An image of the heavily-cratered far side of the Moon.
NASA.
The Moon's Geological History
Scientists have studied the ages of rocks in regions with craters and
determined when in the Moon's past the craters were forming most
quickly. By studying the light-colored regions, called
highlands, they found that from about 4.6 to 3.8
billion years ago rocky debris
rained down on the surface of the young
Moon, forming craters very quickly. Then the rocky rain subsided, and
fewer craters have formed since then.
Rock samples from very large craters (called basins) showed that about
3.8 to 3.1 billion years ago several huge, asteroid-like objects
struck the Moon, just as the rocky rain was ending. This was shortly
followed by lava flows which filled in the basins and formed the dark
maria. This explains why there are so few
craters on the maria, but dense, overlapping craters in the highlands.
No lava flows occurred on the highlands to erase the original blanket
of craters from the time when the Moon's surface was showered with the
debris of the early solar system.
The far side of the Moon has only one small maria. So lunar
geologists believe that the far side is very representative of how the
Moon looked 4 billion years ago.
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