Jeffrey Andrews-Hanna

And because of this, the oldest surfaces of the moon are in something called crater saturation.

Basically, every square inch is covered by craters.

You have craters on top of craters.

Every new crater that forms is destroying an old crater.

The frustrating thing about this is

is that this is the period of time when the moon had the most internally driven activity, volcanism, tectonism, all that cool stuff that I want to know about, but I look at the surface and all I see are the craters, which of course are interesting in their own right, but I want to know what else was happening on the moon, and that's where gravity data is key.

We can take gravity and look through those craters to see what's going on within the crust, within the mantle below, and say something about early lunar evolution.

All right, so now getting to this Grail gravity data.

There's a couple of basic terms and basic concepts that I want to get across first before we really try to interpret this data.

This is a map of the Grail gravity data.

It's something that we call free air gravity.

That's just the gravity as you would measure it.

If you're standing on the surface, if you've got a satellite, this is the gravity field as you measure it.

But when you look at this, what you see is a lot of craters, the stuff that I was saying I want to be able to see through to the interior.

But here, these craters, this is all driven by topography.

Just like Everest and Pratt knew that the topography of the Himalayas should produce a gravitational attraction that they could calculate, we know that the topography of the moon should produce a gravitational attraction that we can similarly calculate.

And so we can do this just like Everson Pratt.

We can calculate the gravity field of the moon as we would expect it to be from topography.

We can subtract that from what we actually observe and see where the differences lie.

That gives us something called Bouguer gravity.