Lunar fault maps reveal how much the moon is shrinking

A new study of young tectonic landforms on the moon offers the most detailed estimate yet of how much the moon has contracted as its interior gradually cooled over time.

Published Oct. 14 in The Planetary Science Journal, the paper, “Thrust Faulting and the Late-Stage Contraction of the Moon,” identifies more than 10,000 young faults distributed across the moon’s surface and estimates that the moon’s radius has decreased by approximately 45 meters (148 feet) over the past 400 to 800 million years.

The study was led by Thomas R. Watters, scientist emeritus in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum, along with co-authors Maria Banks, Cole Nypaver and Mark Robinson. Using observations from the Lunar Reconnaissance Orbiter Camera (LROC), the researchers mapped thousands of contractional landforms known as lobate scarps and small mare ridges across both the lunar highlands and maria, the vast dark plains visible on the moon’s surface.

If the rate of radius decrease has remained constant over this period, it suggests the moon has been shrinking about 6 to 11 centimeters (2.4 to 4.3 inches) per million years or about 0.1 micrometers (0.0001 millimeters) per year. This is much less than the average rate that Earth’s plates move, about 1.5 centimeters (0.6 inches) per year—about the same rate fingernails grow—driven by heat loss from Earth’s interior.

Earth’s plate boundaries localize about 90% of the planet’s tectonic and seismic activity. The moon is a single-plate body with faults that express contraction. Unlike Earth, with its mosaic of more than 15 lithospheric plates, the moon’s single plate acts like an unbroken shell. Thus, as the lunar interior cools and contracts, faults are not concentrated along plate boundaries but are widely distributed over the lunar surface. This means that moonquakes from slip events on existing faults or the formation of new faults can occur anywhere on the moon.

The new study also reveals that some areas have more faults than others, and that the thickest lunar crust appears to have the fewest faults, suggesting these areas may be the most stable. The paper suggests that widespread faulting on the moon did not occur early in lunar history because a heavily fractured, thick, porous crust insulated the interior, inhibiting cooling and contraction until relatively recently.

The Lunar Reconnaissance Orbiter has been orbiting the moon for the past 17 years, and in that time, the LROC has returned millions of images of the lunar surface.

“The number of these young faults revealed in LROC images is much greater than I ever imagined,” Watters said. “As the interior of the moon continues to cool and contract, it is expected that the growth of existing faults and formation of new faults continues today.”

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