Scientists trace tiny moonquakes to Apollo 17 lander – left over from 1972

The leftover lunar descent module, which carried the Apollo 17 crew to the surface of the Moon in 1972, triggers tiny artificial moonquakes that rumble through Earth's natural satellite every lunar morning, according to research.

Despite having no tectonic plates or volcanic activity, the Moon is still geologically active. Its surface expands and contracts due to extreme temperature fluctuations that can range from 250 degrees Fahrenheit (~121.1°C) during the day to -208 degrees Fahrenheit (-133.3°C) at night. It's worth noting that the lunar day-night cycle equates to roughly 30 days on Earth.

These changes can cause thermal moonquakes, or measurable surface vibrations. Other events – such as meteoroids crashing, or gravitational interaction with the Earth, which can squeeze and stretch its interior – can lead to moonquakes too.

Academics at the California Institute of Technology (Caltech) and NASA have found a third source of lunar rumbling: the Apollo 17 lunar module itself.

The astronauts who rode that module down installed seismometers on the lunar surface; thermal moonquakes were measured using this equipment from October 1976 to May 1977, with the readings beamed back to Earth for future analysis. Fast forward to this decade, and scientists have studied that old data with machine learning algorithms and found a set of recordings that stood out.

"We developed algorithms to accurately determine the arrival timing of the waves, measure the strength of the seismic signal, and find the direction of the moonquake source," they explained in their study published in the Journal of Geophysical Research – Planets earlier this month.

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They found moonquakes that occurred every morning without fail that were surprisingly uniform, and traced them back to the leftover Apollo 17 base. "Every lunar morning when the sun hits the lander, it starts popping off," Allen Husker, a co-author on the paper and research professor of geophysics at Caltech, explained 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 tremors, emitted from the lunar lander swelling in the heat and shrinking in the cold, are tiny and would not be felt by astronauts, but the seismometers are sensitive enough to detect them since they're only located a few hundred metres away. The university team believes that the artificial moonquakes are important to study as space agencies and companies look to build infrastructure on the Moon. They will have to take into account the thermal effects the Sun has on the Moon and the stuff we install on it.

Husker said the signals may also help them search for water – a vital resource that will help astronauts live and work on the Moon, which has kickstarted a race to explore the lunar south pole.

"There are also certain regions in craters at the Moon's south pole that never see sunlight; they are permanently shadowed. If we could put up a few seismometers there, we could look for water ice that may be trapped in the subsurface; seismic waves travel slower through water," he explained. 

The Register has asked the lead author of the paper and Caltech for further comment. ®