Crater counting is a method for estimating the age of a planet's surface. The method is based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known. Consequently, counting how many craters of various sizes there are in a given area allows determining how long they have accumulated and, consequently, how long ago the surface has formed. The method has been calibrated using the ages obtained by radiometric dating of samples returned from the Moon by the Luna and Apollo missions. It has been used to estimate the age of areas on Mars and other planets that were covered by lava flows, on the Moon of areas covered by giant mares, and how long ago areas on the icy moons of Jupiter and Saturn flooded with new ice.
What one needs to count using the crater counting method are independent craters. However, not all craters on a given surface are independent: Secondary craters are craters formed by materialexcavated by a primary impact that falls back to the surface seconds or minutes later. A way to distinguish primary and secondary craters is to consider their geometric arrangement; for example, large craters often have rays of secondary craters. Secondaries can sometimes also be recognized by their particular shape different from primary craters; this is due to the fact that the excavated material is slower and impacts at a lower angle than asteroids that arrive from space to create the primary crater. The accuracy of age estimates of geologically young surfaces based on crater counting on Mars has been questioned due to formation of large amounts of secondary craters. In one case, the impact that created Zunil crater produced about a hundred secondary craters, some more than 1000 km from the primary impact. If similar impacts also produced comparable amounts of secondaries, it would mean a particular crater-free area of Mars had not been "splattered by a large, infrequent primary crater", as opposed to suffering relatively few small primary impacts since its formation.
