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