It’s true, the moon is shrinking. And astronomers now know how fast it’s contracting. While only satellite observations and the most delicate instruments can detect these shifts, the consequences of this shrinkage means moonquakes can occur anywhere on the lunar surface at a moment’s notice.
A team led by Smithsonian National Air and Space Museum planetary geologist Thomas Watters detailed their findings in a study recently published in The Planetary Science Journal. Researchers used data from NASA’s Lunar Reconnaissance Orbiter Camera (LROC), which has spent over 17 years generating millions of images of the moon’s surface. A shriveling lunar landscape may sound dramatic, but it’s occurring at a glacial pace. Based on these new calculations, the moon’s overall radius has decreased nearly 148 feet over the last 400–800 million years as it continues to cool. That’s about 2.4 to 4.3 inches per million years, or just 0.1 micrometer annually.
Although slow, the incremental activity’s effects are visible everywhere. On Earth, about 90 percent of all seismic activity occurs near the boundaries between lithospheric plates. But unlike its host planet, our moon is encased by a single, shell-like plate. This means interior cooling can generate slips and faults anywhere at any given time. After reviewing images taken by the LROC, the study’s authors tallied over 10,000 visible lunar faults.
“The number of these young faults revealed in LROC images is much greater than I ever imagined,” said Watters. “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.”
While the faults can happen anywhere, some lunar regions are more likely than others to feature them. Areas with the thickest crust have fewer faults, which indicates they may be more stable than other, thinner sections. In addition to better understanding lunar history, the distribution and frequency of moonquakes may also help NASA better plan for a long-term human presence on the moon.
