Episode 37 - Lunar craters, primordial black holes & growing galaxies

So my first paper is Lunar Ejecta as the Missing Piece to Resolving Lunar Cratering Asymmetry.

So this is led by Haliang Li at Macau University of Science and Technology in China, and it's been accepted for publication in ANA.

So this is obviously on the topic of craters on the Moon, which I have to say, before reading this paper, I haven't thought enough about obviously.

[laughs] Um, so the first thing is, what's interesting about them is that they end up being preserved for billions of years, and I, I did kinda know that, but it's just because they, it doesn't have an atmosphere there to mess it up.

Mm-hmm.

And there's also no active geology, so we have a lot of them on Earth.

A lot of impacts, yeah.

Exactly.

So, so these craters are actually really interesting because they are sort of responsible for encoding all the stuff that's happened to the Earth-Moon system over the last few billion years, 'cause the Earth just loses all that information.

So you can't use the Earth, but you can use the Moon.

So it has obviously a distribution of craters across the Moon surface, and it's not actually random.

So that's another thing I thought, I had always thought it was random, but it's not, because it preserves all this information, and there's a few other bits on top of that that I'll talk about as well.

So because the Moon is tidally locked to the Earth, it means that, that the leading hemisphere is constantly, um, facing the direction of orbital motion.

Um, and so it sort of acts a bit like a windshield [laughs] um, and protects the other side from getting too many collisions with, you know, bits and pieces that are, are flying around in the Earth-Moon system.

You also have high impact velocity collisions, and this will end up with having larger craters.

And both of these mean that, uh, the density on the leading side ends up being quite a lot higher, so the crater density.

And this was actually first found on other satellites before it was found back on the Moon.

I guess that's because we've always only had, like, one, you know, one image of the Moon, one side of the Moon that we can see.

But I guess as we've been able to see the other side, we can see that there's this difference.

Yeah, so the asymmetry on the Moon, it's quantified by something called the crater impact ratio, um, and that's just the ratio of craters between the leading and trailing sides, so L over T, or I think I'll just call it asymmetry because that's easy to remember.

So this asymmetry has actually been measured quite a few times by lots of different studies that use, I think, relatively different methods, and what's nice is the value that they get is actually largely consistent between each other, so it's a value of about 1.4.

But the problem is that there's quite a significant discrepancy with what's predicted from theory.

[laughs]

That's true.

So the observations agree, but the theory doesn't.