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

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Andrew Dunkley: Hi there. Thanks for joining us on a Q and A

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edition of Space Nuts. Andrew Dunkley here,

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your host. Great to have your company. Coming

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up, we've got a few questions. Nick is going

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to ask about supercharged neutrinos.

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Andrea is making a return appearance.

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She's got a couple of questions about the

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dark side of the moon and shallow craters.

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And Eli is asking about elements

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and the speed of objects. And if we've got

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time, we'll chuck another question into the

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mix as well. All coming up on this edition of

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Space Nuts. Seconds. Guidance is

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internal. 10, 9.

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Ignition sequence start.

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Jonti Horner: Space nuts.

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Andrew Dunkley: 5, 4, 3, 2. 1. 2, 3, 4,

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5, 5, 4, 3, 2, 1. Space

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nuts. Astronauts report it feels good.

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And with Fred away, Jonti can play.

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It's professor, uh, Jonti

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Horner, professor of Astrophysics at the

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University of Southern Queensland. Jonti,

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hello again. Good afternoon.

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Jonti Horner: How are you going to.

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Andrew Dunkley: I am well. Great to see you. I think we

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should just go straight into it and, uh,

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hit you with our first question.

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It's a topic I'm not overly familiar

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with, but, uh, this one comes from Nick.

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Uh, I just read that a supercharged neutrino

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was detected by the Kilometer Cube

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Neutrino Telescope, and a theory was

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put forward that it came from an exploding

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black hole. Please explain how a black hole

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can explode. Love the show, Nick. Thank you,

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Nick, we love that you love the show.

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Thank you for sending in a question. Um,

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exploding black holes. Um,

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I seem to remember Fred might have written a

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book about something like that once. Um, but

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anyway, um, do they explode or do they

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merge or do they collapse? They eventually

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disappear.

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Jonti Horner: I know that now. You know straight up,

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I'm not a cosmologist or a cosmetologist,

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which I always used to joke about

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cosmologists being cosmetologists. And it

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turns out a cosmetologist is a real thing. So

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never mind. Um, that's, uh,

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further from my area of expertise. So any

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answer I give, take with a larger grain of

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salt, you know, as is always the way you

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know, you. The further you go from your

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expertise, the more out of date your

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knowledge is. My knowledge on

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exploding or rather evaporating black

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holes goes back to basically when I was an

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undergrad and I was doing lots of courses in

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lots of different things. And this goes back

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to some of the work that made Stephen

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Hawking so world renowned. Now, obviously,

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for a lot of people, Stephen Hawking became a

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global name with the publication of A Brief

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History of Time, which did a very good job of

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explaining very complicated things in A way

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that people could at least feel like they had

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a grasp of. Um, I remember reading it as a

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kid, and it made my head hurt. But it, in a

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good way, I could actually follow it. It was

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well explained. One of the things that

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Stephen Hawking did fairly early in his

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career, I think in, like, 1974 or something,

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was do some very theoretical work on, um,

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black holes, where he postulated that black

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holes could lose we through a process called

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Hawking radiation. And the idea is

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that black holes can effectively be

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considered to have a temperature and to

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radiate energy and therefore mass away

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into space. And the smaller the black hole,

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the hotter it is, so the quicker it would

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radiate. And this is all backed up by

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ridiculously complex physics and mathematics

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that is way beyond my level of full

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understanding. But part of the idea behind it

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is what we think of as the empty vacuum of

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space is actually not a true vacuum, but is

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instead constantly populated by pairs

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of matter and antimatter particles that

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spontaneously create and then collide with

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each other and disappear again. And if these,

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if such an event happens near the event

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horizon of a black hole, one of the particles

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falls into the black hole, the other escapes,

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and it's seen to lose mass, lose energy, and,

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um, radiation going along with that. Now,

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the bigger the black hole, the colder it

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would be. So the slower it radiates anyway,

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but also the bigger it is, the more

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effectively it can feed from its environment.

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Even if that's little bits of dust falling

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in. Or if it's near a star, star, it can feed

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off that star, get an accretion disk. So the

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black holes that form in the modern universe

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are formed by stars reaching the end of their

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lives and are massive. They're more massive

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than the sun by L1, where they're formed from

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stars much more massive than the Sun. You get

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massive black holes, you get intermediate

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mass black holes, and you get supermassive

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black holes. And they're all the big whopping

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ones. And, um, the time scale, as I

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understand it, m for those black holes to

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decay through Hawking radiation is

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ridiculously, ridiculously, ridiculously

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longer than the edge of the universe. Yes.

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And, um, they're probably not emitting

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Hawking radiation at a level that we could

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detect because they are very cold. In his

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quantification of it, however, at, uh,

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the birth of the universe, when the

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temperature and pressure was immense after

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the Big Bang, there were

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theoretically a class of black holes created

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called primordial black holes. So these were

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black holes that were not born from the fiery

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Death of a star, but were instead born

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out of the Big Bang and the pressures and the

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temperatures. And um, these could be black

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holes down to the mass of a thumbnail or down

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really, really tiny ones. Planet mass black

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holes.

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Andrew Dunkley: Yep.

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Jonti Horner: The smaller you are as a black hole, the more

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quickly you radiate things away, so the

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shorter your lifetime. And so you have this

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idea that these primordial black holes

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evaporated over time and effectively

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none of them will survive to the current day.

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Those evaporating black holes would

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evaporate over time and give off radiation

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that we have never yet detected. But a black

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hole coming to the end of its life will

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evaporate faster and m faster. There's a

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quote on an article I found recently which

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may be tied to this, um, article entitled

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An Exploding Black Hole Could Reveal the

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Foundations of the Universe, published from

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September last year, talking about

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the predictions that as our technology gets

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better in the coming years, we may be able to

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detect this Hawking radiation in an event

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where the black hole reaches its critical

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phase and evaporates entirely within the next

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few years. So not quite the neutrino

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discovery that we were talking about in the

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question, but a related thing. And there's a

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quote here from Andrea Tham, who

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Associate Professor Andrea Tham, I, I do hate

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it when articles don't give people's well

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earned titles until later in the sentence or

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don't give them at all. Um, which is another

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ont. I could go on, that's separate, but it

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particularly affects my, um,

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early career colleagues, um, affects

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colleagues from non traditional backgrounds

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and stuff. And it's a very

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diminutizing thing, diminishing thing. It

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lowers their expertise. Anyway, this is a

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quote from Associate Professor Andrea Tham

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from University of Massachusetts Amherst,

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says as primordial black holes evaporate,

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they become ever lighter, uh, so hotter.

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They therefore emit even more radiation. It's

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a runaway process until they explode.

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Um, it's that Hawking radiation that our

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telescopes can detect.

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Andrew Dunkley: Yeah.

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Jonti Horner: So what's happening is you've got these

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primordial black holes that are really itty

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bitty diddy ones that are therefore

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evaporating quicker than they can gain mass.

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They'll be on this critical threshold. And so

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you get this runaway death where the more

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massive ones live longer before they get

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small enough to finally evaporate and then

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explode. And so I would guess

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that the observation of this super

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neutrino that has been linked potentially to

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an exploding black hole is not two black

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holes colliding. It's not a modern Black hole

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formed from the death of sars, but rather is

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the death of a primordial black hole, as

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would be predicted by this research by

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Stephen hawking more than 50 years ago in the

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form of Hawking radiation. So that's my

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thinking on what's happening here. Now,

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obviously, I am, um, not an expert.

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Um, I've said previously on many places,

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uh, in a lot of disciplines, and when we're

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teaching our undergrads, we often say, avoid

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Wikipedia. Wikipedia is not a static

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resource. It's a fluid resource and it's

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often wrong. And I know for journalists, it's

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probably often something you caution, don't

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get your facts from Wikipedia. For

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astrophysics, and particularly the more

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technical and hardcore ends of astrophysics,

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Wikipedia is actually very reliable because

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very few people will be interested in

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maliciously editing a webpage because,

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frankly, they'll go off, uh, after other

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topics that are more triggering. But also,

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people who are interested in this stuff and

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have the knowledge tend to be very obsessive.

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And if they spot something wrong, they fix it

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very quickly. The result of that is if you

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Google Hawking radiation. The

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Wikipedia page is very lengthy, goes into a

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lot of detail, includes some of the maths.

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That makes my head hurt, huh? And makes me

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want to cry a little bit.

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I'm talking about black hole evaporation and

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things like this. Now,

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the equation for black hole evaporation

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that's on here, which is based on the Hawking

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work, gives a evaporation

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time for a black hole of

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2.14 times 10 to the 67

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years. So that's 2.14

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multiplied by 10 with 67 zeros

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multiplied by the mass of the black hole

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divided by the mass of the sun to the power

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three. So if you've got a black

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hole that is one solar mass, it will take

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2.14 times 10 to the 67 years to

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evaporate. And the M more massive it is, the

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larger that number gets to the power three.

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So the multiplier here is get the mass of the

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black hole as measured in units of the mass

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of the sun, cube that number, and

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then multiply it by 2.14 times 10 to the

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67, and you get a headache, but

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you get a number. Now, the mass of the sun

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is what, 2 times 10 to the 30

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kilos? Right? Mass of the

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Earth is 5.97 times 10

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to the 24 kilos. So that is

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effectively, um, 2 times 10 to the minus

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6 solar masses, about a millionth of a solar

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mass. So we'll just say it's 1 millionth of a

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solar mass. 1 millionth

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cubed is 1 times 10 to the minus

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18. That means a, uh, black hole, the mass

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of the Earth, would decay much more quickly.

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It would decay in only 10 to the 49 years,

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which is still much, much, much, much longer

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than the edge of the universe. But you can

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play this game with everything. I. I'm

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too fat. You know, we talk about health and

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everything on the show before. I am a fair

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bit more than 100 kilos. But let's assume I

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was 100 kilos, um, just because that's an

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aspirational goal. And it would be nice if it

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were true one day. In fact, I'm 100 kilos and

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you make me a black hole. Um, I would be sad,

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but probably wouldn't have long to think

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about it. At 100 kilos,

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I will be 10 to the 28 times

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less massive than the sun, roughly. The sun

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is 10 to the 30. I'm 10 to the 2. 10 to the

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28 is a difference. 10 to the 28 cubed

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is 28, 56,

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84. So that's 10 to the 84.

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So that means I would disintegrate in two

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times 10 to the 67 times 10 to the minus

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84, which is about 10 to the minus 17

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years. So suddenly, a drum t mass black

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hole would disintegrate and evaporate in

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a tiny fraction of the millisecond.

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So these primordial mass black holes that

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evaporate are, uh, doing

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so because they're very small. You could, if

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you wanted to. And I'll leave this as an

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exercise to the reader, because me doing

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mental arithmetic is not the most exciting

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thing. You could work out what massive black

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hole would have to be to evaporate

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after 13.8 billion years,

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which is about how old the universe is. The

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reason that's an interesting one is if there

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were any primordial mass black holes of that

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mass and they were to evaporate,

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they will be evaporating in the very near

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universe. And that would make them much

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easier to detect because the intensity of

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radiation we detect is proportional to 1

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over the square of the distance. So if

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something's twice as far away, it's four

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times fainter. If it's three times as far

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away, it's nine times fainter. So

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I don't know. I'm not a black hole expert by

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any means. I say that all the time.

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But if there were a black hole of that mass

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formed at the Big Bang, Then maybe

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they will be evaporating in the relatively

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local universe, and they're the ones that

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have been most likely to detect. I do not,

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however, know what the distribution

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of masses for primordial black

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holes would be. It's possibly on this

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Wikipedia page, but have a

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look and find out if it's your kind of thing.

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But hopefully that explains why there's a

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turnover point where things would decay in

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less than the edge of the universe or more

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than the edge of the universe. Uh, and that

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mass is somewhere between the mass of ajonti

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and the mass of the Earth.

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Andrew Dunkley: Okay, fascinating. Yeah. All right, thank

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you, Nick. Uh, and Nick, uh, you might have

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heard us talking a, uh, week or two or

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three or four back, uh, about, uh,

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what they think might be the discovery of a

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primordial black hole. So that's a story

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worth looking up as well.

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Thanks for your question. This is Space Nuts,

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Q and A edition with Andrew Dunkley and John

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de Horner.

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Jonti Horner: 0G and I feel fine. Space Nuts.

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Andrew Dunkley: Uh, now, Jonti, we've got an audio question

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that comes from a repeat offender.

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Uh, her name's Andrea.

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Andrea: Hi, guys. Got, um, a couple of questions I'm

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hoping you can help me with. Um, the first

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question I have is, um,

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why does the dark side of the Moon

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not have anywhere near as much damage as the

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face of the Moon? Um,

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my second question is,

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um, why are the

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craters so shallow on the moon? Considering

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the size of some of the impact

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zones and craters, um, they all seem to be

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the same depth, which is quite shallow, um,

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especially if you look at TAO, which is

347
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3,000 miles wide, um, with an

348
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incredibly shallow crater. Um, if you could

349
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explain for me why that occurs,

350
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that would be absolutely amazing. Thank you

351
00:14:07.380 --> 00:14:09.500
very much. Oh, and this is Andrea from

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Wanneroo and Andrew. Uh, Wanneroo is actually

353
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a noongar, or whadjuk? Noongar. Ah,

354
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people word. Um, that actually

355
00:14:19.460 --> 00:14:22.220
means the area of the

356
00:14:22.220 --> 00:14:25.060
digging stick. Unfortunately, not

357
00:14:25.060 --> 00:14:26.860
pet kangaroo, although I have had one of

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those as well. Thanks, guys. Take care.

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Jonti Horner: Bye.

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Andrew Dunkley: Thanks, Andrea. Lovely to hear from you. I'm

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glad she explained that. Um, you probably

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don't know what she's talking about, Jonti,

363
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but, um, when Andrea last sent us an audio

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question and she said she was from Wanneroo,

365
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I translated that to an indigenous word

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meaning, I want a pet kangaroo. So,

367
00:14:45.030 --> 00:14:47.630
yeah, I know I was being silly, but, um, no,

368
00:14:47.630 --> 00:14:50.270
it's, um, place of the digging stick. Didn't

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know that. So, um, of course, the digging

370
00:14:52.510 --> 00:14:55.190
stick was one of the implements, uh, that the

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ancient indigenous peoples of Australia used

372
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to use to, to uh, dig up,

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um, grubs and other, other bush

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tucker as we call it these days.

