Gravity tends to consolidate material into larger and larger clumps over time. When the solar system first began to form, it was less like the neat and tidy collection of round planets it is today, and more of a disk of material and smaller bodies. So the further back in time you go, the more impacts there were on a surface, because there were more small bodies available for a planet or moon to collide with.
This pattern can be observed in the age-distribution of lunar craters - there are relatively more old craters than you would expect if the rate of bombardment was constant over time.
Mar 21st, 2015
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