# Supercharged Neutrinos, Exploding Black Holes & Lunar Mysteries Uncovered | Q&A

An episode of [Space Nuts: Astronomy Insights & Cosmic Discoveries](https://invisiblenetwork.ai/podcast/1080090608), 18 May 2026, 59 min.

Audio: https://dts.podtrac.com/redirect.mp3/pscrb.fm/rss/p/op3.dev/e/api.spreaker.com/download/episode/72040511/supercharged_neutrinos_exploding_black_holes_lunar_mysteries_uncovered_q_a.mp3

Exploding Black Holes, Lunar Mysteries, and Cosmic Questions In this enlightening Q&A edition of Space Nuts, hosts Andrew Dunkley and Professor Jonti Horner tackle an array of fascinating questions from listeners. From the enigmatic nature of supercharged neutrinos linked to black holes to the mysteries of the Moon's surface, this episode is a deep dive into the cosmos. 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. For more Space Nuts, including our continuously updating newsfeed and to listen to all our episodes, visit our website. Follow us on social media at SpaceNutsPod on Facebook, Instagram, and more. We love engaging with our community, so be sure to drop us a message or comment on your favorite platform. If you’d like to help support Space Nuts and join our growing family of insiders for commercial-free episodes and more, visit spacenutspodcast.com/about. Stay curious, keep looking up, and join us next time for more stellar insights and cosmic wonders. Until then, clear skies and happy stargazing. Become a supporter of this podcast: https://www.spreaker.com/podcast/space-nuts-astronomy-insights-cosmic-discoveries--2631155/support.

## Transcript

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

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