Jeffrey Andrews-Hanna

One of the things we see, glance up at the full moon, the man on the moon, those dark spots are volcanic plains.

Lava flows called maria.

And so this is something you can just see with your naked eye.

So there's been volcanic activity on the moon.

For me as a geophysicist, as a geodynamicist, I really want to understand the dynamic activity of the moon.

And for that, I'm interested in heat.

Because the terrestrial planets, Mercury, Venus, Earth, Mars, the moon, internally driven activity is driven by heat.

And that heat is produced by radioactive elements that are present in virtually all rocks.

So the big ones for the interior of the terrestrial planets are uranium, thorium, and potassium.

Trace amounts of these radioactive elements give off heat.

It's not a lot of heat, but add that up over the entire interior of a planet, and it's enough to drive things like volcanoes, earthquakes, plate tectonics, the Catalina Mountains.

All of these are ultimately driven by heat produced in the Earth, and the moon would have had radioactive heat as well.

But it's the nature of radioactivity that it decreases over time.

So in the earliest history of the moon, four and a half billion years ago, you had a lot more heat being generated in its interior than we have today.

And so if we're interested in dynamic, internally driven activity, it's that first half a billion, billion years is really where all the action should be.

But we also know that there's craters on the Moon.

And we know craters are still forming today.

We see them forming today.

But the rate of cratering was much, much higher as you go back in time.

In particular, about 4 billion years ago, the crater flux was orders of magnitude greater than it is today.