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Jonti Horner: So it's probably worth mentioning for the

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listeners who are not in Australia that many

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of the Australian places have names

378
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that derive from the languages of the

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traditional owners of the land, the

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indigenous people of Australia, who had many

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different countries with many different

382
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language groups. And the origin of the

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00:15:22.970 --> 00:15:24.970
names is not always that well known or

384
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understood because during the invasion of

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Australia and during the events that happened

386
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all the way through to the 1970s, there was a

387
00:15:31.710 --> 00:15:34.710
fairly aggressive attempt to, even if

388
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you weren't wiping out the people, to get rid

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00:15:36.350 --> 00:15:37.710
of the culture and to get rid of the

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00:15:37.710 --> 00:15:40.070
knowledge. I've just looked up Toowoomba

391
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where I am T o uh o uh w o uh o uh m b

392
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a I live about 20 ks west of there.

393
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Toowoomba is an indigenous name. It's a

394
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really interesting town because it's like the

395
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Florida of Queensland. All the old people

396
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come here to retire. It's a lovely place.

397
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It's a beautiful place because Queensland has

398
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a particular climate. But Toowoomba is

399
00:16:00.620 --> 00:16:02.300
a moderated version of that climate because

400
00:16:02.300 --> 00:16:04.060
it sits on the Great Dividing range at about

401
00:16:04.060 --> 00:16:06.620
700 meters above sea level. So it's not as

402
00:16:06.620 --> 00:16:08.460
humid as the coast. It doesn't get as hot as

403
00:16:08.460 --> 00:16:10.780
the coast. It has very lovely dry winters.

404
00:16:10.780 --> 00:16:13.380
Anyway, the name of Toowoomba is

405
00:16:13.700 --> 00:16:16.500
probably based on a word

406
00:16:16.500 --> 00:16:19.380
from likely the Gable or Jarawar

407
00:16:19.380 --> 00:16:21.820
peoples. Not entirely sure. But if you look

408
00:16:21.820 --> 00:16:24.730
around for the origin of Toowoomba as a word,

409
00:16:25.440 --> 00:16:27.040
there's lots of suggestions. There is a

410
00:16:27.040 --> 00:16:29.120
suggestion that it was a, ah, word for swamp

411
00:16:29.120 --> 00:16:31.000
because Toowoomba sits in this swampy area on

412
00:16:31.000 --> 00:16:33.200
top of the hills. According to the Toowoomba

413
00:16:33.200 --> 00:16:35.400
Regional Council, it may have been named

414
00:16:35.400 --> 00:16:38.000
after a property in the area in the 1850s,

415
00:16:38.560 --> 00:16:41.160
or it may have come from an Aboriginal word

416
00:16:41.160 --> 00:16:43.000
meaning either place where water sits, which

417
00:16:43.000 --> 00:16:45.680
will be the swamp thing or place of melon, or

418
00:16:45.680 --> 00:16:48.080
place where reeds grow or berries place or

419
00:16:48.080 --> 00:16:50.440
white man. There are other things saying

420
00:16:50.440 --> 00:16:53.050
meeting of the waters or. Or saying. The name

421
00:16:53.050 --> 00:16:54.810
of Toowoomba may be an anglicized version of

422
00:16:54.810 --> 00:16:57.770
the word bu wonga, which meant thunder in

423
00:16:57.770 --> 00:16:59.810
the dialect of the upper Burnett and Gaynda

424
00:16:59.810 --> 00:17:02.770
tribes. So we just don't know. And it does

425
00:17:02.770 --> 00:17:04.210
make me a little bit sad. We talk about

426
00:17:04.210 --> 00:17:06.170
indigenous astronomy a bit and the wonderful

427
00:17:06.170 --> 00:17:08.570
work that, um, Professor Duane Hamaker and

428
00:17:08.570 --> 00:17:10.250
his students have done over the years working

429
00:17:10.250 --> 00:17:12.130
with the indigenous people of Australia. But

430
00:17:12.130 --> 00:17:13.410
it does make me sad how much of this

431
00:17:13.410 --> 00:17:15.010
knowledge is lost where you don't even know

432
00:17:15.010 --> 00:17:16.930
the origin of the name. So it's wonderful

433
00:17:16.930 --> 00:17:19.439
that in this case we actually know where the

434
00:17:19.439 --> 00:17:21.879
name comes from. We can talk to that. So when

435
00:17:21.879 --> 00:17:23.119
you're looking at the map of Australia and

436
00:17:23.119 --> 00:17:25.439
think a lot of the places are unusual from

437
00:17:25.439 --> 00:17:27.639
the perspective of someone from an Anglo

438
00:17:27.639 --> 00:17:29.559
background or from a European background,

439
00:17:29.799 --> 00:17:31.919
it's because even though it's a primarily

440
00:17:31.919 --> 00:17:34.599
English speaking country nowadays with a,

441
00:17:35.159 --> 00:17:38.039
with that, you know, Anglo heritage, a lot of

442
00:17:38.039 --> 00:17:39.519
the names are actually from the traditional

443
00:17:39.519 --> 00:17:42.159
owners, even if the heritage of that name

444
00:17:42.159 --> 00:17:42.999
itself is lost.

445
00:17:43.479 --> 00:17:45.959
Andrew Dunkley: Yes. Uh, where I live, Dubbo is

446
00:17:45.959 --> 00:17:48.599
supposedly a Wiradjuri word for red

447
00:17:48.599 --> 00:17:50.900
earth, because the soil here is red,

448
00:17:51.740 --> 00:17:54.180
uh, which might sound horrifying to people.

449
00:17:54.460 --> 00:17:56.980
Uh, it is when you get a dust storm and

450
00:17:56.980 --> 00:17:58.800
everything turns red, uh,

451
00:17:59.060 --> 00:18:01.700
Jonti Horner: when it gets wet and you're bringing it in

452
00:18:01.700 --> 00:18:03.940
because the red soil marks horrible

453
00:18:03.940 --> 00:18:06.340
everything up, you know. Yeah. Dog goes out

454
00:18:06.340 --> 00:18:08.180
and gets their paws muddy and brings in red

455
00:18:08.180 --> 00:18:08.820
footprints.

456
00:18:09.140 --> 00:18:12.140
Andrew Dunkley: Red footprints on a light colored carpet. No,

457
00:18:12.140 --> 00:18:14.900
uh, terrible stuff. And of course one that

458
00:18:14.900 --> 00:18:17.540
relates to astronomy is warmera,

459
00:18:18.040 --> 00:18:20.680
which is an indigenous word for uh, the,

460
00:18:20.840 --> 00:18:23.800
the implement they used to launch a spear

461
00:18:24.120 --> 00:18:26.080
rather than just throw the spear. They used

462
00:18:26.080 --> 00:18:29.050
to have a specially made, um,

463
00:18:29.720 --> 00:18:32.040
I suppose you'd call it a, like a handheld

464
00:18:32.040 --> 00:18:33.850
catapult. And it, um,

465
00:18:35.240 --> 00:18:37.680
and, and it. Yeah, and it flung the spear at

466
00:18:37.680 --> 00:18:40.240
greater speed and distance. And that's uh, it

467
00:18:40.240 --> 00:18:42.040
was called a woomera. And of course woomera

468
00:18:42.040 --> 00:18:44.850
rocket range is where Australia's, uh,

469
00:18:44.980 --> 00:18:47.740
early space efforts were, uh, were

470
00:18:47.740 --> 00:18:49.660
launched from in South Australia. So yeah,

471
00:18:49.660 --> 00:18:51.940
it's um, fascinating history really is.

472
00:18:51.940 --> 00:18:54.100
Jonti Horner: And um, of course the aquilatl that I

473
00:18:54.100 --> 00:18:55.300
mentioned there, I just double checked

474
00:18:55.300 --> 00:18:57.780
because it's like I remember an aquilateral

475
00:18:57.780 --> 00:18:59.020
being a thing that used for throwing space.

476
00:18:59.020 --> 00:19:00.860
It turns out that that was an Aztec implement

477
00:19:00.860 --> 00:19:02.460
that served the same kind of process. So the

478
00:19:02.460 --> 00:19:04.660
word applatl apparently comes from Aztec.

479
00:19:05.060 --> 00:19:08.060
Andrew Dunkley: Oh, wow. Didn't know that. Back

480
00:19:08.060 --> 00:19:08.940
to you, Andrea.

481
00:19:08.940 --> 00:19:09.380
Jonti Horner: Yes.

482
00:19:10.660 --> 00:19:12.900
Andrew Dunkley: Okay, two questions. Dark side of the moon?

483
00:19:13.370 --> 00:19:16.270
Uh, smoother. Now I've always been aware that

484
00:19:16.430 --> 00:19:18.990
the side we can see is so rugged and

485
00:19:19.230 --> 00:19:20.830
pockmarked and mountainous.

486
00:19:20.910 --> 00:19:21.390
Jonti Horner: Yes.

487
00:19:21.710 --> 00:19:24.090
Andrew Dunkley: But the side that we cannot see that, uh,

488
00:19:24.090 --> 00:19:26.790
Artemis 2 recently had a look at, uh, and

489
00:19:26.790 --> 00:19:28.750
where the Chinese have been running around on

490
00:19:28.750 --> 00:19:31.490
their little scooters. Um,

491
00:19:31.490 --> 00:19:32.830
it's smoother. Why?

492
00:19:33.470 --> 00:19:35.630
Jonti Horner: Well, this is a weird one. So

493
00:19:36.350 --> 00:19:39.350
it looks more uniform when

494
00:19:39.350 --> 00:19:40.950
you look at it. And I'm saying that very

495
00:19:40.950 --> 00:19:42.630
carefully rather than smoother, because

496
00:19:42.630 --> 00:19:45.380
smoother invokes polished or smooth.

497
00:19:45.380 --> 00:19:47.340
Like your skin when you're a kid is a lot

498
00:19:47.340 --> 00:19:48.700
smoother than your skin when you get to my

499
00:19:48.700 --> 00:19:50.100
edge and you've got all the wrinkles right.

500
00:19:50.420 --> 00:19:52.220
Yeah. Um, or the scars.

501
00:19:52.220 --> 00:19:52.820
Andrew Dunkley: See this one?

502
00:19:52.820 --> 00:19:53.220
Jonti Horner: Yeah.

503
00:19:53.540 --> 00:19:56.060
Andrew Dunkley: Uh, that's from a golf club. My neighbor hit

504
00:19:56.060 --> 00:19:58.460
me in the face with a seven iron. Yeah. It

505
00:19:58.460 --> 00:20:00.380
wasn't malicious. It was the backswing. I was

506
00:20:00.380 --> 00:20:01.140
standing too close.

507
00:20:01.140 --> 00:20:02.340
Jonti Horner: I was going to say it sounds like the

508
00:20:02.340 --> 00:20:03.860
adventures you have in double. You know,

509
00:20:04.100 --> 00:20:06.620
something to pass the time. The reason that

510
00:20:06.620 --> 00:20:08.300
I'm being careful in my wording here and

511
00:20:08.300 --> 00:20:10.140
saying it looks more uniform rather than it's

512
00:20:10.140 --> 00:20:12.440
smoother is actually, I don't think it is

513
00:20:12.440 --> 00:20:14.120
smoother, but I think it definitely does look

514
00:20:14.120 --> 00:20:17.000
more uniform on the near side of the

515
00:20:17.000 --> 00:20:18.360
Moon. It should be said that we're talking

516
00:20:18.360 --> 00:20:20.320
near side and far side. The dark side of the

517
00:20:20.320 --> 00:20:21.880
Moon is simply the side of the Moon pointed

518
00:20:21.880 --> 00:20:24.120
away from the Sun. And, um, that rotates

519
00:20:24.120 --> 00:20:25.960
around as the Moon goes around the Earth,

520
00:20:25.960 --> 00:20:27.360
which is why we get the faces right. If

521
00:20:27.360 --> 00:20:29.200
you're stood on the Moon, you'll get at a

522
00:20:29.200 --> 00:20:31.240
given location two weeks of daytime and two

523
00:20:31.240 --> 00:20:33.200
weeks of nighttime. And when it's nighttime

524
00:20:33.200 --> 00:20:34.720
for you, you'd be on the dark side of the

525
00:20:34.720 --> 00:20:36.880
Moon. But when the Moon's new, the dark side

526
00:20:36.880 --> 00:20:38.750
points towards us. The far side of the Moon

527
00:20:39.140 --> 00:20:41.340
always points away from the Earth. Now, on

528
00:20:41.340 --> 00:20:42.900
the near side of the Moon, which is a side

529
00:20:42.900 --> 00:20:45.620
we're familiar with, the view we get is

530
00:20:45.700 --> 00:20:48.660
very non uniform because we've got the

531
00:20:48.660 --> 00:20:51.500
mare and the non mare regions. So the mare

532
00:20:51.500 --> 00:20:53.300
are the seas which make up the man in the

533
00:20:53.300 --> 00:20:55.380
Moon or whatever picture you have, which are

534
00:20:55.380 --> 00:20:57.820
these flood basalt areas. And, um, then

535
00:20:57.820 --> 00:21:00.500
you've got the non mari areas, which are more

536
00:21:00.500 --> 00:21:02.260
traditionally rocky object looking.

537
00:21:03.540 --> 00:21:05.920
Andrew Dunkley: He's the drunk man in the Moon,

538
00:21:06.310 --> 00:21:08.000
uh, here because he's upside down.

