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 areas with craters and
determined when in the Moon's past the craters were forming most
quickly. By studying the light-colored areas of the Moon, called
highlands, they found that from about 4.6 to 3.8
billion years ago pieces of rock
rained down on the surface of the
young Moon, forming craters very quickly. Then the rocky rain slowed
down, 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 hit
the Moon, just as the rocky rain was ending. Lava flowed up to fill
in the basins and formed the dark maria.
This explains why there are so few craters on the maria, but lots of
craters in the highlands. No lava flows occurred on the highlands to
erase the original craters from the rocky rain.
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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