7:24
holes colliding. It's not a modern Black hole
7:27
formed from the death of sars, but rather is
7:30
the death of a primordial black hole, as
7:32
would be predicted by this research by
7:34
Stephen hawking more than 50 years ago in the
7:37
form of Hawking radiation. So that's my
7:40
thinking on what's happening here. Now,
7:42
obviously, I am, um, not an expert.
7:45
Um, I've said previously on many places,
7:47
uh, in a lot of disciplines, and when we're
7:49
teaching our undergrads, we often say, avoid
7:51
Wikipedia. Wikipedia is not a static
7:53
resource. It's a fluid resource and it's
7:55
often wrong. And I know for journalists, it's
7:57
probably often something you caution, don't
7:59
get your facts from Wikipedia. For
8:01
astrophysics, and particularly the more
8:03
technical and hardcore ends of astrophysics,
8:07
Wikipedia is actually very reliable because
8:09
very few people will be interested in
8:11
maliciously editing a webpage because,
8:13
frankly, they'll go off, uh, after other
8:15
topics that are more triggering. But also,
8:18
people who are interested in this stuff and
8:19
have the knowledge tend to be very obsessive.
8:21
And if they spot something wrong, they fix it
8:23
very quickly. The result of that is if you
8:25
Google Hawking radiation. The
8:28
Wikipedia page is very lengthy, goes into a
8:31
lot of detail, includes some of the maths.
8:33
That makes my head hurt, huh? And makes me
8:35
want to cry a little bit.
8:36
I'm talking about black hole evaporation and
8:38
things like this. Now,
8:41
the equation for black hole evaporation
8:43
that's on here, which is based on the Hawking
8:46
work, gives a evaporation
8:49
time for a black hole of
8:51
2.14 times 10 to the 67
8:54
years. So that's 2.14
8:57
multiplied by 10 with 67 zeros
9:01
multiplied by the mass of the black hole
9:03
divided by the mass of the sun to the power
9:05
three. So if you've got a black
9:08
hole that is one solar mass, it will take
9:11
2.14 times 10 to the 67 years to
9:14
evaporate. And the M more massive it is, the
9:15
larger that number gets to the power three.
9:19
So the multiplier here is get the mass of the
9:21
black hole as measured in units of the mass
9:23
of the sun, cube that number, and
9:26
then multiply it by 2.14 times 10 to the
9:29
67, and you get a headache, but
9:32
you get a number. Now, the mass of the sun
9:35
is what, 2 times 10 to the 30
9:38
kilos? Right? Mass of the
9:41
Earth is 5.97 times 10
9:44
to the 24 kilos. So that is
9:46
effectively, um, 2 times 10 to the minus
9:49
6 solar masses, about a millionth of a solar
9:52
mass. So we'll just say it's 1 millionth of a
9:53
solar mass. 1 millionth
9:57
cubed is 1 times 10 to the minus
10:00
18. That means a, uh, black hole, the mass
10:03
of the Earth, would decay much more quickly.
10:05
It would decay in only 10 to the 49 years,
10:09
which is still much, much, much, much longer
10:11
than the edge of the universe. But you can
10:12
play this game with everything. I. I'm
10:15
too fat. You know, we talk about health and
10:17
everything on the show before. I am a fair
10:20
bit more than 100 kilos. But let's assume I
10:22
was 100 kilos, um, just because that's an
10:25
aspirational goal. And it would be nice if it
10:27
were true one day. In fact, I'm 100 kilos and
10:30
you make me a black hole. Um, I would be sad,
10:32
but probably wouldn't have long to think
10:34
about it. At 100 kilos,
10:37
I will be 10 to the 28 times
10:40
less massive than the sun, roughly. The sun
10:43
is 10 to the 30. I'm 10 to the 2. 10 to the
10:46
28 is a difference. 10 to the 28 cubed
10:49
is 28, 56,
10:51
84. So that's 10 to the 84.
10:54
So that means I would disintegrate in two
10:57
times 10 to the 67 times 10 to the minus
10:59
84, which is about 10 to the minus 17
11:02
years. So suddenly, a drum t mass black
11:05
hole would disintegrate and evaporate in
11:08
a tiny fraction of the millisecond.
11:11
So these primordial mass black holes that
11:13
evaporate are, uh, doing
11:16
so because they're very small. You could, if
11:19
you wanted to. And I'll leave this as an
11:20
exercise to the reader, because me doing
11:22
mental arithmetic is not the most exciting
11:24
thing. You could work out what massive black
11:26
hole would have to be to evaporate
11:29
after 13.8 billion years,
11:32
which is about how old the universe is. The
11:35
reason that's an interesting one is if there
11:37
were any primordial mass black holes of that
11:39
mass and they were to evaporate,
11:42
they will be evaporating in the very near
11:44
universe. And that would make them much
11:47
easier to detect because the intensity of
11:49
radiation we detect is proportional to 1
11:52
over the square of the distance. So if
11:54
something's twice as far away, it's four
11:55
times fainter. If it's three times as far
11:57
away, it's nine times fainter. So
12:00
I don't know. I'm not a black hole expert by
12:02
any means. I say that all the time.
12:06
But if there were a black hole of that mass
12:08
formed at the Big Bang, Then maybe
12:12
they will be evaporating in the relatively
12:14
local universe, and they're the ones that
12:15
have been most likely to detect. I do not,
12:18
however, know what the distribution
12:21
of masses for primordial black
12:24
holes would be. It's possibly on this
12:25
Wikipedia page, but have a
12:28
look and find out if it's your kind of thing.
12:30
But hopefully that explains why there's a
12:33
turnover point where things would decay in
12:34
less than the edge of the universe or more
12:36
than the edge of the universe. Uh, and that
12:38
mass is somewhere between the mass of ajonti
12:40
and the mass of the Earth.
12:42
Andrew Dunkley: Okay, fascinating. Yeah. All right, thank
12:44
you, Nick. Uh, and Nick, uh, you might have
12:47
heard us talking a, uh, week or two or
12:49
three or four back, uh, about, uh,
12:52
what they think might be the discovery of a
12:55
primordial black hole. So that's a story
12:57
worth looking up as well.
12:58
Thanks for your question. This is Space Nuts,
13:01
Q and A edition with Andrew Dunkley and John
13:04
de Horner.
13:07
Jonti Horner: 0G and I feel fine. Space Nuts.
13:10
Andrew Dunkley: Uh, now, Jonti, we've got an audio question
13:13
that comes from a repeat offender.
13:15
Uh, her name's Andrea.
13:17
Andrea: Hi, guys. Got, um, a couple of questions I'm
13:20
hoping you can help me with. Um, the first
13:22
question I have is, um,
13:25
why does the dark side of the Moon
13:28
not have anywhere near as much damage as the
13:31
face of the Moon? Um,
13:35
my second question is,
13:37
um, why are the
13:40
craters so shallow on the moon? Considering
13:43
the size of some of the impact
13:46
zones and craters, um, they all seem to be
13:49
the same depth, which is quite shallow, um,
13:52
especially if you look at TAO, which is
13:54
3,000 miles wide, um, with an
13:57
incredibly shallow crater. Um, if you could
14:01
explain for me why that occurs,
14:04
that would be absolutely amazing. Thank you
14:07
very much. Oh, and this is Andrea from
14:09
Wanneroo and Andrew. Uh, Wanneroo is actually
14:12
a noongar, or whadjuk? Noongar. Ah,
14:15
people word. Um, that actually
14:19
means the area of the
14:22
digging stick. Unfortunately, not
14:25
pet kangaroo, although I have had one of
14:26
those as well. Thanks, guys. Take care.
