The coolest thing about the moon is that it's not just a rock. It's Earth. It's a piece of the Earth. Yes, literally. Long science rambling under the readmore.
Like, we figured out that because of how planets and moons form, they'd get different isotype ratios based on where in the solar system they formed. So we were excited to do some core samples when we went to the moon, as we could figure out if the moon formed closer or farther from the sun and got captured into a moon, or if it formed near us and has always been a moon.
The answer? Neither! It's the same as the earth. You'd think that would mean it formed near the earth, but no. It's literally THE SAME. The moon is a big chunk of the earth that somehow ended up in the damn sky.
That is weird and unexpected.
So the scientists start trying to build models to figure out how a chunk of the planet could end up in the sky and the answer that makes the most amount of sense is this:
At some point, the earth got hit by something approximately the size of FUCKING MARS. This impact caused chunks of molten earth to be flung out into orbit, which slowly coalesced into the moon. The thing that hit us? It either bounced off into deep space, or it merged in with the earth.
OK, makes sense. Wild, but makes sense.
EXCEPT this theory still has problems that we haven't been able to fix, and remember: it's the best one we have.
Problem 1: getting hit by something that big should have melted the planet. We know the earth was molten early in its formation, but the timing doesn't seem to work out for the moon-creating impact to have been during that time. So this would have caused the planet to melt a second time. And we're pretty sure that didn't happen, because the earth still has a bunch of heavy metals in the crust that should have all ended up in the core if we'd been molten twice. So... What?
Problem 2: the question is not "why do we have a moon", but "why do we only have one?"
Because here's the thing: we can do these simulations to figure out how big the rock that hit us was, what angle it came in at, how fast it was moving, all that crap. And in 99.8% of simulations where we get a moon, we don't just get one. We get two.
There's certain symmetries in how the molten chunks of rock would reform into a solid object, so if the simulation says we'd get a moon, we usually get a second one on the opposite side of the orbit.
And yes, it is possible to get a simulation where we end up with one big one, but it's super rare. And you never want to bet on super rare probabilities. So either we are in the universe where we just happened to roll a natural 20 on "only one moon", or one of two things are true:
1. Despite how otherwise accurate our "moon created by massive impact" theory is, it's incomplete or just wrong, or even weirder...
2. We did have two moons. We somehow lost one.
How? It can't have fallen back to earth, that would have melted the planet a third time and we're still pretty sure it was only molten ONCE.
And moons aren't pool balls. If another big rock was flying through the solar system and hit Moon 2: This Time It's Personal, it wouldn't just go flying out of orbit, it would be turned into a bunch of molten rock, along with its impactor, and we'd expect to see evidence of that splattered all over the earth and remaining moon.
So at the moment, our best theory about the moon is that it's a chunk of earth that ended up in the sky because we got hit by something the size of Mars which would have melted the planet, but also couldn't have, and it ended up in orbit as one moon even though that's really unlikely.
And that's not even getting started on how weird it is as a moon! See, there's like 200 moons in our solar system. None of them are anything like the moon, with one minor exception that's so exceptional that we had to demote a planet over it:
THE MOON IS TOO DAMN BIG.
Planets just don't have moons this big! We think of it as normal, because it's ours and the first moon we learn about, but it's totally weird. It's too big. It barely orbits us! The earth and moon are really like twin planets that are orbiting a central gravitational point that's barely inside the earth.
Compare it to all the other moons in the solar system, and they're all tiny compared to the planet they're orbiting. This is because of the "clearing your orbit" effect. The gravitational influence of a planet causes any rocks nearby to get thrown out or pulled in, unless they're small enough to end up in a specific orbit which is dominated by the planet.
And that's what's happened to every planet! Except for one ex-planet, and one very weird planet we don't realize is weird because we live in it. The moon is too big. But it's time to talk about Pluto.
You know how I said the moon is too big? Charon is even more way too big, despite being smaller. It's tiny. It has a radius of only 600 km. But it's a "moon" of a planet that's only got a radius of 1200km! And that makes even less sense. They both orbit around a central point between them, the differences in their mass is so small that Charon is 12% of Pluto's mass, and this was argument #1 why Pluto needed to be demoted to a dwarf planet.
Planets just don't have moons that big. I say, standing on a planet with a moon that's TOO DAMN BIG.
Our moon is only a bit more than 1% the mass of the Earth, but that's still too big.
It's too big, it's made of the earth, it's weird that it's there at all, and if it is there, it's weird that we don't have two moons.
It makes me happy that we, as a species, have spent hundreds of thousands of years going "huh, what is the moon?" and with all our science and ACTUALLY VISITING THE DAMN THING MULTIPLE TIMES, we still have to say "dunno, it's weird."
We may know leagues and bounds more now, but every explanation brings up more questions.
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