An image of the heavily-cratered far side of the Moon.
The Moon's Geological History
Scientists have studied the ages of rocks in cratered regions and have
determined the rate of cratering at various epochs in the Moon's past.
By studying the light-colored highlands
found that there was a period of heavy bombardment very early
Moon's history, from about 4.6 to 3.8 billion years ago. After that,
the rate dropped off substantially and much less cratering has
occurred since then.
Rock samples from large impact basins show that about 3.1 to 3.8
billion years ago several asteroid-size objects impacted the Moon,
just as the period of heavy bombardment was ending. This was shortly
followed by lava flows which filled in the basins and formed the dark
maria. This explains why craters are
relatively few and isolated on the dark maria, but dense and
overlapping 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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