14:29
Jonti Horner: Bye.
14:29
Andrew Dunkley: Thanks, Andrea. Lovely to hear from you. I'm
14:31
glad she explained that. Um, you probably
14:33
don't know what she's talking about, Jonti,
14:34
but, um, when Andrea last sent us an audio
14:37
question and she said she was from Wanneroo,
14:39
I translated that to an indigenous word
14:42
meaning, I want a pet kangaroo. So,
14:45
yeah, I know I was being silly, but, um, no,
14:47
it's, um, place of the digging stick. Didn't
14:50
know that. So, um, of course, the digging
14:52
stick was one of the implements, uh, that the
14:55
ancient indigenous peoples of Australia used
14:58
to use to, to uh, dig up,
15:01
um, grubs and other, other bush
15:04
tucker as we call it these days.
15:05
Jonti Horner: So it's probably worth mentioning for the
15:07
listeners who are not in Australia that many
15:09
of the Australian places have names
15:12
that derive from the languages of the
15:15
traditional owners of the land, the
15:16
indigenous people of Australia, who had many
15:18
different countries with many different
15:20
language groups. And the origin of the
15:22
names is not always that well known or
15:24
understood because during the invasion of
15:27
Australia and during the events that happened
15:29
all the way through to the 1970s, there was a
15:31
fairly aggressive attempt to, even if
15:34
you weren't wiping out the people, to get rid
15:36
of the culture and to get rid of the
15:37
knowledge. I've just looked up Toowoomba
15:40
where I am T o uh o uh w o uh o uh m b
15:42
a I live about 20 ks west of there.
15:45
Toowoomba is an indigenous name. It's a
15:47
really interesting town because it's like the
15:49
Florida of Queensland. All the old people
15:51
come here to retire. It's a lovely place.
15:53
It's a beautiful place because Queensland has
15:57
a particular climate. But Toowoomba is
16:00
a moderated version of that climate because
16:02
it sits on the Great Dividing range at about
16:04
700 meters above sea level. So it's not as
16:06
humid as the coast. It doesn't get as hot as
16:08
the coast. It has very lovely dry winters.
16:10
Anyway, the name of Toowoomba is
16:13
probably based on a word
16:16
from likely the Gable or Jarawar
16:19
peoples. Not entirely sure. But if you look
16:21
around for the origin of Toowoomba as a word,
16:25
there's lots of suggestions. There is a
16:27
suggestion that it was a, ah, word for swamp
16:29
because Toowoomba sits in this swampy area on
16:31
top of the hills. According to the Toowoomba
16:33
Regional Council, it may have been named
16:35
after a property in the area in the 1850s,
16:38
or it may have come from an Aboriginal word
16:41
meaning either place where water sits, which
16:43
will be the swamp thing or place of melon, or
16:45
place where reeds grow or berries place or
16:48
white man. There are other things saying
16:50
meeting of the waters or. Or saying. The name
16:53
of Toowoomba may be an anglicized version of
16:54
the word bu wonga, which meant thunder in
16:57
the dialect of the upper Burnett and Gaynda
16:59
tribes. So we just don't know. And it does
17:02
make me a little bit sad. We talk about
17:04
indigenous astronomy a bit and the wonderful
17:06
work that, um, Professor Duane Hamaker and
17:08
his students have done over the years working
17:10
with the indigenous people of Australia. But
17:12
it does make me sad how much of this
17:13
knowledge is lost where you don't even know
17:15
the origin of the name. So it's wonderful
17:16
that in this case we actually know where the
17:19
name comes from. We can talk to that. So when
17:21
you're looking at the map of Australia and
17:23
think a lot of the places are unusual from
17:25
the perspective of someone from an Anglo
17:27
background or from a European background,
17:29
it's because even though it's a primarily
17:31
English speaking country nowadays with a,
17:35
with that, you know, Anglo heritage, a lot of
17:38
the names are actually from the traditional
17:39
owners, even if the heritage of that name
17:42
itself is lost.
17:43
Andrew Dunkley: Yes. Uh, where I live, Dubbo is
17:45
supposedly a Wiradjuri word for red
17:48
earth, because the soil here is red,
17:51
uh, which might sound horrifying to people.
17:54
Uh, it is when you get a dust storm and
17:56
everything turns red, uh,
17:59
Jonti Horner: when it gets wet and you're bringing it in
18:01
because the red soil marks horrible
18:03
everything up, you know. Yeah. Dog goes out
18:06
and gets their paws muddy and brings in red
18:08
footprints.
18:09
Andrew Dunkley: Red footprints on a light colored carpet. No,
18:12
uh, terrible stuff. And of course one that
18:14
relates to astronomy is warmera,
18:18
which is an indigenous word for uh, the,
18:20
the implement they used to launch a spear
18:24
rather than just throw the spear. They used
18:26
to have a specially made, um,
18:29
I suppose you'd call it a, like a handheld
18:32
catapult. And it, um,
18:35
and, and it. Yeah, and it flung the spear at
18:37
greater speed and distance. And that's uh, it
18:40
was called a woomera. And of course woomera
18:42
rocket range is where Australia's, uh,
18:44
early space efforts were, uh, were
18:47
launched from in South Australia. So yeah,
18:49
it's um, fascinating history really is.
18:51
Jonti Horner: And um, of course the aquilatl that I
18:54
mentioned there, I just double checked
18:55
because it's like I remember an aquilateral
18:57
being a thing that used for throwing space.
18:59
It turns out that that was an Aztec implement
19:00
that served the same kind of process. So the
19:02
word applatl apparently comes from Aztec.
19:05
Andrew Dunkley: Oh, wow. Didn't know that. Back
19:08
to you, Andrea.
19:08
Jonti Horner: Yes.
19:10
Andrew Dunkley: Okay, two questions. Dark side of the moon?
19:13
Uh, smoother. Now I've always been aware that
19:16
the side we can see is so rugged and
19:19
pockmarked and mountainous.
19:20
Jonti Horner: Yes.
19:21
Andrew Dunkley: But the side that we cannot see that, uh,
19:24
Artemis 2 recently had a look at, uh, and
19:26
where the Chinese have been running around on
19:28
their little scooters. Um,
19:31
it's smoother. Why?
19:33
Jonti Horner: Well, this is a weird one. So
19:36
it looks more uniform when
19:39
you look at it. And I'm saying that very
19:40
carefully rather than smoother, because
19:42
smoother invokes polished or smooth.
19:45
Like your skin when you're a kid is a lot
19:47
smoother than your skin when you get to my
19:48
edge and you've got all the wrinkles right.
19:50
Yeah. Um, or the scars.
19:52
Andrew Dunkley: See this one?
19:52
Jonti Horner: Yeah.
19:53
Andrew Dunkley: Uh, that's from a golf club. My neighbor hit
19:56
me in the face with a seven iron. Yeah. It
19:58
wasn't malicious. It was the backswing. I was
20:00
standing too close.
20:01
Jonti Horner: I was going to say it sounds like the
20:02
adventures you have in double. You know,
20:04
something to pass the time. The reason that
20:06
I'm being careful in my wording here and
20:08
saying it looks more uniform rather than it's
20:10
smoother is actually, I don't think it is
20:12
smoother, but I think it definitely does look
20:14
more uniform on the near side of the
20:17
Moon. It should be said that we're talking
20:18
near side and far side. The dark side of the
20:20
Moon is simply the side of the Moon pointed
20:21
away from the Sun. And, um, that rotates
20:24
around as the Moon goes around the Earth,
20:25
which is why we get the faces right. If
20:27
you're stood on the Moon, you'll get at a
20:29
given location two weeks of daytime and two
20:31
weeks of nighttime. And when it's nighttime
20:33
for you, you'd be on the dark side of the
20:34
Moon. But when the Moon's new, the dark side
20:36
points towards us. The far side of the Moon
20:39
always points away from the Earth. Now, on
20:41
the near side of the Moon, which is a side
20:42
we're familiar with, the view we get is
20:45
very non uniform because we've got the
20:48
mare and the non mare regions. So the mare
20:51
are the seas which make up the man in the
20:53
Moon or whatever picture you have, which are
20:55
these flood basalt areas. And, um, then
20:57
you've got the non mari areas, which are more
21:00
traditionally rocky object looking.
