This is a really interesting article, though I thought the similarity of the moon and the Earth's crust was taken as support for the giant impact, as Theia vaporized much of the outer part of the Earth and merged with it, the ejecta from that impact (not Theia itself) becoming the moon.
https://www.youtube.com/watch?v=ibV4MdN5wo0
In the past five years, a bombardment of studies has exposed a problem: The canonical giant impact hypothesis rests on assumptions that do not match the evidence. If Theia hit Earth and later formed the moon, the moon should be made of Theia-type material. But the moon does not look like Theia — or like Mars, for that matter. Down to its atoms, it looks almost exactly like Earth.
As I understand it, we have no idea what Theia was like chemically (assuming it existed, of course). The first thought I had as I read the article is that possibly the Earth's crust and the moon are both mixtures of Theian and early Earth material. I guess that's what they're proposing as the synestia hypothesis, but wouldn't this simply be a variant of the giant impact?
Another possibility (also mentioned) is that Theia and Earth were very much alike chemically. Yes, this challenges a lot of what we currently understand about planetary formation, but in a galaxy where every new extrasolar planet we discover seems to do the same thing, well...
Another possibility they mentioned is a series of impacts with smaller planetoids (so no Theia), but that seems improbable, unless the early solar system was even messier than we think it was. If this were true, though, I'd expect to see Mars and Venus with real moons of their own, since they'd likely also be hit with a series of bodies of the same size during the early development of the solar system. And I'd expect less homogeneity between the Earth's crust and the moon's, since not all of the lesser impacts would completely pulverize the crust. And wouldn't the moon have layers of rocks of different composition, like an onion?. Not sure if there's a good way to test that yet.
One thing we're learning recently is that the interior of planetary bodies are very different from what we've expected them to be (and the surfaces of many bodies, such as Pluto are much younger than we've expected too).
The biggest fly in the ointment I'm aware of for the current giant impact theory lately is that the moon has far more water in its rocks, and even in its interior, than makes sense in light of the hypothesis that the Earth got most of its water later, from water-bearing comets and so on (which could not have impregnated the moon with water below its lithosphere). But
one explanation is that the Earth already had water, some of which ended up on/in the moon as a consequence of the giant impact (or perhaps Theia had water?)
We still have a lot to learn about our solar system.