539
00:21:08.000 --> 00:21:11.000
Jonti Horner: Absolutely, yeah. Those areas

540
00:21:11.000 --> 00:21:13.420
that make the drunk man are, uh,

541
00:21:13.600 --> 00:21:16.480
flood basalt outpourings on the near side of

542
00:21:16.480 --> 00:21:18.320
the Moon that were formed early in the Moon's

543
00:21:18.320 --> 00:21:20.719
formation. If you ascribe to the idea that

544
00:21:20.719 --> 00:21:22.720
there was a Late Heavy Bombardment when the

545
00:21:22.720 --> 00:21:25.400
impact rate spiked, then they are thought to

546
00:21:25.400 --> 00:21:27.600
have formed there. But in actuality, evidence

547
00:21:27.600 --> 00:21:29.200
for the Late Heavy Bombardment has pretty

548
00:21:29.200 --> 00:21:31.520
much dissipated. So the closer you are to

549
00:21:32.250 --> 00:21:34.410
impact studies and studies of the Moon, the

550
00:21:34.410 --> 00:21:36.330
less strongly you hold to the idea that heavy

551
00:21:36.330 --> 00:21:38.170
bombardment was a thing. But as with all

552
00:21:38.170 --> 00:21:40.170
science, the further you get from a certain

553
00:21:40.170 --> 00:21:41.570
expertise, the more out of date your

554
00:21:41.570 --> 00:21:44.050
knowledge is. So the heavy bombardment is

555
00:21:44.050 --> 00:21:45.970
quite often still viewed as canon in a Lot of

556
00:21:45.970 --> 00:21:47.969
areas, whereas those who are closest to the

557
00:21:47.969 --> 00:21:49.890
topic have a lot more doubt that it ever

558
00:21:49.890 --> 00:21:52.010
happened. But anyway, on the near side of the

559
00:21:52.010 --> 00:21:54.970
Moon, you've got areas

560
00:21:54.970 --> 00:21:57.450
of the Moon that didn't have

561
00:21:57.920 --> 00:22:00.040
Amare, didn't have a flood basalt

562
00:22:00.040 --> 00:22:02.880
outpouring. And, um, you've got areas that

563
00:22:02.880 --> 00:22:04.680
did. And then overlaid on that, you've got

564
00:22:04.680 --> 00:22:06.800
some more recent impacts, which are the rare

565
00:22:06.800 --> 00:22:09.560
craters where you've got weathered material

566
00:22:09.560 --> 00:22:11.320
on the surface that looks darker and an

567
00:22:11.320 --> 00:22:13.160
impact comes along, digs through the darker

568
00:22:13.160 --> 00:22:14.760
material to the unweathered material below

569
00:22:14.760 --> 00:22:17.000
and splashes it across the surface. So the

570
00:22:17.000 --> 00:22:18.920
near side of the Moon looks very non uniform

571
00:22:18.920 --> 00:22:20.960
because you've got that disparity between the

572
00:22:21.120 --> 00:22:24.050
flood basalts and the non flood basalts. And

573
00:22:24.050 --> 00:22:26.210
the non flood basalt is an older surface

574
00:22:26.210 --> 00:22:28.090
because the flood basalt erases the evidence

575
00:22:28.090 --> 00:22:30.570
of what happened before. So there are

576
00:22:30.650 --> 00:22:32.970
slightly fewer impacts on the mare than there

577
00:22:32.970 --> 00:22:35.650
are on the non mare because it's younger

578
00:22:35.650 --> 00:22:38.170
surface. Prior to any

579
00:22:38.170 --> 00:22:39.850
spacecraft going to the Moon, the assumption

580
00:22:39.850 --> 00:22:41.330
was the far side of the Moon would look like

581
00:22:41.330 --> 00:22:43.810
the near side. But when we sent spacecraft

582
00:22:43.810 --> 00:22:45.490
there, we realized it doesn't. And that was a

583
00:22:45.490 --> 00:22:47.250
big puzzle for astronomers for a very long

584
00:22:47.250 --> 00:22:50.050
time in that there were effectively no mare

585
00:22:50.050 --> 00:22:51.970
on the far side. There's little bits, but not

586
00:22:51.970 --> 00:22:54.890
very much. Now, the idea here is

587
00:22:54.890 --> 00:22:57.850
that, uh, when the Moon formed, it formed as

588
00:22:57.850 --> 00:23:00.170
a result of a giant impact on the Earth. The

589
00:23:00.170 --> 00:23:02.170
Moon accreted, uh, and initially was fully

590
00:23:02.170 --> 00:23:03.570
molten and then it cooled from the outside

591
00:23:03.570 --> 00:23:06.530
in. So at a certain time in the Moon's youth,

592
00:23:07.010 --> 00:23:09.650
the surface was very thin above a magma

593
00:23:09.650 --> 00:23:12.170
ocean, above a molten ocean. And at that

594
00:23:12.170 --> 00:23:14.810
time, small impacts wouldn't penetrate that

595
00:23:14.810 --> 00:23:17.130
crust. And you get normal craters, you get

596
00:23:17.130 --> 00:23:18.610
mountain ranges and all the rest of it

597
00:23:18.610 --> 00:23:20.370
forming. But when you got a really big impact

598
00:23:21.010 --> 00:23:23.130
that would break through the crust, create a

599
00:23:23.130 --> 00:23:25.450
big impact basin that would then be flooded

600
00:23:25.450 --> 00:23:27.330
with flood basalt, which gave you this

601
00:23:27.330 --> 00:23:29.810
incredibly flat, smooth floor and

602
00:23:29.810 --> 00:23:31.890
erased all the evidence of the impacts before

603
00:23:32.930 --> 00:23:35.810
eventually the Moon cooled enough that any

604
00:23:35.810 --> 00:23:37.810
molten material was sufficiently deep that

605
00:23:37.810 --> 00:23:40.410
even the biggest impacts would not cause

606
00:23:40.410 --> 00:23:43.210
these flood basalt outpourings. Coupled with

607
00:23:43.210 --> 00:23:46.090
the fact that as the solar system aged,

608
00:23:46.090 --> 00:23:47.850
it cleaned up very effectively and the big

609
00:23:47.850 --> 00:23:49.530
impactors were effectively gone. So the big

610
00:23:49.530 --> 00:23:52.020
impacts were early on. So the idea was that

611
00:23:52.020 --> 00:23:55.020
the mare are caused by the very biggest

612
00:23:55.020 --> 00:23:57.900
impacts that will create impact basins

613
00:23:57.900 --> 00:23:59.620
that are, uh, hundreds or thousands of

614
00:23:59.620 --> 00:24:02.620
kilometres across, that are broadly

615
00:24:02.780 --> 00:24:05.180
circular in shape before other things happen,

616
00:24:05.500 --> 00:24:07.500
and that they're filled with molten material.

617
00:24:08.060 --> 00:24:10.220
And the areas on the near side that are not

618
00:24:10.220 --> 00:24:12.300
in the mare are the areas that were not

619
00:24:12.860 --> 00:24:15.140
induced into one of these flood basalt outpat

620
00:24:15.140 --> 00:24:16.860
rings. Effectively they escaped being in one

621
00:24:16.860 --> 00:24:19.090
of the craters from the very biggest impact

622
00:24:19.090 --> 00:24:22.010
us. We thought that prior

623
00:24:22.010 --> 00:24:23.650
to going to the far side of the Moon, you

624
00:24:23.650 --> 00:24:25.170
would have assumed that the far side would be

625
00:24:25.170 --> 00:24:26.850
the same, but it turns out that it's not.

626
00:24:27.410 --> 00:24:29.490
That was a real problem because this idea

627
00:24:29.490 --> 00:24:31.250
that the impacts were big enough to punch

628
00:24:31.250 --> 00:24:34.210
through and flood to the surface

629
00:24:35.410 --> 00:24:38.210
should work all across the Moon. So why

630
00:24:38.210 --> 00:24:39.650
then do you not get the flood basalt

631
00:24:39.650 --> 00:24:41.330
outpourings on the far side of the Moon?

632
00:24:41.330 --> 00:24:43.850
There are kind of three explanations that

633
00:24:43.850 --> 00:24:46.350
have been put forward for this, the first of

634
00:24:46.350 --> 00:24:49.190
which is, frankly, bunkham. The idea that the

635
00:24:49.190 --> 00:24:50.650
near side of the Moon faced the Earth, uh,

636
00:24:50.790 --> 00:24:52.910
and the Earth, uh, shielded it and so

637
00:24:52.910 --> 00:24:54.990
therefore there'd be more impacts on the far

638
00:24:54.990 --> 00:24:56.470
side. Well, that just kind of

639
00:24:57.830 --> 00:25:00.310
runs counterintuitive. You'd say the far side

640
00:25:00.310 --> 00:25:02.310
experience more hits, it gets more cratering.

641
00:25:02.390 --> 00:25:03.830
Well, I don't believe that from a minute

642
00:25:03.830 --> 00:25:05.270
because the Earth is so small from the Moon's

643
00:25:05.270 --> 00:25:06.870
point of view, it's barely a shield at all.

644
00:25:07.590 --> 00:25:09.790
But if that were the case, surely you'd

645
00:25:09.790 --> 00:25:11.870
expect more mare on the far side because you

646
00:25:11.870 --> 00:25:14.020
get more of these big impacts. So that, to

647
00:25:14.020 --> 00:25:15.620
me, doesn't work. So we can rule that out.

648
00:25:16.420 --> 00:25:19.380
The other answers are, uh, kind of tied

649
00:25:19.380 --> 00:25:22.180
together. But the idea is

650
00:25:22.980 --> 00:25:25.700
that the Moon had a thicker

651
00:25:25.700 --> 00:25:28.300
layer above the molten layer on the far side

652
00:25:28.300 --> 00:25:31.180
of the Moon to the near side. Two

653
00:25:31.180 --> 00:25:33.140
ways you can make that happen. One idea is

654
00:25:33.140 --> 00:25:36.140
that the heat from the young Earth, which

655
00:25:36.140 --> 00:25:37.860
would also have been molten at this time, and

656
00:25:37.860 --> 00:25:39.620
being bigger will keep its heat longer, so

657
00:25:39.620 --> 00:25:42.150
will be molten for longer. The Earth will be

658
00:25:42.150 --> 00:25:44.230
irradiating the Moon. The Moon will be close

659
00:25:44.230 --> 00:25:45.790
to the Earth, uh, when they formed because

660
00:25:45.790 --> 00:25:47.870
it's moved away. Since that

661
00:25:48.110 --> 00:25:50.390
radiative heat would have kept the near side

662
00:25:50.390 --> 00:25:53.110
of the Moon hot for longer, which the molten

663
00:25:53.110 --> 00:25:54.630
material on the surface would have stayed for

664
00:25:54.630 --> 00:25:57.390
longer, but also it would have taken longer

665
00:25:57.390 --> 00:26:00.110
for the crust to thicken on that side. So

666
00:26:00.110 --> 00:26:01.790
therefore the crust on the far side of the

667
00:26:01.790 --> 00:26:03.440
Moon would have formed quicker and, um,

668
00:26:03.440 --> 00:26:06.310
thicker. The other idea is that, ah, you

669
00:26:06.310 --> 00:26:08.350
get the same kind of effect from tidal forces

670
00:26:08.860 --> 00:26:10.620
that the tidal influence of the Earth on the

671
00:26:10.620 --> 00:26:11.980
Moon is stronger on the near side than the

672
00:26:11.980 --> 00:26:14.540
far side because the strength of tides falls

673
00:26:14.540 --> 00:26:16.460
off as distance to the power four. So that's

674
00:26:16.460 --> 00:26:19.100
a very strong, very rapid effect. Yeah,

675
00:26:19.260 --> 00:26:21.580
possibly both of those things combine

676
00:26:22.220 --> 00:26:24.260
give you a crust around the Moon that is

677
00:26:24.260 --> 00:26:26.460
thinner on the near side than the far side at

678
00:26:26.460 --> 00:26:28.300
all times as, ah, the Moon cools on the

679
00:26:28.300 --> 00:26:30.540
interior. Which means that

680
00:26:31.100 --> 00:26:33.700
the far side of the moon, the molten

681
00:26:33.700 --> 00:26:36.140
material was deeply enough buried quickly

682
00:26:36.140 --> 00:26:38.970
enough that no Maori forming impact happened.

683
00:26:38.970 --> 00:26:40.730
You've got the South Pole Ait Kin basin,

684
00:26:40.730 --> 00:26:42.690
which is the biggest impact scar on the Moon,

685
00:26:43.010 --> 00:26:44.930
doesn't really have much flood basalt in it,

686
00:26:44.930 --> 00:26:47.690
which either means that it is younger and

687
00:26:47.690 --> 00:26:50.130
therefore, uh, the interior had cooled enough

688
00:26:50.130 --> 00:26:53.009
that it didn't crack that egg or

689
00:26:53.570 --> 00:26:56.370
that it was an area where the crust

690
00:26:56.370 --> 00:26:59.130
was thicker anyway, you know, so the idea is

691
00:26:59.130 --> 00:27:00.850
that the difference between the near side and

692
00:27:00.850 --> 00:27:02.290
the far side of the Moon is down to the

693
00:27:02.290 --> 00:27:03.840
thickness of the crust when the biggest

694
00:27:03.840 --> 00:27:06.440
impacts were happening. And the idea that

695
00:27:06.440 --> 00:27:08.320
probably due to a combination of tidal

696
00:27:08.320 --> 00:27:10.560
effects and radiative heating from the

697
00:27:10.560 --> 00:27:13.240
incredibly luminous molten Earth, the

698
00:27:13.320 --> 00:27:15.560
near side of the Moon stayed a thicker shell

699
00:27:15.800 --> 00:27:17.960
and therefore was more effectively punctured.

700
00:27:18.200 --> 00:27:20.480
And so the near side got the mare and the far

701
00:27:20.480 --> 00:27:22.360
side looks more like your typical rocky

702
00:27:22.360 --> 00:27:25.280
objects like Mercury, like a lot of the rocky

703
00:27:25.280 --> 00:27:26.840
moons and stuff like that in the outer solar

704
00:27:26.840 --> 00:27:29.800
system. That's the thinking there. But it is

705
00:27:29.800 --> 00:27:32.120
really strikingly obvious when you see photos

706
00:27:32.120 --> 00:27:34.230
of the far side side of the Moon and you're

707
00:27:34.230 --> 00:27:35.750
not told it's the far side of the Moon, you

708
00:27:35.750 --> 00:27:37.510
assume you're looking at an object that is

709
00:27:37.510 --> 00:27:39.590
not the Moon because it's not different to

710
00:27:39.590 --> 00:27:40.750
our experience of the Moon.

711
00:27:41.150 --> 00:27:41.710
Andrew Dunkley: Indeed.

712
00:27:41.870 --> 00:27:44.030
So, so part two of your question, you

713
00:27:44.030 --> 00:27:46.150
basically covered because of the deep

714
00:27:46.150 --> 00:27:49.030
Jonti Horner: impact a little bit. Part two is a bit

715
00:27:49.030 --> 00:27:50.070
more complex.

716
00:27:50.070 --> 00:27:51.910
Andrew Dunkley: This is about, this is the one about shallow

717
00:27:51.910 --> 00:27:52.430
craters.