21:03
Andrew Dunkley: He's the drunk man in the Moon,
21:06
uh, here because he's upside down.
21:08
Jonti Horner: Absolutely, yeah. Those areas
21:11
that make the drunk man are, uh,
21:13
flood basalt outpourings on the near side of
21:16
the Moon that were formed early in the Moon's
21:18
formation. If you ascribe to the idea that
21:20
there was a Late Heavy Bombardment when the
21:22
impact rate spiked, then they are thought to
21:25
have formed there. But in actuality, evidence
21:27
for the Late Heavy Bombardment has pretty
21:29
much dissipated. So the closer you are to
21:32
impact studies and studies of the Moon, the
21:34
less strongly you hold to the idea that heavy
21:36
bombardment was a thing. But as with all
21:38
science, the further you get from a certain
21:40
expertise, the more out of date your
21:41
knowledge is. So the heavy bombardment is
21:44
quite often still viewed as canon in a Lot of
21:45
areas, whereas those who are closest to the
21:47
topic have a lot more doubt that it ever
21:49
happened. But anyway, on the near side of the
21:52
Moon, you've got areas
21:54
of the Moon that didn't have
21:57
Amare, didn't have a flood basalt
22:00
outpouring. And, um, you've got areas that
22:02
did. And then overlaid on that, you've got
22:04
some more recent impacts, which are the rare
22:06
craters where you've got weathered material
22:09
on the surface that looks darker and an
22:11
impact comes along, digs through the darker
22:13
material to the unweathered material below
22:14
and splashes it across the surface. So the
22:17
near side of the Moon looks very non uniform
22:18
because you've got that disparity between the
22:21
flood basalts and the non flood basalts. And
22:24
the non flood basalt is an older surface
22:26
because the flood basalt erases the evidence
22:28
of what happened before. So there are
22:30
slightly fewer impacts on the mare than there
22:32
are on the non mare because it's younger
22:35
surface. Prior to any
22:38
spacecraft going to the Moon, the assumption
22:39
was the far side of the Moon would look like
22:41
the near side. But when we sent spacecraft
22:43
there, we realized it doesn't. And that was a
22:45
big puzzle for astronomers for a very long
22:47
time in that there were effectively no mare
22:50
on the far side. There's little bits, but not
22:51
very much. Now, the idea here is
22:54
that, uh, when the Moon formed, it formed as
22:57
a result of a giant impact on the Earth. The
23:00
Moon accreted, uh, and initially was fully
23:02
molten and then it cooled from the outside
23:03
in. So at a certain time in the Moon's youth,
23:07
the surface was very thin above a magma
23:09
ocean, above a molten ocean. And at that
23:12
time, small impacts wouldn't penetrate that
23:14
crust. And you get normal craters, you get
23:17
mountain ranges and all the rest of it
23:18
forming. But when you got a really big impact
23:21
that would break through the crust, create a
23:23
big impact basin that would then be flooded
23:25
with flood basalt, which gave you this
23:27
incredibly flat, smooth floor and
23:29
erased all the evidence of the impacts before
23:32
eventually the Moon cooled enough that any
23:35
molten material was sufficiently deep that
23:37
even the biggest impacts would not cause
23:40
these flood basalt outpourings. Coupled with
23:43
the fact that as the solar system aged,
23:46
it cleaned up very effectively and the big
23:47
impactors were effectively gone. So the big
23:49
impacts were early on. So the idea was that
23:52
the mare are caused by the very biggest
23:55
impacts that will create impact basins
23:57
that are, uh, hundreds or thousands of
23:59
kilometres across, that are broadly
24:02
circular in shape before other things happen,
24:05
and that they're filled with molten material.
24:08
And the areas on the near side that are not
24:10
in the mare are the areas that were not
24:12
induced into one of these flood basalt outpat
24:15
rings. Effectively they escaped being in one
24:16
of the craters from the very biggest impact
24:19
us. We thought that prior
24:22
to going to the far side of the Moon, you
24:23
would have assumed that the far side would be
24:25
the same, but it turns out that it's not.
24:27
That was a real problem because this idea
24:29
that the impacts were big enough to punch
24:31
through and flood to the surface
24:35
should work all across the Moon. So why
24:38
then do you not get the flood basalt
24:39
outpourings on the far side of the Moon?
24:41
There are kind of three explanations that
24:43
have been put forward for this, the first of
24:46
which is, frankly, bunkham. The idea that the
24:49
near side of the Moon faced the Earth, uh,
24:50
and the Earth, uh, shielded it and so
24:52
therefore there'd be more impacts on the far
24:54
side. Well, that just kind of
24:57
runs counterintuitive. You'd say the far side
25:00
experience more hits, it gets more cratering.
25:02
Well, I don't believe that from a minute
25:03
because the Earth is so small from the Moon's
25:05
point of view, it's barely a shield at all.
25:07
But if that were the case, surely you'd
25:09
expect more mare on the far side because you
25:11
get more of these big impacts. So that, to
25:14
me, doesn't work. So we can rule that out.
25:16
The other answers are, uh, kind of tied
25:19
together. But the idea is
25:22
that the Moon had a thicker
25:25
layer above the molten layer on the far side
25:28
of the Moon to the near side. Two
25:31
ways you can make that happen. One idea is
25:33
that the heat from the young Earth, which
25:36
would also have been molten at this time, and
25:37
being bigger will keep its heat longer, so
25:39
will be molten for longer. The Earth will be
25:42
irradiating the Moon. The Moon will be close
25:44
to the Earth, uh, when they formed because
25:45
it's moved away. Since that
25:48
radiative heat would have kept the near side
25:50
of the Moon hot for longer, which the molten
25:53
material on the surface would have stayed for
25:54
longer, but also it would have taken longer
25:57
for the crust to thicken on that side. So
26:00
therefore the crust on the far side of the
26:01
Moon would have formed quicker and, um,
26:03
thicker. The other idea is that, ah, you
26:06
get the same kind of effect from tidal forces
26:08
that the tidal influence of the Earth on the
26:10
Moon is stronger on the near side than the
26:11
far side because the strength of tides falls
26:14
off as distance to the power four. So that's
26:16
a very strong, very rapid effect. Yeah,
26:19
possibly both of those things combine
26:22
give you a crust around the Moon that is
26:24
thinner on the near side than the far side at
26:26
all times as, ah, the Moon cools on the
26:28
interior. Which means that
26:31
the far side of the moon, the molten
26:33
material was deeply enough buried quickly
26:36
enough that no Maori forming impact happened.
26:38
You've got the South Pole Ait Kin basin,
26:40
which is the biggest impact scar on the Moon,
26:43
doesn't really have much flood basalt in it,
26:44
which either means that it is younger and
26:47
therefore, uh, the interior had cooled enough
26:50
that it didn't crack that egg or
26:53
that it was an area where the crust
26:56
was thicker anyway, you know, so the idea is
26:59
that the difference between the near side and
27:00
the far side of the Moon is down to the
27:02
thickness of the crust when the biggest
27:03
impacts were happening. And the idea that
27:06
probably due to a combination of tidal
27:08
effects and radiative heating from the
27:10
incredibly luminous molten Earth, the
27:13
near side of the Moon stayed a thicker shell
27:15
and therefore was more effectively punctured.
