My expertise is on basically understanding the orbits and the evolution of small bodies in the Solar System. So I worry about asteroids, I worry about comets, I worry about meteorites. I worry about the evolution of the Moon and the bombardment history of all these worlds.
When you think about our Moon, the thing to realise is that it probably has the most complete and clear history available of the last four and half billion years of Solar System evolution. Almost all of the rocks we have in the Earth only go back to about four billion years ago. So we’re missing almost a half a billion years of Earth’s history. And the only way to really get at that information, for the most part, is to some degree go to the Moon.
If you really understand how the Moon has been affected by bombardment-- by comets and asteroids hitting it over time-- in a sense, you’d tell the story of the Solar System. Very early on in the Solar System, the planets were much more like the asteroids we see today, the comets that we see today. They were just small little chunks of rock and ice and such, but there was a lot of them. So over time, they begin to collide with one another. And eventually they grew into larger and larger objects, and eventually grew into the worlds we see today.
There was a lot of leftovers. The leftovers began to bombard the Moon, but the Moon kept a record of that history. And so that early phase of Solar System history-- what we would call planet formation-- the end game of that is recorded on the Moon.