718
00:27:53.550 --> 00:27:56.190
Jonti Horner: And craters are uh, shallow, not just on the

719
00:27:56.190 --> 00:27:58.070
Moon, but everywhere. There is a boundary

720
00:27:58.070 --> 00:28:00.070
between what, what researchers describe as a

721
00:28:00.070 --> 00:28:02.870
simple creator and a complex crater. The

722
00:28:02.870 --> 00:28:05.550
size at which you get that boundary varies

723
00:28:05.550 --> 00:28:07.630
dependent on the strength of material that's

724
00:28:07.630 --> 00:28:10.550
impacted and also the mass of the planet and

725
00:28:10.550 --> 00:28:12.390
therefore the strength of gravity. So I

726
00:28:12.390 --> 00:28:14.709
believe on the Earth it's about 8 or 9

727
00:28:14.709 --> 00:28:16.590
kilometers. For the moon it's about 18

728
00:28:16.590 --> 00:28:19.590
kilometers smaller than that. You get

729
00:28:19.590 --> 00:28:21.470
a simple crater that forms, which looks very

730
00:28:21.470 --> 00:28:23.310
similar to what you'd get if you almost just

731
00:28:23.390 --> 00:28:26.350
threw a rock really hard into sand or

732
00:28:26.350 --> 00:28:28.070
something. You get the typical bowl shaped

733
00:28:28.070 --> 00:28:30.310
crater like Meteor Crater in Arizona. Really

734
00:28:30.310 --> 00:28:33.110
nice example. Yeah, ah, a

735
00:28:33.110 --> 00:28:36.030
size above about like say 8 or 9 kilometers

736
00:28:36.030 --> 00:28:38.670
on Earth or above about 20 kilometers on the

737
00:28:38.670 --> 00:28:40.830
moon. You get to the domain where you get a

738
00:28:40.830 --> 00:28:42.790
complex crater. And complex craters are

739
00:28:42.790 --> 00:28:45.590
characterized by having quite often central

740
00:28:45.750 --> 00:28:48.670
impact peaks, but also having

741
00:28:48.670 --> 00:28:51.550
these much shallower depths compared to

742
00:28:51.550 --> 00:28:54.260
their width, you know, and it's particularly

743
00:28:54.260 --> 00:28:55.620
true for the mare, where they're flood

744
00:28:55.620 --> 00:28:58.540
basalts, where you have a very shallow crater

745
00:28:58.540 --> 00:29:00.860
for the width of the crater. But it's true

746
00:29:00.860 --> 00:29:02.860
even if you look at 20 kilometer craters on

747
00:29:02.860 --> 00:29:04.820
the moon and there's a beautiful photo,

748
00:29:04.820 --> 00:29:06.500
incidentally, if you look on some of the NASA

749
00:29:06.500 --> 00:29:08.139
images, there's a beautiful photo of the

750
00:29:08.139 --> 00:29:10.700
crater Aristarchus taken by the Lunar

751
00:29:10.700 --> 00:29:12.860
Reconnaissance Orbiter, and that shows this

752
00:29:12.860 --> 00:29:15.180
kind of terracing around the walls, the

753
00:29:15.180 --> 00:29:16.980
central peaks. And so there's a few things

754
00:29:16.980 --> 00:29:19.800
going on here that contribute to why what

755
00:29:19.800 --> 00:29:21.480
we describe as being,

756
00:29:22.560 --> 00:29:25.440
um, what I say, complex craters are

757
00:29:25.440 --> 00:29:25.800
actually,

758
00:29:26.960 --> 00:29:29.840
um, shallower compared to

759
00:29:29.840 --> 00:29:31.840
their width than the simple ones. And, uh,

760
00:29:31.840 --> 00:29:33.280
there's a few things that have been suggested

761
00:29:33.280 --> 00:29:36.080
to this. So complex craters have depths that

762
00:29:36.080 --> 00:29:38.520
can be a 15th, a 25th or even less

763
00:29:38.920 --> 00:29:41.400
of the crater width, which looks very, very

764
00:29:41.400 --> 00:29:44.120
shallow. Now, there's a few things proposed

765
00:29:44.120 --> 00:29:47.080
in for this. Firstly, when you form a bigger

766
00:29:47.080 --> 00:29:49.640
crater, uh, the walls can slump in, so

767
00:29:49.640 --> 00:29:52.560
material slides and gradually you get this

768
00:29:53.040 --> 00:29:54.800
material from the edges sliding into the

769
00:29:54.800 --> 00:29:56.040
middle. And if you look at that photo of

770
00:29:56.040 --> 00:29:58.240
Arisarcus, it looks very much like that's

771
00:29:58.240 --> 00:30:01.240
happened. You see evidence of landslides that

772
00:30:01.240 --> 00:30:02.880
have filled in the crater and made it

773
00:30:02.880 --> 00:30:05.880
shallower. The other thing is craters

774
00:30:05.880 --> 00:30:08.600
that big are, ah, large enough to render the

775
00:30:08.600 --> 00:30:11.420
material, where the impact happens, molten.

776
00:30:12.290 --> 00:30:14.050
And in other words, the material can flow

777
00:30:14.050 --> 00:30:16.010
like a liquid rather than behaving like a

778
00:30:16.010 --> 00:30:17.490
solid material of your desk.

779
00:30:17.650 --> 00:30:20.090
Andrew Dunkley: Well, you can still see a rebound point in

780
00:30:20.090 --> 00:30:21.250
the middle of the crater too.

781
00:30:21.250 --> 00:30:23.610
Jonti Horner: That's it. So the stuff at the middle, the

782
00:30:23.610 --> 00:30:25.250
central peaks, are thought to be rebound of

783
00:30:25.250 --> 00:30:28.010
this fluid material springing back before it

784
00:30:28.010 --> 00:30:30.390
freezes solid again effectively. And, um,

785
00:30:30.410 --> 00:30:32.570
then the flat base of these craters that

786
00:30:32.570 --> 00:30:34.690
makes them shallower is because you make a

787
00:30:34.690 --> 00:30:37.250
pool of liquid that spreads out and settles.

788
00:30:38.550 --> 00:30:40.230
And so therefore you get these shower things,

789
00:30:40.230 --> 00:30:43.110
whereas with smaller craters you don't get to

790
00:30:43.110 --> 00:30:45.670
that point. So you get much more material

791
00:30:45.670 --> 00:30:47.670
behaving more as a solid than a liquid,

792
00:30:47.750 --> 00:30:49.990
effectively. So the thinking is for these

793
00:30:49.990 --> 00:30:51.510
complex craters, and like I said for the

794
00:30:51.510 --> 00:30:53.789
moon, I think the size scale is what, 18 to

795
00:30:53.789 --> 00:30:56.350
20 kilometers, something like that. It's a

796
00:30:56.350 --> 00:30:58.030
point at which you transition from simply

797
00:30:58.030 --> 00:31:00.590
behaving as a solid to the surface behaving

798
00:31:00.590 --> 00:31:03.270
in more of a liquid fashion, you get

799
00:31:03.580 --> 00:31:05.620
complex craters having central peaks, they

800
00:31:05.620 --> 00:31:08.500
have terraces, they've got flat floors. The

801
00:31:08.500 --> 00:31:10.860
more massive the object, the smaller the

802
00:31:10.860 --> 00:31:13.820
boundary is, because gravity has a

803
00:31:13.820 --> 00:31:14.540
role in this.

804
00:31:15.020 --> 00:31:15.580
Andrew Dunkley: Yeah.

805
00:31:15.690 --> 00:31:18.300
Jonti Horner: Um, and then you get the basins which are

806
00:31:18.300 --> 00:31:19.940
even bigger and there where you get the

807
00:31:19.940 --> 00:31:22.100
flooding from basalts, which makes them even

808
00:31:22.100 --> 00:31:24.660
shallower compared to their width. So there

809
00:31:24.660 --> 00:31:27.340
is some beautiful complexity of this

810
00:31:28.060 --> 00:31:30.220
where it's all to do with the physical

811
00:31:30.540 --> 00:31:33.260
behavior of material and how that

812
00:31:33.260 --> 00:31:35.260
changes as an impact gets larger and larger

813
00:31:35.260 --> 00:31:37.620
and therefore more and more damaging and

814
00:31:37.620 --> 00:31:40.620
energetic. Now, NASA have a Mars Ed

815
00:31:40.620 --> 00:31:43.020
website, um, where they actually

816
00:31:43.020 --> 00:31:45.460
explicitly say, if you Google for this, it's

817
00:31:45.460 --> 00:31:48.379
marsed Asu. Edu and then

818
00:31:48.379 --> 00:31:50.460
a really long string afterwards. It's a Mars

819
00:31:50.460 --> 00:31:52.340
education thing at Arizona State University.

820
00:31:52.820 --> 00:31:54.580
And I'll just quote here.

821
00:31:55.540 --> 00:31:58.380
Compared to simple craters, complex craters

822
00:31:58.380 --> 00:32:01.040
also generate a lot more impact. Melted

823
00:32:01.040 --> 00:32:03.640
rock, this typically flows and pools like

824
00:32:03.640 --> 00:32:05.880
lava to form a sheet that covers a shattered

825
00:32:05.880 --> 00:32:08.360
rock known as breccia. On the crater floor,

826
00:32:08.760 --> 00:32:11.160
the crater's inner walls may slump downwards,

827
00:32:11.240 --> 00:32:13.600
rotating backwards in blocks, which can widen

828
00:32:13.600 --> 00:32:15.640
the crater's rim and line the inner walls

829
00:32:15.640 --> 00:32:18.040
with terraces. But as a result, complex

830
00:32:18.040 --> 00:32:19.680
craters look shallow. Uh, they have rim

831
00:32:19.680 --> 00:32:21.760
diameters about 30 times greater than their

832
00:32:21.760 --> 00:32:24.240
depths. By comparison, simple craters are

833
00:32:24.240 --> 00:32:26.120
about five times wider than they are deep.

834
00:32:26.910 --> 00:32:28.470
Um, earlier on it said the more energy and

835
00:32:28.470 --> 00:32:30.310
impact delivers, the bigger the cavity on the

836
00:32:30.310 --> 00:32:32.470
ground. But immediately after the blast, the

837
00:32:32.470 --> 00:32:34.590
center of the cavity begins to rise as rocks

838
00:32:34.590 --> 00:32:36.150
rebound from the shock. That's what gives you

839
00:32:36.150 --> 00:32:38.190
the mountains. This uplift gives you a

840
00:32:38.190 --> 00:32:40.670
central peak or cluster of peaks. So that's a

841
00:32:40.670 --> 00:32:42.630
really nice way of condensing my lengthy,

842
00:32:42.630 --> 00:32:45.350
waffly answer into something a bit more

843
00:32:45.350 --> 00:32:46.270
simple and straightforward.

844
00:32:47.150 --> 00:32:49.790
Andrew Dunkley: Fair enough. Okay, uh, now, Andrea, you can

845
00:32:49.790 --> 00:32:51.870
believe all of that, or you can go with my

846
00:32:51.870 --> 00:32:53.930
theory that the lunar city council were on

847
00:32:54.240 --> 00:32:56.080
strike and they didn't finish filling the

848
00:32:56.080 --> 00:32:56.720
potholes.

849
00:32:58.720 --> 00:33:01.530
I go with option two. Um,

850
00:33:01.530 --> 00:33:03.080
Andrea, thanks for your question. Great to

851
00:33:03.080 --> 00:33:04.400
hear from you. Thanks for explaining

852
00:33:04.400 --> 00:33:05.200
Wanneroo.

853
00:33:05.440 --> 00:33:07.800
This is Space Nuts, a Q and A edition with

854
00:33:07.800 --> 00:33:10.800
Professor Jonti Horner and Andrew Dunkley.

855
00:33:13.280 --> 00:33:15.320
Okay, Houston, we've had a problem here. This

856
00:33:15.320 --> 00:33:17.680
is Houston. Say again, please. Houston, we've

857
00:33:17.680 --> 00:33:19.240
had a problem. Is that a main B plus

858
00:33:19.240 --> 00:33:21.320
undervolt? Roger, main B undervolt. Okay,

859
00:33:21.320 --> 00:33:22.880
standby 13. We're looking at it.

860
00:33:23.130 --> 00:33:23.850
Jonti Horner: These butts.

861
00:33:25.210 --> 00:33:27.450
Andrew Dunkley: Our, uh, last question comes, uh, in two

862
00:33:27.450 --> 00:33:30.280
parts, and it comes, uh, from Eli. Uh,

863
00:33:30.280 --> 00:33:32.770
hello from Coachella Valley in

864
00:33:32.770 --> 00:33:34.970
California. Was Coachella in the news

865
00:33:34.970 --> 00:33:37.970
recently for some big soiree that happened

866
00:33:37.970 --> 00:33:40.970
there? Yeah, uh, big event. Uh, anyway, he

867
00:33:40.970 --> 00:33:43.090
says the grasshoppers have decided to invade.

868
00:33:43.090 --> 00:33:45.570
Believe it or not, Eli, exactly the same

869
00:33:45.570 --> 00:33:48.090
thing is happening where I am. We have

870
00:33:48.410 --> 00:33:50.490
locust, uh, plagues. Once in a blue moon. And

871
00:33:50.490 --> 00:33:53.130
we Had a little one recently. Wasn't, uh, too

872
00:33:53.130 --> 00:33:55.750
significant. But I have discovered with

873
00:33:55.910 --> 00:33:57.950
locusts, or grasshoppers or whatever you call

874
00:33:57.950 --> 00:34:00.630
them wherever you are, that if you drive

875
00:34:00.870 --> 00:34:03.510
over 50 kilometers an hour, they splatter.

876
00:34:04.320 --> 00:34:06.910
Uh, if you drive under 50 kilometers an hour,

877
00:34:06.910 --> 00:34:09.430
they bounce off. Important safety tip.

878
00:34:09.430 --> 00:34:12.150
Especially because when they splatter, they

879
00:34:12.150 --> 00:34:14.590
stink and it's very hard to get off when

880
00:34:14.590 --> 00:34:15.110
they're dry.