27:18
And so the near side got the mare and the far
27:20
side looks more like your typical rocky
27:22
objects like Mercury, like a lot of the rocky
27:25
moons and stuff like that in the outer solar
27:26
system. That's the thinking there. But it is
27:29
really strikingly obvious when you see photos
27:32
of the far side side of the Moon and you're
27:34
not told it's the far side of the Moon, you
27:35
assume you're looking at an object that is
27:37
not the Moon because it's not different to
27:39
our experience of the Moon.
27:41
Andrew Dunkley: Indeed.
27:41
So, so part two of your question, you
27:44
basically covered because of the deep
27:46
Jonti Horner: impact a little bit. Part two is a bit
27:49
more complex.
27:50
Andrew Dunkley: This is about, this is the one about shallow
27:51
craters.
27:53
Jonti Horner: And craters are uh, shallow, not just on the
27:56
Moon, but everywhere. There is a boundary
27:58
between what, what researchers describe as a
28:00
simple creator and a complex crater. The
28:02
size at which you get that boundary varies
28:05
dependent on the strength of material that's
28:07
impacted and also the mass of the planet and
28:10
therefore the strength of gravity. So I
28:12
believe on the Earth it's about 8 or 9
28:14
kilometers. For the moon it's about 18
28:16
kilometers smaller than that. You get
28:19
a simple crater that forms, which looks very
28:21
similar to what you'd get if you almost just
28:23
threw a rock really hard into sand or
28:26
something. You get the typical bowl shaped
28:28
crater like Meteor Crater in Arizona. Really
28:30
nice example. Yeah, ah, a
28:33
size above about like say 8 or 9 kilometers
28:36
on Earth or above about 20 kilometers on the
28:38
moon. You get to the domain where you get a
28:40
complex crater. And complex craters are
28:42
characterized by having quite often central
28:45
impact peaks, but also having
28:48
these much shallower depths compared to
28:51
their width, you know, and it's particularly
28:54
true for the mare, where they're flood
28:55
basalts, where you have a very shallow crater
28:58
for the width of the crater. But it's true
29:00
even if you look at 20 kilometer craters on
29:02
the moon and there's a beautiful photo,
29:04
incidentally, if you look on some of the NASA
29:06
images, there's a beautiful photo of the
29:08
crater Aristarchus taken by the Lunar
29:10
Reconnaissance Orbiter, and that shows this
29:12
kind of terracing around the walls, the
29:15
central peaks. And so there's a few things
29:16
going on here that contribute to why what
29:19
we describe as being,
29:22
um, what I say, complex craters are
29:25
actually,
29:26
um, shallower compared to
29:29
their width than the simple ones. And, uh,
29:31
there's a few things that have been suggested
29:33
to this. So complex craters have depths that
29:36
can be a 15th, a 25th or even less
29:38
of the crater width, which looks very, very
29:41
shallow. Now, there's a few things proposed
29:44
in for this. Firstly, when you form a bigger
29:47
crater, uh, the walls can slump in, so
29:49
material slides and gradually you get this
29:53
material from the edges sliding into the
29:54
middle. And if you look at that photo of
29:56
Arisarcus, it looks very much like that's
29:58
happened. You see evidence of landslides that
30:01
have filled in the crater and made it
30:02
shallower. The other thing is craters
30:05
that big are, ah, large enough to render the
30:08
material, where the impact happens, molten.
30:12
And in other words, the material can flow
30:14
like a liquid rather than behaving like a
30:16
solid material of your desk.
30:17
Andrew Dunkley: Well, you can still see a rebound point in
30:20
the middle of the crater too.
30:21
Jonti Horner: That's it. So the stuff at the middle, the
30:23
central peaks, are thought to be rebound of
30:25
this fluid material springing back before it
30:28
freezes solid again effectively. And, um,
30:30
then the flat base of these craters that
30:32
makes them shallower is because you make a
30:34
pool of liquid that spreads out and settles.
30:38
And so therefore you get these shower things,
30:40
whereas with smaller craters you don't get to
30:43
that point. So you get much more material
30:45
behaving more as a solid than a liquid,
30:47
effectively. So the thinking is for these
30:49
complex craters, and like I said for the
30:51
moon, I think the size scale is what, 18 to
30:53
20 kilometers, something like that. It's a
30:56
point at which you transition from simply
30:58
behaving as a solid to the surface behaving
31:00
in more of a liquid fashion, you get
31:03
complex craters having central peaks, they
31:05
have terraces, they've got flat floors. The
31:08
more massive the object, the smaller the
31:10
boundary is, because gravity has a
31:13
role in this.
31:15
Andrew Dunkley: Yeah.
31:15
Jonti Horner: Um, and then you get the basins which are
31:18
even bigger and there where you get the
31:19
flooding from basalts, which makes them even
31:22
shallower compared to their width. So there
31:24
is some beautiful complexity of this
31:28
where it's all to do with the physical
31:30
behavior of material and how that
31:33
changes as an impact gets larger and larger
31:35
and therefore more and more damaging and
31:37
energetic. Now, NASA have a Mars Ed
31:40
website, um, where they actually
31:43
explicitly say, if you Google for this, it's
31:45
marsed Asu. Edu and then
31:48
a really long string afterwards. It's a Mars
31:50
education thing at Arizona State University.
31:52
And I'll just quote here.
31:55
Compared to simple craters, complex craters
31:58
also generate a lot more impact. Melted
32:01
rock, this typically flows and pools like
32:03
lava to form a sheet that covers a shattered
32:05
rock known as breccia. On the crater floor,
32:08
the crater's inner walls may slump downwards,
32:11
rotating backwards in blocks, which can widen
32:13
the crater's rim and line the inner walls
32:15
with terraces. But as a result, complex
32:18
craters look shallow. Uh, they have rim
32:19
diameters about 30 times greater than their
32:21
depths. By comparison, simple craters are
32:24
about five times wider than they are deep.
32:26
Um, earlier on it said the more energy and
32:28
impact delivers, the bigger the cavity on the
32:30
ground. But immediately after the blast, the
32:32
center of the cavity begins to rise as rocks
32:34
rebound from the shock. That's what gives you
32:36
the mountains. This uplift gives you a
32:38
central peak or cluster of peaks. So that's a
32:40
really nice way of condensing my lengthy,
32:42
waffly answer into something a bit more
32:45
simple and straightforward.
32:47
Andrew Dunkley: Fair enough. Okay, uh, now, Andrea, you can
32:49
believe all of that, or you can go with my
32:51
theory that the lunar city council were on
32:54
strike and they didn't finish filling the
32:56
potholes.
32:58
I go with option two. Um,
33:01
Andrea, thanks for your question. Great to
33:03
hear from you. Thanks for explaining
33:04
Wanneroo.
33:05
This is Space Nuts, a Q and A edition with
33:07
Professor Jonti Horner and Andrew Dunkley.
33:13
Okay, Houston, we've had a problem here. This
33:15
is Houston. Say again, please. Houston, we've
33:17
had a problem. Is that a main B plus
33:19
undervolt? Roger, main B undervolt. Okay,
33:21
standby 13. We're looking at it.
33:23
Jonti Horner: These butts.
33:25
Andrew Dunkley: Our, uh, last question comes, uh, in two
33:27
parts, and it comes, uh, from Eli. Uh,
33:30
hello from Coachella Valley in
33:32
California. Was Coachella in the news
33:34
recently for some big soiree that happened
33:37
there? Yeah, uh, big event. Uh, anyway, he
33:40
says the grasshoppers have decided to invade.
