A team of scientists led by Thomas Watters announced in the August 20, 2010 issue of Science that the Moon is shrinking. According to their paper, the radius of the Moon has decreased by about 100 meters in the last billion years or less.
Watters' team studied images of the Moon's surface made with the Lunar Reconnaissance Orbiter Camera (LROC). The astronomers studied geologic features on the Moon known as lobate scarps with the LROC. Scarps are cliffs caused by fault lines or vertical movement of the crust. Lobate scarps are curved and are indicative that the Moon or planet is shrinking.
The planet Mercury, whose surface is very similar to the cratered highland areas of the Moon, also has many scarps indicating that Mercury has shrunk. Mars also has scarps. The lunar scarps are smaller scale structures than the scarps on Mercury or Mars. Scarps on Mercury or Mars can be more than a kilometer high and extend for hundreds of kilometers. Lunar scarps are less than 100 meters high and a few dozen kilometers long at most.
The Apollo missions imaged scarps near the Moon's equator, but did not study the entire Moon. The new LROC images discovered 14 new scarps distributed over the entire Moon. The global distribution indicates that the Moon is shrinking globally.
Astronomers estimate the age of geologic features on the Moon by the number of craters. More heavily cratered areas are geologically older. The lunar scarps are the youngest tectonic features on the Moon and are less than a billion years old. The scarps most likely formed from the stress on the surface as the Moon contracted, and their scale indicates that the Moon's radius has decreased by about 100 meters.
The Moon has shrunk by about 100 meters in less than the last 1 billion years. Why?
The Moon was hot and molten when it first formed. The Moon gradually cooled and solidified. The lunar interior, while not as hot as Earth's interior, is still warm. So the Moon continues to cool and shrink slightly as it cools.
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