881
00:34:15.110 --> 00:34:17.550
Jonti Horner: I'd love to have a swapsy where we've had an

882
00:34:17.550 --> 00:34:19.670
incredibly dry last few months here. It's

883
00:34:19.670 --> 00:34:21.590
been our wet season. We've had 40 mil of rain

884
00:34:21.590 --> 00:34:24.450
in four months. Months, wow. Which is hooray,

885
00:34:24.450 --> 00:34:25.930
you know, that's really what you want in your

886
00:34:25.930 --> 00:34:27.530
wet season when the dry season is about to

887
00:34:27.530 --> 00:34:29.410
start. But what that means is that we're

888
00:34:29.410 --> 00:34:31.090
probably going to see yet another mouse

889
00:34:31.090 --> 00:34:33.170
plague. And mouse plagues are sad because in

890
00:34:33.170 --> 00:34:34.930
the times that are good, mice reproduce like

891
00:34:34.930 --> 00:34:37.450
crazy. But then you get the boom busting and

892
00:34:37.450 --> 00:34:39.130
so you start getting lots of them coming to

893
00:34:39.130 --> 00:34:41.250
your house. And I'm soft hearted. I don't

894
00:34:41.250 --> 00:34:43.010
want to hurt them or do anything, but at the

895
00:34:43.010 --> 00:34:44.370
same time, I don't want them pooing on

896
00:34:44.370 --> 00:34:46.370
everything in my kitchen. Yeah. So we're

897
00:34:46.370 --> 00:34:49.070
getting mouth plague times. Uh, I, I, I

898
00:34:49.070 --> 00:34:51.350
suspect that locust plagues are horrible, but

899
00:34:51.350 --> 00:34:53.350
mouse plague is an entirely different horror.

900
00:34:53.350 --> 00:34:54.710
Andrew Dunkley: And mouse plagues are worse.

901
00:34:54.870 --> 00:34:55.270
Jonti Horner: Yeah.

902
00:34:55.270 --> 00:34:57.830
Andrew Dunkley: Because the mice try to find somewhere to

903
00:34:57.830 --> 00:35:00.710
hide inside. Locusts only get in if they've

904
00:35:00.870 --> 00:35:03.310
got an open avenue, otherwise they just stay

905
00:35:03.310 --> 00:35:06.230
outside. And you, you know. And they also are

906
00:35:06.230 --> 00:35:08.790
very disturbing when you're trying to putt on

907
00:35:08.790 --> 00:35:09.350
a golf cart.

908
00:35:09.350 --> 00:35:10.310
Jonti Horner: Oh, absolutely.

909
00:35:10.550 --> 00:35:11.670
Andrew Dunkley: It's getting away.

910
00:35:11.910 --> 00:35:12.710
Jonti Horner: They must see.

911
00:35:12.950 --> 00:35:13.430
Andrew Dunkley: Sorry.

912
00:35:13.780 --> 00:35:15.340
Jonti Horner: You walk into the kitchen at night or you

913
00:35:15.340 --> 00:35:16.500
walk somewhere and you just see something

914
00:35:16.500 --> 00:35:18.660
move in the periphery, and that's always a

915
00:35:18.660 --> 00:35:21.500
little disturbing. Yeah, well, uh, we, yeah,

916
00:35:21.500 --> 00:35:21.980
we talk,

917
00:35:21.980 --> 00:35:24.060
Andrew Dunkley: we're talking mouse plague here as well. So

918
00:35:24.060 --> 00:35:26.700
we could have, we could have both. But our

919
00:35:26.700 --> 00:35:29.500
last big locust plague, it was so

920
00:35:29.500 --> 00:35:31.260
big, the birds just got fed up with eating

921
00:35:31.260 --> 00:35:33.220
them, so they gave up as well.

922
00:35:33.220 --> 00:35:34.260
Jonti Horner: It's very weird.

923
00:35:35.040 --> 00:35:37.500
Andrew Dunkley: Um, Eli, what are you asking us? Uh, since

924
00:35:37.500 --> 00:35:40.420
you mentioned a paucity of questions,

925
00:35:40.420 --> 00:35:43.300
I hope you don't mind, um, a

926
00:35:43.300 --> 00:35:43.780
twofer.

927
00:35:43.780 --> 00:35:46.380
Okay, well, we've got two questions then. Uh,

928
00:35:46.380 --> 00:35:48.420
when the solar system formed, I always

929
00:35:48.420 --> 00:35:51.420
imagined the inner rockier planets as having

930
00:35:51.420 --> 00:35:53.380
more heavier elements due to their greater

931
00:35:53.380 --> 00:35:55.820
mass and gravity, and with lighter elements

932
00:35:56.220 --> 00:35:59.100
collecting more in the outer gas giants.

933
00:35:59.100 --> 00:36:01.860
But then I realized, isn't the sun mostly

934
00:36:01.860 --> 00:36:04.700
hydrogen the lightest element? Now I'm

935
00:36:04.700 --> 00:36:06.700
Confused. That's his first question.

936
00:36:06.780 --> 00:36:07.260
Jonti Horner: Yeah.

937
00:36:08.140 --> 00:36:10.850
Andrew Dunkley: This is your bullpen, isn't it? This is your

938
00:36:10.850 --> 00:36:11.850
area of expertise.

939
00:36:11.850 --> 00:36:14.050
Jonti Horner: This is much more my comfort zone. So this is

940
00:36:14.770 --> 00:36:15.130
really.

941
00:36:15.130 --> 00:36:16.930
Andrew Dunkley: I hope you realize I did try to find

942
00:36:16.930 --> 00:36:18.370
questions that worked for you.

943
00:36:18.370 --> 00:36:20.650
Jonti Horner: No, no, that's all good. And it means you can

944
00:36:20.650 --> 00:36:22.450
leave the cosmology ones for when Fred gets

945
00:36:22.450 --> 00:36:23.410
back as well, which is great.

946
00:36:24.210 --> 00:36:26.610
This is a lovely question. And it speaks to

947
00:36:26.930 --> 00:36:29.050
how our understanding of how planets form has

948
00:36:29.050 --> 00:36:31.530
changed over time. And we've now got quite a

949
00:36:31.530 --> 00:36:34.490
high level of complexity in the ideas we have

950
00:36:34.490 --> 00:36:36.170
behind planet formation. But a really

951
00:36:36.170 --> 00:36:39.000
fundamental part of it is that everything

952
00:36:39.320 --> 00:36:41.880
in the solar system to first order

953
00:36:42.280 --> 00:36:45.000
has the same composition as the sun, because

954
00:36:45.000 --> 00:36:46.400
we're all formed from the same material,

955
00:36:46.400 --> 00:36:48.240
formed from an enormous cloud of gas and dust

956
00:36:48.240 --> 00:36:50.720
called a giant molecular cloud that collapsed

957
00:36:50.720 --> 00:36:53.040
under its own gravity. You got effectively

958
00:36:53.040 --> 00:36:55.040
the protostarsome forming in the middle with

959
00:36:55.040 --> 00:36:56.760
a disk of material around it we call a

960
00:36:56.760 --> 00:36:59.640
protoplanetary disk. And in that disk you

961
00:36:59.640 --> 00:37:02.040
have solid material and gaseous

962
00:37:02.040 --> 00:37:04.880
material going around the sun, orbiting the

963
00:37:04.880 --> 00:37:06.740
sun, collapsing to a disk because of the

964
00:37:06.740 --> 00:37:08.580
conservation of angular momentum. So kind of

965
00:37:08.580 --> 00:37:10.740
where the Earth is, material was whizzing

966
00:37:10.740 --> 00:37:13.660
around at about 30 kilometers a second. But

967
00:37:13.660 --> 00:37:15.580
individual dust grains that were next to each

968
00:37:15.580 --> 00:37:17.700
other were both moving at about the same

969
00:37:17.700 --> 00:37:20.580
speed. So very little difference in speed

970
00:37:20.580 --> 00:37:22.020
between the particles, even though they're

971
00:37:22.020 --> 00:37:24.740
going really quickly. Now, the further you

972
00:37:24.740 --> 00:37:26.820
are from the sun, the colder the temperature

973
00:37:26.820 --> 00:37:29.780
is in that disk. And every single material

974
00:37:29.780 --> 00:37:31.960
you can think of has a

975
00:37:32.120 --> 00:37:34.920
sublimation temperature. Below that

976
00:37:34.920 --> 00:37:37.120
temperature it will be solid, and above that

977
00:37:37.120 --> 00:37:39.480
temperature it will be gas. Reason I'm not

978
00:37:39.480 --> 00:37:41.080
talking about liquid is in order to have

979
00:37:41.080 --> 00:37:43.240
liquid you need pressure. And in this case

980
00:37:43.240 --> 00:37:45.000
you don't have any or you don't have enough

981
00:37:45.160 --> 00:37:46.680
to either have solid or gas.

982
00:37:47.079 --> 00:37:47.559
Andrew Dunkley: Yep.

983
00:37:47.880 --> 00:37:50.800
Jonti Horner: If you are gas, then you

984
00:37:50.800 --> 00:37:52.720
don't form planets initially. If you're

985
00:37:52.720 --> 00:37:55.280
solid, you can do. So what happens all

986
00:37:55.280 --> 00:37:57.680
through this disk? For a variety of different

987
00:37:57.680 --> 00:38:00.000
bits of physics going on, you get whatever

988
00:38:00.000 --> 00:38:01.880
solid material you have at that distance

989
00:38:02.540 --> 00:38:05.340
colliding, sticking together, forming

990
00:38:05.340 --> 00:38:07.980
bigger bits. And so you get from millimeter

991
00:38:07.980 --> 00:38:10.620
to meter to kilometer to planet sized

992
00:38:10.700 --> 00:38:13.340
bits of debris. As you get bigger,

993
00:38:13.340 --> 00:38:16.220
gravity can start taking on a role and start

994
00:38:16.220 --> 00:38:17.620
pulling in a bit of extra stuff so you can

995
00:38:17.620 --> 00:38:19.500
feed quicker. Plus if you're bigger, you've

996
00:38:19.500 --> 00:38:21.220
got a bigger cross section, so you hit more

997
00:38:21.220 --> 00:38:23.260
things to devour them. So you get this

998
00:38:23.260 --> 00:38:25.060
process where you get lots of small things,

999
00:38:25.060 --> 00:38:26.820
making a few bigger things, and the big ones

1000
00:38:26.820 --> 00:38:29.720
tend to dominate um, so a thing called

1001
00:38:29.720 --> 00:38:32.360
oligarchic growth is the idea. And you form

1002
00:38:32.360 --> 00:38:34.290
planetesimals, and then oligarchs, which are,

1003
00:38:34.290 --> 00:38:37.160
uh, protoplanets, and a few of them collide

1004
00:38:37.240 --> 00:38:40.040
all the rest of it. If you are far

1005
00:38:40.040 --> 00:38:42.560
enough from the sun, you're beyond what's

1006
00:38:42.560 --> 00:38:45.200
known as the water ice line. Now, that's the

1007
00:38:45.200 --> 00:38:47.600
point at which the temperature is below the

1008
00:38:47.600 --> 00:38:49.800
sublimation point of water. So instead of

1009
00:38:49.800 --> 00:38:52.200
water being a gas or vapor, it's a solid.

1010
00:38:52.840 --> 00:38:54.600
Now, we always imagine water being quite

1011
00:38:54.600 --> 00:38:56.000
scarce. And I just said we've had 40

1012
00:38:56.000 --> 00:38:57.750
millimeters of rain in the last four months.

1013
00:38:58.090 --> 00:39:00.860
Uh, water is very scarce here. But in terms

1014
00:39:00.860 --> 00:39:03.020
of compounds in the universe, water is one of

1015
00:39:03.020 --> 00:39:05.420
the most abundant things there is because

1016
00:39:05.420 --> 00:39:07.300
it's a combination of hydrogen, which is the

1017
00:39:07.300 --> 00:39:09.940
most common atom with 74, 75% of all

1018
00:39:09.940 --> 00:39:12.300
atoms, and oxygen, which is the second most

1019
00:39:12.300 --> 00:39:15.260
common atom with about 1% of all atoms. Put

1020
00:39:15.260 --> 00:39:17.100
hydrogen, oxygen together, and you get water.

1021
00:39:17.660 --> 00:39:19.980
So in the protoplanetary disk around the sun,

1022
00:39:20.220 --> 00:39:23.180
water was probably about the most

1023
00:39:23.180 --> 00:39:25.180
common species other than molecular hydrogen,

1024
00:39:25.180 --> 00:39:27.330
molecular and helium atoms.

1025
00:39:27.970 --> 00:39:29.890
Lots and lots of water. Now, where the Earth

1026
00:39:29.890 --> 00:39:32.810
formed, it was too hot. So you don't

1027
00:39:32.810 --> 00:39:35.170
have water as a solid, so you form the Earth

1028
00:39:35.170 --> 00:39:38.090
dry. There's no solid water to accrete. You

1029
00:39:38.090 --> 00:39:40.490
might get a little bit of water as a gas that

1030
00:39:40.490 --> 00:39:43.450
is trapped in the solid material, which is

1031
00:39:43.450 --> 00:39:45.170
why people think most of the Earth's water

1032
00:39:45.170 --> 00:39:46.770
was delivered from further out. Because if

1033
00:39:46.770 --> 00:39:49.770
far enough out, you form from primarily water

1034
00:39:49.770 --> 00:39:52.570
with everything else added in. So the inner

1035
00:39:52.570 --> 00:39:55.160
solar system, you don't have that water to

1036
00:39:55.160 --> 00:39:57.080
accrete. So you're limited to the things that

1037
00:39:57.080 --> 00:39:59.920
are solid at, uh, higher temperatures. So

1038
00:39:59.920 --> 00:40:01.920
you're limited to accreting from rock and

1039
00:40:01.920 --> 00:40:04.800
metal. So you get telluric planets, or

1040
00:40:04.800 --> 00:40:06.440
is the archaic way of saying it, or

1041
00:40:06.440 --> 00:40:09.120
terrestrial planets beyond the ice line.

1042
00:40:09.439 --> 00:40:12.240
Water ice dominates the solid material. So

1043
00:40:12.240 --> 00:40:14.040
you've got a lot more to feed from, so you

1044
00:40:14.040 --> 00:40:16.440
grow more quickly, and you can get more

1045
00:40:16.440 --> 00:40:17.960
massive planets more quickly, which is where

1046
00:40:17.960 --> 00:40:20.240
Jupiter and Saturn come in. Now, there's a

1047
00:40:20.240 --> 00:40:21.960
lot of discussion about how they may have

1048
00:40:21.960 --> 00:40:23.960
migrated through the nebula, all the rest of

1049
00:40:23.960 --> 00:40:26.200
it, and the subtleties of the formation in

1050
00:40:26.200 --> 00:40:28.360
other planetary systems. We have planets like

1051
00:40:28.360 --> 00:40:30.120
Jupiter orbiting their stars every four or

1052
00:40:30.120 --> 00:40:32.120
five hours even, but we don't think they

1053
00:40:32.120 --> 00:40:34.480
formed there. We think they migrated in. So

1054
00:40:34.480 --> 00:40:37.120
you form beyond the ice line

1055
00:40:37.200 --> 00:40:39.760
more quickly because you've got more food,

1056
00:40:39.760 --> 00:40:42.240
and you can grow to masses like 10 or 12.

