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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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