33:43
Believe it or not, Eli, exactly the same
33:45
thing is happening where I am. We have
33:48
locust, uh, plagues. Once in a blue moon. And
33:50
we Had a little one recently. Wasn't, uh, too
33:53
significant. But I have discovered with
33:55
locusts, or grasshoppers or whatever you call
33:57
them wherever you are, that if you drive
34:00
over 50 kilometers an hour, they splatter.
34:04
Uh, if you drive under 50 kilometers an hour,
34:06
they bounce off. Important safety tip.
34:09
Especially because when they splatter, they
34:12
stink and it's very hard to get off when
34:14
they're dry.
34:15
Jonti Horner: I'd love to have a swapsy where we've had an
34:17
incredibly dry last few months here. It's
34:19
been our wet season. We've had 40 mil of rain
34:21
in four months. Months, wow. Which is hooray,
34:24
you know, that's really what you want in your
34:25
wet season when the dry season is about to
34:27
start. But what that means is that we're
34:29
probably going to see yet another mouse
34:31
plague. And mouse plagues are sad because in
34:33
the times that are good, mice reproduce like
34:34
crazy. But then you get the boom busting and
34:37
so you start getting lots of them coming to
34:39
your house. And I'm soft hearted. I don't
34:41
want to hurt them or do anything, but at the
34:43
same time, I don't want them pooing on
34:44
everything in my kitchen. Yeah. So we're
34:46
getting mouth plague times. Uh, I, I, I
34:49
suspect that locust plagues are horrible, but
34:51
mouse plague is an entirely different horror.
34:53
Andrew Dunkley: And mouse plagues are worse.
34:54
Jonti Horner: Yeah.
34:55
Andrew Dunkley: Because the mice try to find somewhere to
34:57
hide inside. Locusts only get in if they've
35:00
got an open avenue, otherwise they just stay
35:03
outside. And you, you know. And they also are
35:06
very disturbing when you're trying to putt on
35:08
a golf cart.
35:09
Jonti Horner: Oh, absolutely.
35:10
Andrew Dunkley: It's getting away.
35:11
Jonti Horner: They must see.
35:12
Andrew Dunkley: Sorry.
35:13
Jonti Horner: You walk into the kitchen at night or you
35:15
walk somewhere and you just see something
35:16
move in the periphery, and that's always a
35:18
little disturbing. Yeah, well, uh, we, yeah,
35:21
we talk,
35:21
Andrew Dunkley: we're talking mouse plague here as well. So
35:24
we could have, we could have both. But our
35:26
last big locust plague, it was so
35:29
big, the birds just got fed up with eating
35:31
them, so they gave up as well.
35:33
Jonti Horner: It's very weird.
35:35
Andrew Dunkley: Um, Eli, what are you asking us? Uh, since
35:37
you mentioned a paucity of questions,
35:40
I hope you don't mind, um, a
35:43
twofer.
35:43
Okay, well, we've got two questions then. Uh,
35:46
when the solar system formed, I always
35:48
imagined the inner rockier planets as having
35:51
more heavier elements due to their greater
35:53
mass and gravity, and with lighter elements
35:56
collecting more in the outer gas giants.
35:59
But then I realized, isn't the sun mostly
36:01
hydrogen the lightest element? Now I'm
36:04
Confused. That's his first question.
36:06
Jonti Horner: Yeah.
36:08
Andrew Dunkley: This is your bullpen, isn't it? This is your
36:10
area of expertise.
36:11
Jonti Horner: This is much more my comfort zone. So this is
36:14
really.
36:15
Andrew Dunkley: I hope you realize I did try to find
36:16
questions that worked for you.
36:18
Jonti Horner: No, no, that's all good. And it means you can
36:20
leave the cosmology ones for when Fred gets
36:22
back as well, which is great.
36:24
This is a lovely question. And it speaks to
36:26
how our understanding of how planets form has
36:29
changed over time. And we've now got quite a
36:31
high level of complexity in the ideas we have
36:34
behind planet formation. But a really
36:36
fundamental part of it is that everything
36:39
in the solar system to first order
36:42
has the same composition as the sun, because
36:45
we're all formed from the same material,
36:46
formed from an enormous cloud of gas and dust
36:48
called a giant molecular cloud that collapsed
36:50
under its own gravity. You got effectively
36:53
the protostarsome forming in the middle with
36:55
a disk of material around it we call a
36:56
protoplanetary disk. And in that disk you
36:59
have solid material and gaseous
37:02
material going around the sun, orbiting the
37:04
sun, collapsing to a disk because of the
37:06
conservation of angular momentum. So kind of
37:08
where the Earth is, material was whizzing
37:10
around at about 30 kilometers a second. But
37:13
individual dust grains that were next to each
37:15
other were both moving at about the same
37:17
speed. So very little difference in speed
37:20
between the particles, even though they're
37:22
going really quickly. Now, the further you
37:24
are from the sun, the colder the temperature
37:26
is in that disk. And every single material
37:29
you can think of has a
37:32
sublimation temperature. Below that
37:34
temperature it will be solid, and above that
37:37
temperature it will be gas. Reason I'm not
37:39
talking about liquid is in order to have
37:41
liquid you need pressure. And in this case
37:43
you don't have any or you don't have enough
37:45
to either have solid or gas.
37:47
Andrew Dunkley: Yep.
37:47
Jonti Horner: If you are gas, then you
37:50
don't form planets initially. If you're
37:52
solid, you can do. So what happens all
37:55
through this disk? For a variety of different
37:57
bits of physics going on, you get whatever
38:00
solid material you have at that distance
38:02
colliding, sticking together, forming
38:05
bigger bits. And so you get from millimeter
38:07
to meter to kilometer to planet sized
38:10
bits of debris. As you get bigger,
38:13
gravity can start taking on a role and start
38:16
pulling in a bit of extra stuff so you can
38:17
feed quicker. Plus if you're bigger, you've
38:19
got a bigger cross section, so you hit more
38:21
things to devour them. So you get this
38:23
process where you get lots of small things,
38:25
making a few bigger things, and the big ones
38:26
tend to dominate um, so a thing called
38:29
oligarchic growth is the idea. And you form
38:32
planetesimals, and then oligarchs, which are,
38:34
uh, protoplanets, and a few of them collide
38:37
all the rest of it. If you are far
38:40
enough from the sun, you're beyond what's
38:42
known as the water ice line. Now, that's the
38:45
point at which the temperature is below the
38:47
sublimation point of water. So instead of
38:49
water being a gas or vapor, it's a solid.
38:52
Now, we always imagine water being quite
38:54
scarce. And I just said we've had 40
38:56
millimeters of rain in the last four months.
38:58
Uh, water is very scarce here. But in terms
39:00
of compounds in the universe, water is one of
39:03
the most abundant things there is because
39:05
it's a combination of hydrogen, which is the
39:07
most common atom with 74, 75% of all
39:09
atoms, and oxygen, which is the second most
39:12
common atom with about 1% of all atoms. Put
39:15
hydrogen, oxygen together, and you get water.
39:17
So in the protoplanetary disk around the sun,
39:20
water was probably about the most
39:23
common species other than molecular hydrogen,
39:25
molecular and helium atoms.
