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Most of the impact basins---craters measured in hundreds of kilometers---were made during this time.It is noteworthy that about the time the heavy bombardment ended, life took hold.The last stage of that "sweeping up", called the occurred from about 4.1 to 3.8 billion years ago.Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month.Victoria Crater on Mars as seen by the Mars Reconnaissance Orbiter. It is a simple crater that has been partially filled in with sand---note the sand dunes on its floor.If you download the full-resolution version of this picture from the link on the picture, you will be able to see the rover, Opportunity next to the top left crater rim.

Select the image to go to the Selene website for a great flyover of the crater.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. Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics. Links to external sites will be displayed in another window.