Jeffrey Andrews-Hanna

It's been agreed for quite some time now that the moon formed in a giant impact.

And for a very long time, the whole community more or less agreed on what that impact looked like.

You may have heard the story of a Mars-sized object slammed into the Earth, blasted ejecta out, and that ejecta coalesced to form the Earth's moon.

That explains many of the observations of the Moon, but there's a few key ones that it doesn't even come close to explaining.

In particular, the Earth and Moon are almost identical in composition, and that's not what this model predicts.

And so, just in the last several years, new theories have come out, also revolving around giant impacts, but very, very different impacts.

In one view, there are two almost identically sized objects strike one another.

And in that, the result of that impact, the material blasted out again forms the moon.

But this is a completely different type of impact than was envisioned before with very different consequences.

In another very different scenario, the early Earth was rotating incredibly fast.

It was spinning very quickly, almost as fast as it's possible for it to spin before it becomes unstable and breaks apart.

And at that point in time, an object strikes the Earth, and that smaller, faster impact blasts material into orbit, which forms the moon.

These are very different scenarios with very different predictions in a lot of respects, but they can both give us a moon-like object in orbit around the Earth.

We don't know the answer, and both of these theories have problems.

Now, the formation of the moon is not just the formation of the moon, it's also the biggest thing that happened to the Earth.

We don't understand the Earth until we understand where the moon came from, and that's still an area of intense debate in the field.

For me personally, the one thing I struggle with most in lunar science is the fact that the moon is lopsided in just a fundamental way.

So this is a visible image map of the moon centered on the far side.

What you see, the near side has all those dark patches, that basaltic maria.

The far side has very few.