39:27
Lots and lots of water. Now, where the Earth
39:29
formed, it was too hot. So you don't
39:32
have water as a solid, so you form the Earth
39:35
dry. There's no solid water to accrete. You
39:38
might get a little bit of water as a gas that
39:40
is trapped in the solid material, which is
39:43
why people think most of the Earth's water
39:45
was delivered from further out. Because if
39:46
far enough out, you form from primarily water
39:49
with everything else added in. So the inner
39:52
solar system, you don't have that water to
39:55
accrete. So you're limited to the things that
39:57
are solid at, uh, higher temperatures. So
39:59
you're limited to accreting from rock and
40:01
metal. So you get telluric planets, or
40:04
is the archaic way of saying it, or
40:06
terrestrial planets beyond the ice line.
40:09
Water ice dominates the solid material. So
40:12
you've got a lot more to feed from, so you
40:14
grow more quickly, and you can get more
40:16
massive planets more quickly, which is where
40:17
Jupiter and Saturn come in. Now, there's a
40:20
lot of discussion about how they may have
40:21
migrated through the nebula, all the rest of
40:23
it, and the subtleties of the formation in
40:26
other planetary systems. We have planets like
40:28
Jupiter orbiting their stars every four or
40:30
five hours even, but we don't think they
40:32
formed there. We think they migrated in. So
40:34
you form beyond the ice line
40:37
more quickly because you've got more food,
40:39
and you can grow to masses like 10 or 12.
40:42
Earth matters while there is still an
40:44
Abundance of gas around. That gas doesn't
40:47
hang around long because once the sun fully
40:49
turns on after a few million years, it blows
40:51
the dust and the gas away and you're left
40:53
with what, whatever's left over. But if you
40:56
form to be 10 or 12 earth masses
40:59
before the gas is blown away, suddenly your
41:01
gravitational ah, pull is strong enough to
41:04
hold on to hydrogen and helium. If you're
41:06
less massive than that, then the escape
41:09
velocity of a hydrogen or helium atom will be
41:11
higher. Sorry, the escape velocity of
41:14
your object with that mass will be lower than
41:17
the speed at which hydrogen and helium atoms
41:19
move at that temperature. So you can't hold
41:21
on to them, they just escape because of their
41:23
motion, because of the temperature they're
41:25
at. When you get to 10 or 12 earth masses,
41:27
the escape velocity from your core
41:30
is higher than the speed at which hydrogen
41:32
and helium is moving. So you can start to
41:34
capture that. And like I said, 75% of all
41:36
atoms are hydrogen, 24% of all atoms are
41:39
helium. 99% of the mass of the
41:41
protoplanetary disk, or 98%
41:44
maybe is unaccessible till you get to that
41:46
mass and suddenly you've got this whole new
41:48
food source. So you quickly devour all the
41:51
gas around you until you open a gap in the
41:53
disk. And that's how you get the gas giant
41:55
planet shoot from Saturn with Uranus and
41:57
Neptune they formed further out, they had a
41:59
lot of abundant volatile material, but they
42:02
didn't really get massive enough to devour
42:04
the gas before the gas was blown away.
42:07
So that's why you get the ice giants. Uh, and
42:10
that is partially because they're further
42:11
away, they form slower. There are some
42:13
arguments that Uranus and Neptune may have
42:15
formed between Jupiter and Saturn and been
42:17
scattered out. But on a broad
42:19
brushstrokes sense, in our solar
42:22
system we don't think a huge amount of
42:24
migration happened, which is probably down to
42:26
the mass of the protoplanetary disk, not
42:28
compared to the hot Jupiter systems we
42:31
find elsewhere. So the planets we see today
42:34
are within a factor of two or three times
42:37
the same distance they were when they formed.
42:39
Jupiter might have migrated in and back out.
42:41
Uranus and Neptune probably formed
42:43
significantly closer to the sun and migrated
42:45
outwards. But you've got Jupiter and
42:48
outwards forming in the ice dominated area,
42:51
the terrestrial planets forming in the, in
42:53
the area without ice and therefore they're
42:56
dominated by the rock and the metal. So
42:57
you've like got this filter. So if you look
43:00
at the fraction of iron compared to carbon
43:03
in the Earth, or pick any Two things that
43:06
would have been solid silicon versus iron,
43:09
phosphorus for whatever, you know, things
43:11
that were solid. The abundances of those
43:13
things in all of the planets relative
43:16
to one another will be effectively the same
43:18
as the abundance in the Sun. But the
43:20
terrestrial planets weren't able to capture
43:22
the things that would have been gas at their
43:24
distances. Other than that, what was
43:26
delivered later on and weren't able to hold
43:29
on to hydrogen and helium. So you get that
43:30
chemical differentiation as a
43:33
result of the location of the solar system,
43:35
There's a bit of added complexity because
43:37
chemistry happens, and you'll get isotopic
43:39
variations and stuff. But in broad brush
43:41
strokes, the reason the terrestrial planets
43:43
are dominated by rocky and metallic material
43:46
is they never got massive enough to capture
43:47
the gas, and they formed close in where ice
43:50
wasn't around. That's effectively how it
43:53
happens. So what this question from Eli is
43:55
doing is actually effectively describing
43:59
the logic process that went into how
44:02
we first began to understand planet
44:04
formation. Because I said before, I think,
44:07
um, on a previous episode, astronomy is not
44:09
an experimental science in the way that every
44:11
other science is. You know, biology,
44:12
chemistry, physics. You want to figure out
44:15
how something works, you can do experiments.
44:17
Astronomy is an observational science.
44:18
Everything's so big and so far away, we can't
44:20
put it in a lab and smash it it. We instead
44:23
play detective. We look out at the universe
44:24
and we gather clues and we ask questions,
44:26
exactly like the question Eli has asked here,
44:28
which in its fundamental sense is, why do we
44:30
have rocky planets close in and gaseous ones
44:33
further out? Why are there different
44:34
compositions when we should be the same
44:36
composition of the sun? We then come up with
44:38
explanations for that that are our theories.
44:41
And to be a good theory, you can't just say,
44:43
I explain everything we see. You've got to
44:45
make predictions. As we find more things, we
44:48
will observe this. And that's how we test
44:50
that theory. And we test it by this interplay
44:52
between observation on the one theory on the
44:55
other. And what Eli's asked here is
44:57
essentially the questions that people are
44:59
asking that led to our current understanding
45:01
of planet formation.
45:03
Andrew Dunkley: And yet, uh, we
45:06
see other solar systems with exoplanets that
45:09
defy what we think is normal. Uh, you
45:12
have gas giants close to the parent star and
45:15
rocky planets further out.
45:17
Jonti Horner: Um, and that's how we develop.
45:19
Andrew Dunkley: Is that because they've just drifted that
45:21
way.
45:22
Jonti Horner: It's complicated. So our
45:25
ideas planet formation happened
45:28
have undergone quite a few major revolutions
45:30
as we found planets around other stars. So
45:33
in the early 1990s,
45:36
had a couple of talks at my local astronomy
45:38
society in the UK from um, Professor Wolfson
45:41
of York University. And Professor Wolfson
45:44
was an advocate of an entirely different
45:45
formation scenario for the solar system. I
45:48
think he was someone who argued that, that
45:50
the solar system formed through an encounter
45:52
between the sun and a young proto star where
45:54
materials pulled out of the sun into a
45:56
massive tongue and that tongue condensed into
45:58
planets.
46:01
Back then, um,
46:04
that idea was going out of fashion because
46:06
we'd found a few debris disks around stars
46:08
like Vega formal heartbeat pictoris but it
46:10
was still considered possible. Yeah, such an
46:12
event would be incredibly vanishingly
46:15
rare because stars getting that close
46:17
together within one another's hills sphere
46:20
is incredibly unusual. Very, very rare.
