Smithsonian Study Maps 10,000 Young Moon Faults and Finds Our Neighbor Shrank 45 Meters
Lunar Reconnaissance Orbiter images show the Moon's radius has dropped about 148 feet over the past 400 to 800 million years. Moonquakes could strike anywhere on its surface.
The Moon is slowly shrinking, and a new study has put the most detailed number yet on how much. Published Oct. 14 in The Planetary Science Journal, the paper identifies more than 10,000 young faults spread across the lunar surface and estimates that the Moon's radius has decreased by about 45 meters, or 148 feet, over the past 400 to 800 million years.
The study was led by Thomas R. Watters, scientist emeritus at the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum, with co-authors Maria Banks, Cole Nypaver and Mark Robinson. Its title is "Thrust Faulting and the Late-Stage Contraction of the Moon." The team used images from the Lunar Reconnaissance Orbiter Camera to map thousands of contractional landforms called lobate scarps and small mare ridges, in both the lunar highlands and the maria, the vast dark plains visible from Earth.
If the rate of shrinking has stayed constant over that period, the Moon has been contracting by about 6 to 11 centimeters (2.4 to 4.3 inches) per million years, or about 0.1 micrometers a year. That is far slower than the average speed of Earth's tectonic plates, about 1.5 centimeters (0.6 inches) a year, roughly the rate fingernails grow. Earth's plate motion is driven by heat loss from the planet's interior.
The Moon works differently. Earth has more than 15 lithospheric plates, and its plate boundaries localize about 90% of the planet's tectonic and seismic activity. The Moon is a single-plate body, so its crust acts like an unbroken shell. As the interior cools and contracts, faults are not concentrated along boundaries but spread widely over the surface. That means moonquakes, whether from slip on existing faults or from the formation of new ones, can occur anywhere on the Moon.
The study also found that some areas have more faults than others. The thickest lunar crust appears to have the fewest faults, which suggests those regions may be the most stable. That detail matters to anyone planning long-term bases on the surface.
One puzzle the paper addresses is timing. The researchers suggest widespread faulting did not happen early in lunar history because a thick, heavily fractured, porous crust insulated the interior and held in heat, inhibiting cooling and contraction until relatively recently. The authors describe a hypothetical heat trap that has kept the interior warm, with the liquid outer core surrounding a solid inner core, and that delayed the crustal faulting that continues today.
The Lunar Reconnaissance Orbiter has circled the Moon for 17 years, and its camera has returned millions of images of the surface. That archive made the survey possible. "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."
The finding gives mission planners a clearer picture of a Moon that is still geologically active, if only slightly, and shows where the ground is least likely to move.
