Sensors placed on the surface of the Moon during the Apollo 17 mission picked up a mysterious tremor, one that regularly occurred as the Sun rose to its peak position over the lunar surface. Unlike regular moonquakes that are triggered by the varying temperatures of the lunar environment, however, this one had a rather peculiar, human-made source.
During the Apollo 17 mission in 1972, astronauts placed seismometers on the lunar surface to measure moonquakes. More than 50 years later, a group of scientists reanalyzed the data collected by the last crewed mission to the Moon using new techniques such as machine learning. The reanalysis revealed a new kind of seismic activity that took place at the same time during the lunar mornings, which turned out to be coming from the Apollo 17 lunar lander.
Moonquakes are the result of temperature swings on the Moon, which vary from 121 degrees Celsius during the day to -133 C at night. A full day on the Moon, including both day and night, is roughly equivalent to the same amount of time as 29.5 Earth days. These extreme temperature variations cause the Moon’s surface to expand when it’s hot and contract when it’s cold, causing it to shake and crack.
The Apollo 17 sensors collected data from October 1976 to May 1977. Thermal moonquakes transpired during the afternoon as the Sun began to descend from its peak position over the lunar surface. The additional tremors, on the other hand, were regularly set off as the Sun reached its peak position during the lunar morning. The scientists behind the new study were able to trace the source of the morning moonquakes to the Apollo 17 lunar lander located just a few hundred meters away from the sensors.
“Every lunar morning when the sun hits the lander, it starts popping off,” Allen Husker, research professor of geophysics at Caltech, and co-author of the study published in the Journal of Geophysical Research: Planets, said in a statement. “Every five to six minutes another one, over a period of five to seven Earth hours. They were incredibly regular and repeating.”
The newly detected tremors on the Moon, while not originating from the Moon itself, could provide valuable insights into the thermal expansion and contraction of the Apollo 17 lander, potentially guiding the design of future lunar landers.
Studying moonquakes also helps scientists better understand what goes on beneath the lunar surface since seismic waves travel at different speeds through different material. Husker hopes to be able to place seismometers on the Moon’s permanently shadowed regions where there may be reservoirs of water ice beneath the surface, measuring seismic waves as they travel slower through water.
“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,” Husker said.