46:24
And so what that would predict is
46:27
if that theory were correct, we would be
46:29
almost unique. There will be vanishingly
46:32
few planets round of the stars because the
46:34
scenario you need to form planets would only
46:36
happen very rarely. On the other hand, there
46:39
was the idea which dated back to uh,
46:41
initially the 1700s and beyond the
46:44
Laplacian model, the circum solar disk
46:47
model, which has evolved into what we have
46:48
now, which suggested that as part of star
46:51
formation you get a disk of material around a
46:53
star and planets form from that disk. Disks
46:55
are a natural byproduct of the formation of
46:57
stars. Therefore planetary systems should be
46:59
common. Both scenarios, with a bit of
47:02
fudging and fiddling, could perfectly explain
47:04
how the solar system looked and have been
47:05
finessed to reproduce the solar system. But
47:08
the test was always going to be which of
47:10
these series is correct will depend on how
47:12
many planets we found on other stars. If
47:15
planets are rare, then maybe the solar system
47:17
is the result of a tongue being pulled out on
47:19
the sun. If planetary systems are common,
47:22
that cannot be the case. So that was a test
47:24
that was done there. So when we found the
47:27
first planetary systems around other stars
47:28
and we found that planets are ubiquitous,
47:30
that was kind of the death knell for the
47:32
Wolfson type model of a tongue being sucked
47:35
out of the sun and forming planets. But
47:38
it kind of confirmed the Laplace model. But
47:40
it also threw a spanner into the work in that
47:42
the variation of planet formation of that
47:45
disk model suggested that you would always
47:47
form planetary systems with rocky planets in
47:49
the middle and gas planets on the outside.
47:50
Because it had been developed to explain the
47:52
solar system. When you found planets
47:55
that were hot Jupiters, they don't fit. Their
47:57
planets are massive Jupiter close to their
47:59
star, which brought in the concept of inward
48:02
migration. Now it's an interesting time
48:04
because in the same few years
48:06
people had started to realize that in the
48:08
Solar system, there was clear evidence of
48:10
planetary migration for the giant planets,
48:12
primarily that Neptune had migrated outwards,
48:15
carrying Pluto with it from the Plutinos.
48:18
So you've got these seminal papers by Renu
48:20
Malhotra talking about the outward migration
48:23
of Neptune being evidenced in Pluto and the
48:26
Plutinos, predating the discovery of the
48:28
first exoplanet. And one of my gripes through
48:30
my career has been that the exoplanet
48:32
community primarily came from binary star
48:35
astronomers, not from solar system
48:37
astronomers. So reinvented migration to some
48:39
degree and assumed that we had no evidence
48:41
for it in the solar system. And in parallel,
48:43
the solar system community was working on
48:45
migration separately. But the
48:48
discoveries of planet stars over the last
48:51
30 years and more, which is a great
48:54
scientific revolution we've lived through.
48:55
You know, you and I grew up in a world where
48:57
the only planetary system we knew was our
48:58
own. And kids today grew up in a world where
49:00
we know planets are ubiquitous. That's a
49:02
cataclysmic shift to have lived through.
49:05
Yeah, living through that has proven
49:07
an incredibly fertile testing ground for our
49:09
theories of planet formation. Turns out that
49:12
that Laplace theory, the disk theory, was
49:15
a good way of the way there. So it hasn't
49:18
been totally discarded, but it's been refined
49:20
and we've learned more about it, and that
49:21
continues to the current day. The refinements
49:23
are leading to all sorts of complexities,
49:25
like invoking streaming instabilities to
49:28
concentrate pebbles at certain distances and
49:30
all sorts of subtleties to try and address
49:33
some of the pitfalls of how on Earth do you
49:35
get from millimeter size to meter sized
49:37
objects when collisions should become
49:38
disruptive? All sorts of things like
49:41
this. And it's through those observations
49:44
that we get to improve and refine our models.
49:47
We're not going to end up throwing out the
49:49
disk model now because we can see the disks
49:51
that form planets around other stars. Because
49:52
our telescopes have got that good. Yep. Um,
49:55
and one of the predictions would have been
49:57
prior to them getting that good, if the disk
49:59
model is right. When we look at places like
50:00
the Orion Nebula with a sufficiently good
50:02
telescope, we should see protoplanetary
50:04
disks, propolids. Then the telescope's got
50:07
good enough and we can see them now. We've
50:09
even got to the point now where we can
50:11
actually even observe fine structure within
50:14
them to see the gaps that giant planets open
50:16
up, to see the spiral waves that are
50:18
sometimes induced by a massive planet being
50:20
born. So we're now not only
50:22
inferring planet formation from the plethora
50:25
of planets that we're discovering around
50:27
other stars and from the fine details of what
50:29
we know about the solar system. But we're
50:30
actually also getting observations of the
50:32
disks in which it's happening that are
50:34
providing extra information to improve those
50:36
models.
50:37
Andrew Dunkley: It's fascinating. So you can simply
50:40
say there's no one size fits all
50:43
way of this happening.
50:44
Jonti Horner: Well, it's circumstantial. It's a broad
50:46
thing, rather a narrow thing. So in a broad
50:49
sense, planets form in a disc around a star.
50:52
Natural product of star formation. There may
50:54
be occasional ways of the planet formation
50:57
mechanisms happen like the planets around.
51:00
Um, neutron stars are thought
51:02
to probably be second generation planets.
51:05
Probably material formed from a disk that
51:06
formed around the neutron star after the
51:08
supernova and formed a new generation of
51:10
planets. You might eventually one
51:13
day possibly find planets formed from
51:16
material pulled off a star. The very most
51:18
massive planets, some of them will probably
51:20
have been formed more like binary stars
51:23
than actual planets which we talked in the
51:25
past about. When is a brown dwarf not a brown
51:28
dwarf? Yes, but the broad brushstroke thing
51:30
is fairly well established. But n every
51:33
single planetary system is unique. Everyone
51:36
has unique circumstances. Some disks around
51:38
stars are more massive than others. Not every
51:40
cell will have an identical disc. Some disks
51:43
get truncated because passing starship's
51:45
material away. Some disks get ablated away
51:48
because it's a massive star nearby whose
51:50
radiation pushes material away. You then even
51:52
get impacts on the chemistry. So there's
51:54
really fascinating studies looking at the
51:56
solar system that suggests there was a nearby
51:59
supernova when the planets were forming that
52:02
injected highly radioactive short lived
52:04
aluminium 23 I think it is that gave an
52:07
extra spike to the melting of planetesimals
52:10
that led to some of the subtleties of how the
52:13
solar system looks. There are indications
52:15
even I think that the amount of gold in the
52:17
solar system is unusually high compared to
52:19
the standard metallicity. The amounts of
52:21
everything else with indication of pollution
52:24
from two neutron stars colliding within
52:26
10,000 light years of where the solar system
52:29
would form about 100 million years before we
52:31
formed. So even that level of injection
52:34
of material is unique from one system to the
52:37
next. And that's why every planetary system,
52:39
like every person is unique.
52:41
Andrew Dunkley: Fascinating.
52:42
Jonti Horner: Fascinating.
52:43
Andrew Dunkley: Aren't you glad you asked Eli and Eli's
52:45
second question?
52:46
I recently read that some star systems
52:49
are zipping through their galaxy orbits at
52:51
incredible speeds of 1200. I'm
52:54
assuming that is kilometers per second. Uh,
52:57
that's 0.4% the speed of
53:00
light. That got me wondering how fast could
53:03
our solar system get going before
53:05
we started noticing Things going wrong, you
53:08
know, the windows rattling and such.
