Oldest thing found carbon dating
The longer the surface has been exposed to space, the more craters it will have.If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area.Also the pressure from the surrounding solid rock squeezes the molten rock upward.Molten rock contains trapped gases that expand as it rises causing it to rise even faster.Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets. Worlds with less volcanism or erosion or tectonic activity in their history will retain more impact craters since the planet formed.Worlds with more geological or erosional activity will have newer surfaces or craters that have been so worn away as to be unrecognizable.
Select the image to go to the Selene website for a great flyover of the crater.
The explosion is what carves out the crater so almost all craters are round (otherwise the great majority would be oblong in shape).
See the "Not Round" page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).
Some of them have orbits that cross the orbits of the planets and moons.
When they get close enough to a planet or moon, they will be pulled in by the large body's gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet.