Jeffrey Andrews-Hanna

And that's what GRAIL was doing.

We can't do this on the Earth because the Earth has this pesky, annoying atmosphere that just ruins all of our gravity observations.

On the Moon, without an atmosphere, we can orbit as low as we want, and eventually, those two satellites orbited at zero kilometer altitude, and they just smacked into the lunar surface, and that was the end of the mission.

It was planned, not an accident.

But because of that low orbit altitude, we've got better gravity data for the moon than we have for any other body in the solar system, including Earth.

We have a better global gravity model for the moon than Earth.

And that to me is pretty impressive.

This is a map of the gravity field of the moon.

But before I talk about this and talk about what we learn about the moon from this, I want to give you just kind of like a five slide summary of the moon.

So, introducing the Moon, a few things everyone should know about the Moon.

One is that we think the Moon formed in a giant impact about four and a half billion years ago, when a giant proto-planet slammed into the Earth as the Earth was forming, launched material out into space.

That material formed our Moon.

The Moon began hot, at least the outer parts were molten early in its evolution, something we call a magma ocean.

And this was the state of the Moon until it started to solidify, and eventually this magma ocean solidified to make, similar to what we have on the Earth, a low density crust and a higher density mantle.

And we need to know that to understand the gravity data we see.

When we look at the surface of the moon, we see craters.

We see craters, craters everywhere.

Craters of all sizes ranging from millimeters to thousands of kilometers.

But craters are not all we see.

There's a lot of other things on the moon.