Scientists have presented the most detailed assessment to date of how much the Moon has shrunk over the past hundreds of millions of years. The study was published on October 14 in the journal The Planetary Science Journal.
The authors, led by Thomas R. Watters of the Smithsonian National Air and Space Museum, used images from the Lunar Reconnaissance Orbiter Camera (LROC), a camera aboard the Lunar Reconnaissance Orbiter probe, which has been orbiting the Moon for 17 years. They mapped thousands of young faults, so-called lobate scarps and small ridges on the lunar maria (dark plains).
According to the researchers' estimate, over the past 400-800 million years the Moon's radius has decreased by about 45 meters. If the rate remained constant, the Moon contracted by 6-11 centimeters per million years, or about 0.1 micrometers per year. For comparison: Earth's plates move on average about 1.5 centimeters per year, roughly the rate at which fingernails grow.
Earth is covered by a mosaic of more than 15 lithospheric plates, and about 90% of tectonic and seismic activity is concentrated at their boundaries. The Moon, however, is a single plate, so its faults are distributed across the entire surface. This means moonquakes can occur anywhere.
The study also showed that the density of faults is uneven: in areas with the thickest crust there are the fewest, which makes such zones the most stable. The authors suggest that mass fault formation did not occur in the Moon's early history, since a heavily fractured and porous crust hindered the cooling of the interior. Only relatively recently, when internal heat began to escape faster, did active cracking begin.
"The number of these young faults in the LROC images turned out to be far greater than I could have imagined," Watters noted. "Since the Moon's interior continues to cool and contract, the growth of existing faults and the formation of new ones is probably continuing today as well."
The work does not prove that contraction is occurring at exactly this rate; it is an estimate based on the assumption of a constant rate. Nevertheless, it provides the most complete picture to date of the Moon's young tectonics.