Figure 3.4 is an image of the Argyre Basin, which is an ancient impact crater about 1,000 km in diameter. Notice that the original crater rim has been obliterated by numerous subsequent smaller impact events. This view was taken from an orbiter. Crater density on Mars is significantly lower than that of the Moon and Mercury. This is probably because: (a) the number of asteroids and meteorites decreases the further a planet is from the Sun, and (b) subsequent geological processes on Mars, such as hydro-processes, aeolian processes and volcanism, have significantly destroyed the earlier impact history of the planet.