Is it true that the reason the far side of the Moon has more impact craters than the near side is because the Earth shields the near side?
Thank you very much.
The Earth partially shields the near side of the Moon from incoming asteroids, but that is not a large enough effect to influence crater densities. Just using simple straight-line geometry, you can calculate how much of the lunar sky is obscured by the Earth, about 4 square degrees out of 41,000 sq degrees for the whole sky. This makes the Earth negligible as a shield for the Moon. The real reason there are more impact craters on the far side of the Moon is that the near side has a much thinner crust which has allowed volcanoes to erupt and fill in ancient large basins (or large impact craters). These large lava flows have covered craters that were formed early in the Moon’s history through the late heavy bombardment, which is when the largest percentage of impacts were occurring in the inner solar system. It is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though the both sides have received the same number of impacts. Further, the oldest areas in both near and far side are saturated, meaning that they have reached equilibrium (each new crater, on average, destroys one old one). In this case, the density of craters is no longer an accurate measure of the number of hits the surface has received.
David Morrison, Senior Scientist
Brad Bailey, Staff Scientist