The surface of the planets can explain the dynamics of planetary interiors (for example, how often volcanoes erupt or how often earthquakes occur). In order to see how much the surface has changed over time the age of the surface should be obtained. Scientists can estimate the age of a planetary surface by counting craters. The logic is that the largest bodies (the ones that would form the largest craters) were used up before the smaller ones, since there were fewer of the larger ones to start with. So the larger a crater is, the older the planet surface.