A new estimate puts the Moon’s shrinkage at a few inches per million years

The Moon is shrinking, but at a pace far too slow to notice from year to year. A Popular Science report published October 14 describes new calculations that put the change at a few inches per million years. The estimate offers a long-term measure of lunar cooling and contraction, while the distribution of surface faults may help researchers understand which regions are more geologically active.

The report describes a study led by planetary geologist Thomas Watters of the Smithsonian National Air and Space Museum and published in The Planetary Science Journal. The researchers used images from NASA’s Lunar Reconnaissance Orbiter Camera (LROC), which the report says has produced millions of lunar images over more than 17 years. Reviewing those images, the study’s authors counted more than 10,000 visible lunar faults.

According to the report, the calculations indicate that the Moon’s overall radius has decreased nearly 148 feet over a period of 400–800 million years. The report gives a rate of about 2.4 to 4.3 inches per million years, or roughly 0.1 micrometer annually. These are estimates over immense timescales, not a sign of a sudden change.

Cooling inside the Moon is the proposed driver: as its interior cools, the body contracts, stressing and breaking its outer rock. The report contrasts the Moon’s shell-like single tectonic plate with Earth, where it says about 90 percent of seismic activity occurs near lithospheric plate boundaries. With no network of plate boundaries to concentrate activity in the same way, lunar faults may occur across a broader range of the surface.

That does not establish that moonquakes happen everywhere or at any moment. The report presents quakes as a possible consequence of fault activity, but does not provide quake measurements, probabilities, or a forecast. Nor does its account give enough detail about the calculation to independently evaluate how the images and fault counts produce the stated rate.

The reported fault pattern is uneven. Regions with the thickest crust have fewer visible faults, which the researchers interpret as a possible sign that those areas are more stable than thinner-crust regions. That pattern could help NASA plan for a long-term human presence on the Moon. The report, however, does not identify specific landing sites or quantify the risks to people or structures. A distribution of faults can inform questions about lunar geology; on its own, it is not a prediction of when or where a quake will occur.