1057
00:40:42.240 --> 00:40:44.760
Earth matters while there is still an

1058
00:40:44.760 --> 00:40:47.710
Abundance of gas around. That gas doesn't

1059
00:40:47.710 --> 00:40:49.550
hang around long because once the sun fully

1060
00:40:49.550 --> 00:40:51.870
turns on after a few million years, it blows

1061
00:40:51.870 --> 00:40:53.950
the dust and the gas away and you're left

1062
00:40:53.950 --> 00:40:56.630
with what, whatever's left over. But if you

1063
00:40:56.630 --> 00:40:59.030
form to be 10 or 12 earth masses

1064
00:40:59.750 --> 00:41:01.910
before the gas is blown away, suddenly your

1065
00:41:01.990 --> 00:41:04.510
gravitational ah, pull is strong enough to

1066
00:41:04.510 --> 00:41:06.830
hold on to hydrogen and helium. If you're

1067
00:41:06.830 --> 00:41:09.190
less massive than that, then the escape

1068
00:41:09.190 --> 00:41:11.430
velocity of a hydrogen or helium atom will be

1069
00:41:11.510 --> 00:41:14.430
higher. Sorry, the escape velocity of

1070
00:41:14.430 --> 00:41:17.270
your object with that mass will be lower than

1071
00:41:17.270 --> 00:41:19.470
the speed at which hydrogen and helium atoms

1072
00:41:19.470 --> 00:41:21.710
move at that temperature. So you can't hold

1073
00:41:21.710 --> 00:41:23.590
on to them, they just escape because of their

1074
00:41:23.830 --> 00:41:25.550
motion, because of the temperature they're

1075
00:41:25.550 --> 00:41:27.750
at. When you get to 10 or 12 earth masses,

1076
00:41:27.830 --> 00:41:30.070
the escape velocity from your core

1077
00:41:30.550 --> 00:41:32.510
is higher than the speed at which hydrogen

1078
00:41:32.510 --> 00:41:34.110
and helium is moving. So you can start to

1079
00:41:34.110 --> 00:41:36.990
capture that. And like I said, 75% of all

1080
00:41:36.990 --> 00:41:39.150
atoms are hydrogen, 24% of all atoms are

1081
00:41:39.150 --> 00:41:41.920
helium. 99% of the mass of the

1082
00:41:41.920 --> 00:41:44.480
protoplanetary disk, or 98%

1083
00:41:44.480 --> 00:41:46.800
maybe is unaccessible till you get to that

1084
00:41:46.800 --> 00:41:48.360
mass and suddenly you've got this whole new

1085
00:41:48.680 --> 00:41:51.280
food source. So you quickly devour all the

1086
00:41:51.280 --> 00:41:53.040
gas around you until you open a gap in the

1087
00:41:53.040 --> 00:41:55.520
disk. And that's how you get the gas giant

1088
00:41:55.520 --> 00:41:57.880
planet shoot from Saturn with Uranus and

1089
00:41:57.880 --> 00:41:59.560
Neptune they formed further out, they had a

1090
00:41:59.560 --> 00:42:02.280
lot of abundant volatile material, but they

1091
00:42:02.280 --> 00:42:04.440
didn't really get massive enough to devour

1092
00:42:04.440 --> 00:42:07.380
the gas before the gas was blown away.

1093
00:42:07.380 --> 00:42:09.980
So that's why you get the ice giants. Uh, and

1094
00:42:10.140 --> 00:42:11.660
that is partially because they're further

1095
00:42:11.660 --> 00:42:13.780
away, they form slower. There are some

1096
00:42:13.780 --> 00:42:15.500
arguments that Uranus and Neptune may have

1097
00:42:15.500 --> 00:42:17.060
formed between Jupiter and Saturn and been

1098
00:42:17.060 --> 00:42:19.580
scattered out. But on a broad

1099
00:42:19.580 --> 00:42:22.580
brushstrokes sense, in our solar

1100
00:42:22.580 --> 00:42:24.420
system we don't think a huge amount of

1101
00:42:24.420 --> 00:42:26.220
migration happened, which is probably down to

1102
00:42:26.220 --> 00:42:28.660
the mass of the protoplanetary disk, not

1103
00:42:28.660 --> 00:42:31.620
compared to the hot Jupiter systems we

1104
00:42:31.620 --> 00:42:34.210
find elsewhere. So the planets we see today

1105
00:42:34.530 --> 00:42:37.490
are within a factor of two or three times

1106
00:42:37.570 --> 00:42:39.490
the same distance they were when they formed.

1107
00:42:39.570 --> 00:42:41.730
Jupiter might have migrated in and back out.

1108
00:42:41.890 --> 00:42:43.610
Uranus and Neptune probably formed

1109
00:42:43.610 --> 00:42:45.370
significantly closer to the sun and migrated

1110
00:42:45.370 --> 00:42:48.330
outwards. But you've got Jupiter and

1111
00:42:48.330 --> 00:42:50.770
outwards forming in the ice dominated area,

1112
00:42:51.410 --> 00:42:53.770
the terrestrial planets forming in the, in

1113
00:42:53.770 --> 00:42:56.450
the area without ice and therefore they're

1114
00:42:56.450 --> 00:42:57.970
dominated by the rock and the metal. So

1115
00:42:57.970 --> 00:43:00.260
you've like got this filter. So if you look

1116
00:43:00.260 --> 00:43:03.180
at the fraction of iron compared to carbon

1117
00:43:03.500 --> 00:43:06.380
in the Earth, or pick any Two things that

1118
00:43:06.380 --> 00:43:08.780
would have been solid silicon versus iron,

1119
00:43:09.020 --> 00:43:11.220
phosphorus for whatever, you know, things

1120
00:43:11.220 --> 00:43:13.660
that were solid. The abundances of those

1121
00:43:13.660 --> 00:43:16.540
things in all of the planets relative

1122
00:43:16.540 --> 00:43:18.580
to one another will be effectively the same

1123
00:43:18.580 --> 00:43:20.740
as the abundance in the Sun. But the

1124
00:43:20.740 --> 00:43:22.740
terrestrial planets weren't able to capture

1125
00:43:22.740 --> 00:43:24.380
the things that would have been gas at their

1126
00:43:24.380 --> 00:43:26.640
distances. Other than that, what was

1127
00:43:26.640 --> 00:43:29.000
delivered later on and weren't able to hold

1128
00:43:29.000 --> 00:43:30.800
on to hydrogen and helium. So you get that

1129
00:43:30.800 --> 00:43:33.720
chemical differentiation as a

1130
00:43:33.720 --> 00:43:35.360
result of the location of the solar system,

1131
00:43:35.360 --> 00:43:37.000
There's a bit of added complexity because

1132
00:43:37.000 --> 00:43:39.840
chemistry happens, and you'll get isotopic

1133
00:43:39.840 --> 00:43:41.439
variations and stuff. But in broad brush

1134
00:43:41.439 --> 00:43:43.920
strokes, the reason the terrestrial planets

1135
00:43:43.920 --> 00:43:46.040
are dominated by rocky and metallic material

1136
00:43:46.120 --> 00:43:47.840
is they never got massive enough to capture

1137
00:43:47.840 --> 00:43:50.680
the gas, and they formed close in where ice

1138
00:43:50.680 --> 00:43:53.430
wasn't around. That's effectively how it

1139
00:43:53.430 --> 00:43:55.870
happens. So what this question from Eli is

1140
00:43:55.870 --> 00:43:58.870
doing is actually effectively describing

1141
00:43:59.110 --> 00:44:02.110
the logic process that went into how

1142
00:44:02.110 --> 00:44:04.110
we first began to understand planet

1143
00:44:04.110 --> 00:44:07.030
formation. Because I said before, I think,

1144
00:44:07.380 --> 00:44:09.270
um, on a previous episode, astronomy is not

1145
00:44:09.270 --> 00:44:11.190
an experimental science in the way that every

1146
00:44:11.190 --> 00:44:12.950
other science is. You know, biology,

1147
00:44:12.950 --> 00:44:15.110
chemistry, physics. You want to figure out

1148
00:44:15.110 --> 00:44:16.790
how something works, you can do experiments.

1149
00:44:17.110 --> 00:44:18.790
Astronomy is an observational science.

1150
00:44:18.790 --> 00:44:20.790
Everything's so big and so far away, we can't

1151
00:44:20.790 --> 00:44:23.160
put it in a lab and smash it it. We instead

1152
00:44:23.160 --> 00:44:24.960
play detective. We look out at the universe

1153
00:44:24.960 --> 00:44:26.640
and we gather clues and we ask questions,

1154
00:44:26.640 --> 00:44:28.600
exactly like the question Eli has asked here,

1155
00:44:28.840 --> 00:44:30.760
which in its fundamental sense is, why do we

1156
00:44:30.760 --> 00:44:33.040
have rocky planets close in and gaseous ones

1157
00:44:33.040 --> 00:44:34.720
further out? Why are there different

1158
00:44:34.720 --> 00:44:36.280
compositions when we should be the same

1159
00:44:36.280 --> 00:44:38.600
composition of the sun? We then come up with

1160
00:44:38.600 --> 00:44:41.000
explanations for that that are our theories.

1161
00:44:41.320 --> 00:44:43.320
And to be a good theory, you can't just say,

1162
00:44:43.320 --> 00:44:45.560
I explain everything we see. You've got to

1163
00:44:45.560 --> 00:44:48.560
make predictions. As we find more things, we

1164
00:44:48.560 --> 00:44:50.600
will observe this. And that's how we test

1165
00:44:50.600 --> 00:44:52.560
that theory. And we test it by this interplay

1166
00:44:52.560 --> 00:44:55.060
between observation on the one theory on the

1167
00:44:55.060 --> 00:44:57.300
other. And what Eli's asked here is

1168
00:44:57.300 --> 00:44:59.580
essentially the questions that people are

1169
00:44:59.580 --> 00:45:01.220
asking that led to our current understanding

1170
00:45:01.220 --> 00:45:02.100
of planet formation.

1171
00:45:03.620 --> 00:45:06.580
Andrew Dunkley: And yet, uh, we

1172
00:45:06.580 --> 00:45:09.540
see other solar systems with exoplanets that

1173
00:45:09.540 --> 00:45:12.540
defy what we think is normal. Uh, you

1174
00:45:12.540 --> 00:45:15.100
have gas giants close to the parent star and

1175
00:45:15.100 --> 00:45:16.580
rocky planets further out.

1176
00:45:17.510 --> 00:45:19.900
Jonti Horner: Um, and that's how we develop.

1177
00:45:19.900 --> 00:45:21.580
Andrew Dunkley: Is that because they've just drifted that

1178
00:45:21.580 --> 00:45:21.860
way.

1179
00:45:22.370 --> 00:45:25.330
Jonti Horner: It's complicated. So our

1180
00:45:25.490 --> 00:45:28.490
ideas planet formation happened

1181
00:45:28.490 --> 00:45:30.370
have undergone quite a few major revolutions

1182
00:45:30.370 --> 00:45:32.530
as we found planets around other stars. So

1183
00:45:33.090 --> 00:45:35.250
in the early 1990s,

1184
00:45:36.370 --> 00:45:38.329
had a couple of talks at my local astronomy

1185
00:45:38.329 --> 00:45:41.170
society in the UK from um, Professor Wolfson

1186
00:45:41.250 --> 00:45:44.130
of York University. And Professor Wolfson

1187
00:45:44.130 --> 00:45:45.810
was an advocate of an entirely different

1188
00:45:45.810 --> 00:45:48.690
formation scenario for the solar system. I

1189
00:45:48.690 --> 00:45:50.440
think he was someone who argued that, that

1190
00:45:50.440 --> 00:45:52.920
the solar system formed through an encounter

1191
00:45:52.920 --> 00:45:54.680
between the sun and a young proto star where

1192
00:45:54.680 --> 00:45:56.320
materials pulled out of the sun into a

1193
00:45:56.320 --> 00:45:58.640
massive tongue and that tongue condensed into

1194
00:45:58.640 --> 00:45:59.200
planets.

1195
00:46:01.520 --> 00:46:04.140
Back then, um,

1196
00:46:04.640 --> 00:46:06.439
that idea was going out of fashion because

1197
00:46:06.439 --> 00:46:08.320
we'd found a few debris disks around stars

1198
00:46:08.320 --> 00:46:10.160
like Vega formal heartbeat pictoris but it

1199
00:46:10.160 --> 00:46:12.520
was still considered possible. Yeah, such an

1200
00:46:12.520 --> 00:46:15.160
event would be incredibly vanishingly

1201
00:46:15.160 --> 00:46:17.520
rare because stars getting that close

1202
00:46:17.520 --> 00:46:20.360
together within one another's hills sphere

1203
00:46:20.840 --> 00:46:23.640
is incredibly unusual. Very, very rare.

