Moonquakes Traced to a Human-Made Source

Sensors placed on the lunar surface during the Apollo 17 mission listened for vibrations over a period of several months and helped establish four types of moonquakes: those triggered by meteorite impacts; shallow quakes, possibly triggered by shrinkage due to a cooling interior; deep quakes, tidal in origin; and thermal quakes, triggered by temperature changes in the transition from lunar day (about 250 degrees Fahrenheit) to night (minus 208 degrees), per Scientific American. Now, more than 50 years after the 1972 moon mission, scientists have made a remarkable find in the data obtained: Once the lunar module's descent stage touched down on the moon, it began triggering an entirely new kind of seismic activity.

It was in reanalyzing data on thermal moonquakes, and recognizing that they occurred at the same time every lunar morning and afternoon, that scientists at the California University of Technology realized some morning tremors came outside the established pattern, but still occurred regularly. An artificial intelligence model indicated they weren't thermal moonquakes. And in triangulating the source, geophysicist Allen Husker and his colleagues realized the quakes were originating hundreds of feet from the seismometers left by the Apollo 17 mission, per WION. In fact, they were originating from the lunar module base. The 14-foot-wide octagonal prism on four legs was warming in the lunar morning sunlight, just like the lunar surface.

"Every lunar morning when the sun hits the lander, it starts popping off," Husker, co-author of a study published in the Journal of Geophysical Research: Planets, says in a statement. "Every five to six minutes another one, over a period of five to seven Earth hours." (One full day on the moon equals 29.5 Earth days.) The insight could guide the design of future lunar landers, per Gizmodo. "It's important to know as much as we can from the existing data so we can design experiments and missions to answer the right questions," says Husker in his statement. We may soon know more. India's Chandrayaan-3 lunar lander, equipped with six high-sensitivity accelerometers, is thought to have detected the first evidence of a moonquake since the 1970s on Aug. 26, its third day on the moon, per Space.com. (More moon stories.)