53:11
Um, yeah, I, I
53:14
think we've had questions similar to this. I
53:17
think we did one recently where we talked
53:19
about how fast the Earth would spin before
53:21
things started to go horribly wrong. Um,
53:24
this is a question of similar ilk. I
53:27
hadn't heard about those sorts of speeds
53:29
being detected by, um.
53:31
Jonti Horner: Uh, there'd be stars very near the
53:34
supermassive black holes at sense of
53:35
galaxies. And that kind of speed surprised
53:37
me. Now, my immediate take on this is that,
53:40
uh, we wouldn't notice
53:43
effectively. So the reason that I'm saying
53:46
that and I, I stand to be proved wrong when
53:48
you get up to relativistic speeds, because my
53:50
knowledge of relativity is not sufficiently
53:52
good to be absolutely certain on this. If you
53:55
are moving at a substantial fraction
53:58
of the speed of light, I don't think we'd
54:00
notice anything wrong in terms of the Earth
54:02
moving around the sun, because we'd still be
54:03
going around the sun at 30 kilometers per
54:04
second while we're both moving around the
54:07
galaxy at relativistic speed and
54:08
accelerating. What we might notice then is
54:11
time dilation in the fact that the external
54:14
universe appears to be moving quicker than it
54:17
should do. So we might see the effect of the
54:19
fact that our time is slowed down if we
54:22
were going around just the same. As, you
54:23
know, you see this stuff about people
54:24
orbiting a black hole at high speed or
54:26
whatever, or falling into a black hole. But
54:28
in terms of us noticing, in terms
54:31
of physical phenomena on Earth that
54:34
we're traveling at a certain speed around the
54:36
galaxy, I don't see a way that that would
54:39
work. And the reason for that is that there's
54:41
no resistive medium. We think about this
54:43
thing happening because when you're driving
54:45
in your car, the quick you get, the more
54:46
obvious your speed is because of the rattling
54:48
and the wind, really resistance and the
54:49
noise. But that's all down to your
54:51
interaction with something that isn't moving
54:53
at the same speed you are. If you're in the
54:56
International Space Station and you're
54:57
orbiting the, uh, Earth at several kilometers
54:59
a second, you don't feel the space station
55:01
rattling because it's going really quick
55:03
because it's moving through the vacuum of
55:05
space, so it's not interacting with anything.
55:07
If you're coming back into the atmosphere,
55:09
you rattle and rumble and all the rest of it.
55:10
We saw this with Artemis 2, because you're
55:13
slowing down, you're experiencing
55:14
acceleration, you're experiencing buffeting.
55:17
So to me, if we are moving as a
55:20
planetary system around the middle of the
55:22
galaxy at very high speed. Our planets would
55:25
still be orbiting the sun in the same way and
55:27
we wouldn't notice any difference. What would
55:30
happen though, is we'd be moving through a m.
55:31
Much, much denser stellar neighborhood. The
55:34
sky would be immeasurably beautiful, but
55:37
challenge. But also close encounters between
55:39
stars will be very common. And so
55:42
it may well be that the stars will be so
55:43
densely packed that eventually we'd have a
55:45
stellar approach that will be so close to
55:47
solar system will be disrupted. And we'd
55:49
certainly notice that also
55:52
if we were injected to there from where we
55:54
are now, There will be a period of adjustment
55:57
where the Oort cloud will be heavily
55:59
destabilized and we'd have catastrophic
56:01
levels of impacts from the comets being
56:02
scattered. But eventually they'd all be gone,
56:04
so it wouldn't be a problem. So we'd notice
56:07
it from the point
56:08
Andrew Dunkley: of view until led to the dinosaurs.
56:10
Jonti Horner: Oh, absolutely. Um, Long may they rest.
56:13
But it's one of those things where
56:15
if we were there and we were transported
56:17
there from now, what we'd notice is that the
56:20
sky looked very different. If we were moving
56:22
at that kind of speed in that denser
56:25
stellar neighborhood, the proper motion of
56:26
stars would be apparent to the naked eye over
56:29
human timescales, which it's not for us.
56:31
Barnard Star, which is the fastest moving
56:33
star across the night sky, will cross the
56:36
diameter of the full Moon in a century. Very
56:38
roughly, that means if Barnard's star was
56:41
bright enough to see with the naked eye, we'd
56:43
have known about proper motion earlier
56:45
because it would be obvious, but it wouldn't
56:48
be the kind of thing you'd notice from one
56:49
year to the next. Whereas if we were in the
56:50
middle of the galaxy Going around the
56:52
supermassive black hole, at that ridiculous
56:54
speed, stars will be closer together, which
56:57
magnifies the effect of motion
57:00
from our perspective. Also, they'd be moving
57:03
quicker, which means that the motion is
57:04
quicker from our perspective. And you
57:06
probably have proper motion being visible on
57:09
human timescales to the point that the
57:10
constellations would move. Rather than being
57:13
fixed patterns that you'd notice,
57:16
you wouldn't feel the acceleration, you
57:18
wouldn't notice anything's wrong. But we
57:21
probably wouldn't be there if the sun had
57:22
been there for a long time. Because it's a
57:24
very intermissal environment for life.
57:26
Because there's a lot of stars close
57:28
together, a lot of massive stars, a lot of
57:30
supernovae. It's probably a bit of a dead
57:32
zone.
57:33
Andrew Dunkley: Aha.
57:33
Jonti Horner: Uh-huh.
57:34
Andrew Dunkley: Okay. All right. Um, thanks
57:37
for your questions, Eli. And, uh, yeah, I
57:40
love that second one. I love what if
57:42
questions. Uh, so, uh, yeah, we've been
57:44
getting a few of those lately. It's. They're
57:47
just such great fun. Thanks, uh, to Nick and
57:49
Andrea as well, for contributing. And if you
57:51
would like to send us a question, please do
57:53
on our website, space nutspodcast.com
57:56
spacenut Click on the AMA
57:59
button at the top. Ask me anything is what
58:01
that stands for. And you can send text and
58:03
audio questions. Don't forget to tell us who
58:05
you are and where you're from. And while
58:07
you're there, have a look around. Check out
58:08
the Space Nuts shop. Maybe you'd like to
58:10
become a supporter. Sign up for the Astronomy
58:13
Daily newsletter, all sorts of things to see
58:15
and do on our website. And please leave
58:17
reviews wherever you listen to
58:20
Space Nuts. We appreciate that as well. Well,
58:22
and we appreciate you, Jonti. Thanks so much
58:25
for, uh, your input today. Fantastic.
58:27
Jonti Horner: Oh, it's always a pleasure. And yeah,
58:28
fabulous questions. Really enjoy them.
58:31
Andrew Dunkley: Me too. And we'll catch up, uh, with you
58:33
very, very soon.
58:34
Jonti Horner: Yeah, I look forward to it. Thank you.
58:36
Andrew Dunkley: Professor Jonti Horner from, uh, the
58:38
University of Southern Queensland, where he
58:40
is a professor of astrophysics. And thanks to
58:43
Huw in the studio, who couldn't be with us
58:45
today because time moves slower for Huw. So,
58:47
uh, he'll be joining us in a couple of
58:50
thousand years. And from me, Andrew Dunkley.
58:52
Thanks for your company. We'll see you on the
58:54
next episode of Space Nuts.
58:55
Jonti Horner: Bye.
58:56
Andrew Dunkley: Bye. Uh, you'll be listening to the
58:58
Space
58:59
Jonti Horner: Nuts podcast,
59:01
available at Apple Podcasts, Spotify,
59:04
iHeartRadio or your favorite podcast
59:07
player. You can also stream on
59:09
demand@bytes.com.
59:11
Andrew Dunkley: this has been another quality podcast
59:13
production from bytes.com.