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The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile.
Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward.
There are still small chunks of rock orbiting the Sun left over from the formation of the solar system.
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.
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.