1204
00:46:24.280 --> 00:46:27.200
And so what that would predict is

1205
00:46:27.200 --> 00:46:29.680
if that theory were correct, we would be

1206
00:46:29.680 --> 00:46:32.520
almost unique. There will be vanishingly

1207
00:46:32.520 --> 00:46:34.640
few planets round of the stars because the

1208
00:46:34.640 --> 00:46:36.960
scenario you need to form planets would only

1209
00:46:36.960 --> 00:46:39.440
happen very rarely. On the other hand, there

1210
00:46:39.440 --> 00:46:41.430
was the idea which dated back to uh,

1211
00:46:41.430 --> 00:46:44.240
initially the 1700s and beyond the

1212
00:46:44.240 --> 00:46:47.220
Laplacian model, the circum solar disk

1213
00:46:47.220 --> 00:46:48.860
model, which has evolved into what we have

1214
00:46:48.860 --> 00:46:51.580
now, which suggested that as part of star

1215
00:46:51.580 --> 00:46:53.300
formation you get a disk of material around a

1216
00:46:53.300 --> 00:46:55.700
star and planets form from that disk. Disks

1217
00:46:55.700 --> 00:46:57.540
are a natural byproduct of the formation of

1218
00:46:57.540 --> 00:46:59.620
stars. Therefore planetary systems should be

1219
00:46:59.620 --> 00:47:02.380
common. Both scenarios, with a bit of

1220
00:47:02.380 --> 00:47:04.260
fudging and fiddling, could perfectly explain

1221
00:47:04.260 --> 00:47:05.700
how the solar system looked and have been

1222
00:47:05.700 --> 00:47:08.500
finessed to reproduce the solar system. But

1223
00:47:08.500 --> 00:47:10.140
the test was always going to be which of

1224
00:47:10.140 --> 00:47:12.700
these series is correct will depend on how

1225
00:47:12.700 --> 00:47:15.140
many planets we found on other stars. If

1226
00:47:15.140 --> 00:47:17.460
planets are rare, then maybe the solar system

1227
00:47:17.460 --> 00:47:19.140
is the result of a tongue being pulled out on

1228
00:47:19.140 --> 00:47:22.140
the sun. If planetary systems are common,

1229
00:47:22.140 --> 00:47:24.740
that cannot be the case. So that was a test

1230
00:47:24.740 --> 00:47:27.100
that was done there. So when we found the

1231
00:47:27.100 --> 00:47:28.780
first planetary systems around other stars

1232
00:47:28.780 --> 00:47:30.500
and we found that planets are ubiquitous,

1233
00:47:30.820 --> 00:47:32.580
that was kind of the death knell for the

1234
00:47:32.580 --> 00:47:35.300
Wolfson type model of a tongue being sucked

1235
00:47:35.300 --> 00:47:38.290
out of the sun and forming planets. But

1236
00:47:38.290 --> 00:47:40.570
it kind of confirmed the Laplace model. But

1237
00:47:40.570 --> 00:47:42.410
it also threw a spanner into the work in that

1238
00:47:42.410 --> 00:47:45.130
the variation of planet formation of that

1239
00:47:45.130 --> 00:47:47.530
disk model suggested that you would always

1240
00:47:47.530 --> 00:47:49.370
form planetary systems with rocky planets in

1241
00:47:49.370 --> 00:47:50.690
the middle and gas planets on the outside.

1242
00:47:50.690 --> 00:47:52.850
Because it had been developed to explain the

1243
00:47:52.850 --> 00:47:55.690
solar system. When you found planets

1244
00:47:55.690 --> 00:47:57.410
that were hot Jupiters, they don't fit. Their

1245
00:47:57.410 --> 00:47:59.290
planets are massive Jupiter close to their

1246
00:47:59.290 --> 00:48:02.290
star, which brought in the concept of inward

1247
00:48:02.290 --> 00:48:04.050
migration. Now it's an interesting time

1248
00:48:04.050 --> 00:48:06.320
because in the same few years

1249
00:48:06.880 --> 00:48:08.600
people had started to realize that in the

1250
00:48:08.600 --> 00:48:10.320
Solar system, there was clear evidence of

1251
00:48:10.320 --> 00:48:12.400
planetary migration for the giant planets,

1252
00:48:12.640 --> 00:48:15.360
primarily that Neptune had migrated outwards,

1253
00:48:15.360 --> 00:48:18.000
carrying Pluto with it from the Plutinos.

1254
00:48:18.480 --> 00:48:20.599
So you've got these seminal papers by Renu

1255
00:48:20.599 --> 00:48:23.120
Malhotra talking about the outward migration

1256
00:48:23.120 --> 00:48:26.040
of Neptune being evidenced in Pluto and the

1257
00:48:26.040 --> 00:48:28.400
Plutinos, predating the discovery of the

1258
00:48:28.400 --> 00:48:30.760
first exoplanet. And one of my gripes through

1259
00:48:30.760 --> 00:48:32.890
my career has been that the exoplanet

1260
00:48:32.890 --> 00:48:35.530
community primarily came from binary star

1261
00:48:35.530 --> 00:48:37.010
astronomers, not from solar system

1262
00:48:37.010 --> 00:48:39.610
astronomers. So reinvented migration to some

1263
00:48:39.610 --> 00:48:41.810
degree and assumed that we had no evidence

1264
00:48:41.810 --> 00:48:43.570
for it in the solar system. And in parallel,

1265
00:48:43.570 --> 00:48:45.290
the solar system community was working on

1266
00:48:45.290 --> 00:48:47.890
migration separately. But the

1267
00:48:48.210 --> 00:48:50.610
discoveries of planet stars over the last

1268
00:48:51.010 --> 00:48:54.010
30 years and more, which is a great

1269
00:48:54.010 --> 00:48:55.610
scientific revolution we've lived through.

1270
00:48:55.610 --> 00:48:57.050
You know, you and I grew up in a world where

1271
00:48:57.050 --> 00:48:58.650
the only planetary system we knew was our

1272
00:48:58.650 --> 00:49:00.960
own. And kids today grew up in a world where

1273
00:49:00.960 --> 00:49:02.760
we know planets are ubiquitous. That's a

1274
00:49:02.760 --> 00:49:04.600
cataclysmic shift to have lived through.

1275
00:49:05.080 --> 00:49:07.720
Yeah, living through that has proven

1276
00:49:07.800 --> 00:49:09.880
an incredibly fertile testing ground for our

1277
00:49:09.880 --> 00:49:12.560
theories of planet formation. Turns out that

1278
00:49:12.560 --> 00:49:15.400
that Laplace theory, the disk theory, was

1279
00:49:15.640 --> 00:49:18.200
a good way of the way there. So it hasn't

1280
00:49:18.200 --> 00:49:20.000
been totally discarded, but it's been refined

1281
00:49:20.000 --> 00:49:21.360
and we've learned more about it, and that

1282
00:49:21.360 --> 00:49:23.480
continues to the current day. The refinements

1283
00:49:23.800 --> 00:49:25.830
are leading to all sorts of complexities,

1284
00:49:25.830 --> 00:49:28.110
like invoking streaming instabilities to

1285
00:49:28.110 --> 00:49:30.470
concentrate pebbles at certain distances and

1286
00:49:30.630 --> 00:49:33.350
all sorts of subtleties to try and address

1287
00:49:33.350 --> 00:49:35.510
some of the pitfalls of how on Earth do you

1288
00:49:35.510 --> 00:49:37.630
get from millimeter size to meter sized

1289
00:49:37.630 --> 00:49:38.950
objects when collisions should become

1290
00:49:38.950 --> 00:49:41.790
disruptive? All sorts of things like

1291
00:49:41.790 --> 00:49:43.590
this. And it's through those observations

1292
00:49:44.230 --> 00:49:46.470
that we get to improve and refine our models.

1293
00:49:47.510 --> 00:49:49.190
We're not going to end up throwing out the

1294
00:49:49.190 --> 00:49:51.190
disk model now because we can see the disks

1295
00:49:51.190 --> 00:49:52.870
that form planets around other stars. Because

1296
00:49:52.870 --> 00:49:55.220
our telescopes have got that good. Yep. Um,

1297
00:49:55.220 --> 00:49:56.700
and one of the predictions would have been

1298
00:49:57.260 --> 00:49:59.300
prior to them getting that good, if the disk

1299
00:49:59.300 --> 00:50:00.700
model is right. When we look at places like

1300
00:50:00.700 --> 00:50:02.500
the Orion Nebula with a sufficiently good

1301
00:50:02.500 --> 00:50:04.900
telescope, we should see protoplanetary

1302
00:50:04.900 --> 00:50:07.300
disks, propolids. Then the telescope's got

1303
00:50:07.300 --> 00:50:09.380
good enough and we can see them now. We've

1304
00:50:09.380 --> 00:50:11.860
even got to the point now where we can

1305
00:50:11.860 --> 00:50:14.020
actually even observe fine structure within

1306
00:50:14.020 --> 00:50:16.580
them to see the gaps that giant planets open

1307
00:50:16.580 --> 00:50:18.900
up, to see the spiral waves that are

1308
00:50:18.900 --> 00:50:20.500
sometimes induced by a massive planet being

1309
00:50:20.500 --> 00:50:22.350
born. So we're now not only

1310
00:50:22.750 --> 00:50:25.750
inferring planet formation from the plethora

1311
00:50:25.750 --> 00:50:27.070
of planets that we're discovering around

1312
00:50:27.070 --> 00:50:29.670
other stars and from the fine details of what

1313
00:50:29.670 --> 00:50:30.990
we know about the solar system. But we're

1314
00:50:30.990 --> 00:50:32.430
actually also getting observations of the

1315
00:50:32.430 --> 00:50:34.430
disks in which it's happening that are

1316
00:50:34.430 --> 00:50:36.270
providing extra information to improve those

1317
00:50:36.270 --> 00:50:36.750
models.

1318
00:50:37.310 --> 00:50:40.270
Andrew Dunkley: It's fascinating. So you can simply

1319
00:50:40.270 --> 00:50:42.350
say there's no one size fits all

1320
00:50:43.150 --> 00:50:44.750
way of this happening.

1321
00:50:44.990 --> 00:50:46.910
Jonti Horner: Well, it's circumstantial. It's a broad

1322
00:50:46.910 --> 00:50:49.710
thing, rather a narrow thing. So in a broad

1323
00:50:49.710 --> 00:50:51.910
sense, planets form in a disc around a star.

1324
00:50:52.070 --> 00:50:54.990
Natural product of star formation. There may

1325
00:50:54.990 --> 00:50:57.750
be occasional ways of the planet formation

1326
00:50:57.750 --> 00:50:59.430
mechanisms happen like the planets around.

1327
00:51:00.040 --> 00:51:02.870
Um, neutron stars are thought

1328
00:51:02.870 --> 00:51:04.950
to probably be second generation planets.

1329
00:51:05.110 --> 00:51:06.830
Probably material formed from a disk that

1330
00:51:06.830 --> 00:51:08.350
formed around the neutron star after the

1331
00:51:08.350 --> 00:51:10.470
supernova and formed a new generation of

1332
00:51:10.470 --> 00:51:13.310
planets. You might eventually one

1333
00:51:13.310 --> 00:51:15.920
day possibly find planets formed from

1334
00:51:16.480 --> 00:51:18.800
material pulled off a star. The very most

1335
00:51:18.800 --> 00:51:20.400
massive planets, some of them will probably

1336
00:51:20.400 --> 00:51:23.160
have been formed more like binary stars

1337
00:51:23.160 --> 00:51:25.360
than actual planets which we talked in the

1338
00:51:25.360 --> 00:51:28.200
past about. When is a brown dwarf not a brown

1339
00:51:28.200 --> 00:51:30.720
dwarf? Yes, but the broad brushstroke thing

1340
00:51:30.720 --> 00:51:33.360
is fairly well established. But n every

1341
00:51:33.360 --> 00:51:36.200
single planetary system is unique. Everyone

1342
00:51:36.200 --> 00:51:38.800
has unique circumstances. Some disks around

1343
00:51:38.800 --> 00:51:40.680
stars are more massive than others. Not every

1344
00:51:40.680 --> 00:51:43.530
cell will have an identical disc. Some disks

1345
00:51:43.530 --> 00:51:45.610
get truncated because passing starship's

1346
00:51:45.610 --> 00:51:48.330
material away. Some disks get ablated away

1347
00:51:48.330 --> 00:51:50.010
because it's a massive star nearby whose

1348
00:51:50.010 --> 00:51:52.890
radiation pushes material away. You then even

1349
00:51:52.890 --> 00:51:54.570
get impacts on the chemistry. So there's

1350
00:51:54.570 --> 00:51:56.970
really fascinating studies looking at the

1351
00:51:56.970 --> 00:51:59.490
solar system that suggests there was a nearby

1352
00:51:59.810 --> 00:52:02.210
supernova when the planets were forming that

1353
00:52:02.210 --> 00:52:04.330
injected highly radioactive short lived

1354
00:52:04.330 --> 00:52:07.290
aluminium 23 I think it is that gave an

1355
00:52:07.290 --> 00:52:10.050
extra spike to the melting of planetesimals

1356
00:52:10.810 --> 00:52:13.170
that led to some of the subtleties of how the

1357
00:52:13.170 --> 00:52:15.410
solar system looks. There are indications

1358
00:52:15.410 --> 00:52:17.290
even I think that the amount of gold in the

1359
00:52:17.290 --> 00:52:19.890
solar system is unusually high compared to

1360
00:52:19.890 --> 00:52:21.810
the standard metallicity. The amounts of

1361
00:52:21.810 --> 00:52:24.330
everything else with indication of pollution

1362
00:52:24.330 --> 00:52:26.930
from two neutron stars colliding within

1363
00:52:26.930 --> 00:52:29.090
10,000 light years of where the solar system

1364
00:52:29.090 --> 00:52:31.850
would form about 100 million years before we

1365
00:52:31.850 --> 00:52:34.810
formed. So even that level of injection

1366
00:52:34.810 --> 00:52:37.770
of material is unique from one system to the

1367
00:52:37.770 --> 00:52:39.770
next. And that's why every planetary system,

1368
00:52:39.770 --> 00:52:41.130
like every person is unique.

1369
00:52:41.850 --> 00:52:42.650
Andrew Dunkley: Fascinating.

1370
00:52:42.730 --> 00:52:43.290
Jonti Horner: Fascinating.

1371
00:52:43.290 --> 00:52:45.970
Andrew Dunkley: Aren't you glad you asked Eli and Eli's

1372
00:52:45.970 --> 00:52:46.610
second question?

1373
00:52:46.610 --> 00:52:49.410
I recently read that some star systems

1374
00:52:49.410 --> 00:52:51.849
are zipping through their galaxy orbits at

1375
00:52:51.849 --> 00:52:54.650
incredible speeds of 1200. I'm

1376
00:52:54.650 --> 00:52:57.340
assuming that is kilometers per second. Uh,

1377
00:52:57.450 --> 00:53:00.290
that's 0.4% the speed of

1378
00:53:00.290 --> 00:53:03.010
light. That got me wondering how fast could

1379
00:53:03.010 --> 00:53:05.520
our solar system get going before

1380
00:53:05.680 --> 00:53:08.520
we started noticing Things going wrong, you

1381
00:53:08.520 --> 00:53:10.320
know, the windows rattling and such.

1382
00:53:11.580 --> 00:53:14.480
Um, yeah, I, I

1383
00:53:14.480 --> 00:53:17.320
think we've had questions similar to this. I

1384
00:53:17.320 --> 00:53:19.280
think we did one recently where we talked

1385
00:53:19.280 --> 00:53:21.080
about how fast the Earth would spin before

1386
00:53:21.080 --> 00:53:23.920
things started to go horribly wrong. Um,

1387
00:53:24.320 --> 00:53:26.960
this is a question of similar ilk. I

1388
00:53:27.040 --> 00:53:29.200
hadn't heard about those sorts of speeds

1389
00:53:29.200 --> 00:53:31.760
being detected by, um.

