The Moon's Geological History

An image of the heavily-cratered far side of the Moon.
Click on image for full size (87K JPEG)
Image courtesy of NASA.

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.

Last modified January 5, 1998 by the Windows Team

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The Moon's Geological History

An image of the heavily-cratered far side of the Moon. Click on image for full size (87K JPEG) Image courtesy of NASA.

An image of the heavily-cratered far side of the Moon.
Click on image for full size (87K JPEG)
Image courtesy of NASA.