Jeffrey Andrews-Hanna

So radio waves can propagate through rock to distances of a kilometer or so.

And we've sent a few different radar instruments to the moon, both on the Apollo missions as well as more recently the Japanese Kaguya mission.

And those radar sounders could probe to depths of about a kilometer

They saw things like layering and basaltic lava flows on the moon.

So this is one way to reveal the lunar interior, but again, we're only getting the top kilometer, so there's a lot more down there to see.

Seismic waves are another way to study the moon's interior.

Seismic waves on the moon can propagate throughout the moon, just like they propagate throughout the Earth.

And we can measure those seismic waves and use their properties to try to understand the structure of the moon.

Where do these seismic waves come from?

Well, the moon has moonquakes similar to the way the Earth has earthquakes.

And there are also impacts on the moon, where either meteorites or man-made spacecraft have impacted into the moon and created seismic waves.

This is another approach.

We can image or at least examine the interior structure of the moon.

Seismic data is a little difficult, though, because there are only a few seismometers put on the moon back in the Apollo days.

They're not functional anymore.

We've got limited coverage.

So my own personal favorite way to study the interior of the moon is with gravity data.

So gravity, in some senses, is kind of an obscure field that's difficult for people to wrap their mind around.

But really, gravity is one of the simplest things out there.

We all understand the basic idea of gravity.