1390
00:53:31.820 --> 00:53:34.300
Jonti Horner: Uh, there'd be stars very near the

1391
00:53:34.300 --> 00:53:35.820
supermassive black holes at sense of

1392
00:53:35.820 --> 00:53:37.900
galaxies. And that kind of speed surprised

1393
00:53:37.900 --> 00:53:40.600
me. Now, my immediate take on this is that,

1394
00:53:40.600 --> 00:53:43.260
uh, we wouldn't notice

1395
00:53:43.260 --> 00:53:46.140
effectively. So the reason that I'm saying

1396
00:53:46.140 --> 00:53:48.340
that and I, I stand to be proved wrong when

1397
00:53:48.340 --> 00:53:50.740
you get up to relativistic speeds, because my

1398
00:53:50.740 --> 00:53:52.740
knowledge of relativity is not sufficiently

1399
00:53:52.740 --> 00:53:55.300
good to be absolutely certain on this. If you

1400
00:53:55.300 --> 00:53:58.270
are moving at a substantial fraction

1401
00:53:58.270 --> 00:54:00.510
of the speed of light, I don't think we'd

1402
00:54:00.510 --> 00:54:02.110
notice anything wrong in terms of the Earth

1403
00:54:02.110 --> 00:54:03.350
moving around the sun, because we'd still be

1404
00:54:03.350 --> 00:54:04.910
going around the sun at 30 kilometers per

1405
00:54:04.910 --> 00:54:07.030
second while we're both moving around the

1406
00:54:07.030 --> 00:54:08.510
galaxy at relativistic speed and

1407
00:54:08.510 --> 00:54:11.110
accelerating. What we might notice then is

1408
00:54:11.430 --> 00:54:14.270
time dilation in the fact that the external

1409
00:54:14.270 --> 00:54:17.150
universe appears to be moving quicker than it

1410
00:54:17.150 --> 00:54:19.230
should do. So we might see the effect of the

1411
00:54:19.230 --> 00:54:22.200
fact that our time is slowed down if we

1412
00:54:22.200 --> 00:54:23.600
were going around just the same. As, you

1413
00:54:23.600 --> 00:54:24.880
know, you see this stuff about people

1414
00:54:24.880 --> 00:54:26.360
orbiting a black hole at high speed or

1415
00:54:26.360 --> 00:54:28.680
whatever, or falling into a black hole. But

1416
00:54:28.680 --> 00:54:31.680
in terms of us noticing, in terms

1417
00:54:31.680 --> 00:54:34.680
of physical phenomena on Earth that

1418
00:54:34.680 --> 00:54:36.600
we're traveling at a certain speed around the

1419
00:54:36.600 --> 00:54:39.160
galaxy, I don't see a way that that would

1420
00:54:39.160 --> 00:54:41.760
work. And the reason for that is that there's

1421
00:54:41.760 --> 00:54:43.640
no resistive medium. We think about this

1422
00:54:43.640 --> 00:54:45.000
thing happening because when you're driving

1423
00:54:45.000 --> 00:54:46.560
in your car, the quick you get, the more

1424
00:54:46.560 --> 00:54:48.320
obvious your speed is because of the rattling

1425
00:54:48.320 --> 00:54:49.640
and the wind, really resistance and the

1426
00:54:49.640 --> 00:54:51.600
noise. But that's all down to your

1427
00:54:51.600 --> 00:54:53.560
interaction with something that isn't moving

1428
00:54:53.560 --> 00:54:56.160
at the same speed you are. If you're in the

1429
00:54:56.160 --> 00:54:57.480
International Space Station and you're

1430
00:54:57.480 --> 00:54:59.280
orbiting the, uh, Earth at several kilometers

1431
00:54:59.280 --> 00:55:01.640
a second, you don't feel the space station

1432
00:55:01.640 --> 00:55:03.160
rattling because it's going really quick

1433
00:55:03.640 --> 00:55:05.240
because it's moving through the vacuum of

1434
00:55:05.240 --> 00:55:07.320
space, so it's not interacting with anything.

1435
00:55:07.480 --> 00:55:09.080
If you're coming back into the atmosphere,

1436
00:55:09.080 --> 00:55:10.760
you rattle and rumble and all the rest of it.

1437
00:55:10.760 --> 00:55:13.080
We saw this with Artemis 2, because you're

1438
00:55:13.080 --> 00:55:14.160
slowing down, you're experiencing

1439
00:55:14.160 --> 00:55:16.270
acceleration, you're experiencing buffeting.

1440
00:55:17.220 --> 00:55:20.140
So to me, if we are moving as a

1441
00:55:20.140 --> 00:55:22.340
planetary system around the middle of the

1442
00:55:22.340 --> 00:55:25.260
galaxy at very high speed. Our planets would

1443
00:55:25.260 --> 00:55:27.860
still be orbiting the sun in the same way and

1444
00:55:27.860 --> 00:55:30.340
we wouldn't notice any difference. What would

1445
00:55:30.340 --> 00:55:31.880
happen though, is we'd be moving through a m.

1446
00:55:31.900 --> 00:55:34.460
Much, much denser stellar neighborhood. The

1447
00:55:34.460 --> 00:55:37.020
sky would be immeasurably beautiful, but

1448
00:55:37.020 --> 00:55:39.300
challenge. But also close encounters between

1449
00:55:39.300 --> 00:55:42.190
stars will be very common. And so

1450
00:55:42.190 --> 00:55:43.910
it may well be that the stars will be so

1451
00:55:43.910 --> 00:55:45.590
densely packed that eventually we'd have a

1452
00:55:45.590 --> 00:55:47.750
stellar approach that will be so close to

1453
00:55:47.750 --> 00:55:49.830
solar system will be disrupted. And we'd

1454
00:55:49.830 --> 00:55:51.790
certainly notice that also

1455
00:55:52.750 --> 00:55:54.990
if we were injected to there from where we

1456
00:55:54.990 --> 00:55:57.590
are now, There will be a period of adjustment

1457
00:55:57.590 --> 00:55:59.110
where the Oort cloud will be heavily

1458
00:55:59.110 --> 00:56:01.030
destabilized and we'd have catastrophic

1459
00:56:01.030 --> 00:56:02.630
levels of impacts from the comets being

1460
00:56:02.630 --> 00:56:04.830
scattered. But eventually they'd all be gone,

1461
00:56:04.830 --> 00:56:07.750
so it wouldn't be a problem. So we'd notice

1462
00:56:07.750 --> 00:56:08.390
it from the point

1463
00:56:08.390 --> 00:56:10.070
Andrew Dunkley: of view until led to the dinosaurs.

1464
00:56:10.070 --> 00:56:12.870
Jonti Horner: Oh, absolutely. Um, Long may they rest.

1465
00:56:13.110 --> 00:56:15.910
But it's one of those things where

1466
00:56:15.910 --> 00:56:17.950
if we were there and we were transported

1467
00:56:17.950 --> 00:56:20.070
there from now, what we'd notice is that the

1468
00:56:20.230 --> 00:56:22.190
sky looked very different. If we were moving

1469
00:56:22.190 --> 00:56:25.150
at that kind of speed in that denser

1470
00:56:25.150 --> 00:56:26.870
stellar neighborhood, the proper motion of

1471
00:56:26.870 --> 00:56:29.470
stars would be apparent to the naked eye over

1472
00:56:29.470 --> 00:56:31.430
human timescales, which it's not for us.

1473
00:56:31.750 --> 00:56:33.830
Barnard Star, which is the fastest moving

1474
00:56:33.830 --> 00:56:36.030
star across the night sky, will cross the

1475
00:56:36.030 --> 00:56:38.560
diameter of the full Moon in a century. Very

1476
00:56:38.560 --> 00:56:41.480
roughly, that means if Barnard's star was

1477
00:56:41.480 --> 00:56:43.920
bright enough to see with the naked eye, we'd

1478
00:56:43.920 --> 00:56:45.280
have known about proper motion earlier

1479
00:56:45.520 --> 00:56:48.040
because it would be obvious, but it wouldn't

1480
00:56:48.040 --> 00:56:49.320
be the kind of thing you'd notice from one

1481
00:56:49.320 --> 00:56:50.960
year to the next. Whereas if we were in the

1482
00:56:50.960 --> 00:56:52.280
middle of the galaxy Going around the

1483
00:56:52.280 --> 00:56:54.160
supermassive black hole, at that ridiculous

1484
00:56:54.160 --> 00:56:57.120
speed, stars will be closer together, which

1485
00:56:57.280 --> 00:56:59.600
magnifies the effect of motion

1486
00:57:00.720 --> 00:57:03.320
from our perspective. Also, they'd be moving

1487
00:57:03.320 --> 00:57:04.930
quicker, which means that the motion is

1488
00:57:04.930 --> 00:57:06.850
quicker from our perspective. And you

1489
00:57:06.850 --> 00:57:09.210
probably have proper motion being visible on

1490
00:57:09.210 --> 00:57:10.930
human timescales to the point that the

1491
00:57:10.930 --> 00:57:13.050
constellations would move. Rather than being

1492
00:57:13.130 --> 00:57:15.770
fixed patterns that you'd notice,

1493
00:57:16.810 --> 00:57:18.970
you wouldn't feel the acceleration, you

1494
00:57:18.970 --> 00:57:21.250
wouldn't notice anything's wrong. But we

1495
00:57:21.250 --> 00:57:22.770
probably wouldn't be there if the sun had

1496
00:57:22.770 --> 00:57:24.210
been there for a long time. Because it's a

1497
00:57:24.210 --> 00:57:26.890
very intermissal environment for life.

1498
00:57:26.890 --> 00:57:28.290
Because there's a lot of stars close

1499
00:57:28.290 --> 00:57:30.010
together, a lot of massive stars, a lot of

1500
00:57:30.010 --> 00:57:32.270
supernovae. It's probably a bit of a dead

1501
00:57:32.270 --> 00:57:32.670
zone.

1502
00:57:33.790 --> 00:57:33.940
Andrew Dunkley: Aha.

1503
00:57:33.940 --> 00:57:34.760
Jonti Horner: Uh-huh.

1504
00:57:34.760 --> 00:57:37.630
Andrew Dunkley: Okay. All right. Um, thanks

1505
00:57:37.630 --> 00:57:40.430
for your questions, Eli. And, uh, yeah, I

1506
00:57:40.430 --> 00:57:42.390
love that second one. I love what if

1507
00:57:42.390 --> 00:57:44.950
questions. Uh, so, uh, yeah, we've been

1508
00:57:44.950 --> 00:57:47.110
getting a few of those lately. It's. They're

1509
00:57:47.110 --> 00:57:49.630
just such great fun. Thanks, uh, to Nick and

1510
00:57:49.630 --> 00:57:51.750
Andrea as well, for contributing. And if you

1511
00:57:51.750 --> 00:57:53.950
would like to send us a question, please do

1512
00:57:53.950 --> 00:57:56.510
on our website, space nutspodcast.com

1513
00:57:56.510 --> 00:57:59.310
spacenut Click on the AMA

1514
00:57:59.310 --> 00:58:01.230
button at the top. Ask me anything is what

1515
00:58:01.230 --> 00:58:03.830
that stands for. And you can send text and

1516
00:58:03.830 --> 00:58:05.790
audio questions. Don't forget to tell us who

1517
00:58:05.790 --> 00:58:07.550
you are and where you're from. And while

1518
00:58:07.550 --> 00:58:08.830
you're there, have a look around. Check out

1519
00:58:08.830 --> 00:58:10.950
the Space Nuts shop. Maybe you'd like to

1520
00:58:10.950 --> 00:58:13.029
become a supporter. Sign up for the Astronomy

1521
00:58:13.029 --> 00:58:15.150
Daily newsletter, all sorts of things to see

1522
00:58:15.150 --> 00:58:17.510
and do on our website. And please leave

1523
00:58:17.510 --> 00:58:19.910
reviews wherever you listen to

1524
00:58:20.390 --> 00:58:22.940
Space Nuts. We appreciate that as well. Well,

1525
00:58:22.940 --> 00:58:25.100
and we appreciate you, Jonti. Thanks so much

1526
00:58:25.100 --> 00:58:27.340
for, uh, your input today. Fantastic.

1527
00:58:27.340 --> 00:58:28.900
Jonti Horner: Oh, it's always a pleasure. And yeah,

1528
00:58:28.900 --> 00:58:30.540
fabulous questions. Really enjoy them.

1529
00:58:31.500 --> 00:58:33.740
Andrew Dunkley: Me too. And we'll catch up, uh, with you

1530
00:58:33.740 --> 00:58:34.420
very, very soon.

1531
00:58:34.420 --> 00:58:35.980
Jonti Horner: Yeah, I look forward to it. Thank you.

1532
00:58:36.540 --> 00:58:38.779
Andrew Dunkley: Professor Jonti Horner from, uh, the

1533
00:58:38.779 --> 00:58:40.740
University of Southern Queensland, where he

1534
00:58:40.740 --> 00:58:43.460
is a professor of astrophysics. And thanks to

1535
00:58:43.460 --> 00:58:45.220
Huw in the studio, who couldn't be with us

1536
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today because time moves slower for Huw. So,

1537
00:58:47.640 --> 00:58:50.340
uh, he'll be joining us in a couple of

1538
00:58:50.340 --> 00:58:52.780
thousand years. And from me, Andrew Dunkley.

1539
00:58:52.780 --> 00:58:54.180
Thanks for your company. We'll see you on the

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next episode of Space Nuts.

1541
00:58:55.700 --> 00:58:56.020
Jonti Horner: Bye.

1542
00:58:56.020 --> 00:58:58.940
Andrew Dunkley: Bye. Uh, you'll be listening to the

1543
00:58:58.940 --> 00:58:59.220
Space

1544
00:58:59.220 --> 00:59:00.340
Jonti Horner: Nuts podcast,

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00:59:01.860 --> 00:59:04.660
available at Apple Podcasts, Spotify,

1546
00:59:04.900 --> 00:59:07.580
iHeartRadio or your favorite podcast

1547
00:59:07.580 --> 00:59:09.300
player. You can also stream on

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00:59:09.300 --> 00:59:10.980
demand@bytes.com.

1549
00:59:11.300 --> 00:59:13.380
Andrew Dunkley: this has been another quality podcast

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00:59:13.380 --> 00:59:15.540
production from bytes.com.