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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 cratered regions and have determined the rate of cratering at various epochs in the Moon's past. By studying the light-colored highlands, they found that there was a period of heavy bombardment very early in the 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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA