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A spacecraft left by US astronauts on the lunar surface could cause small tremors known as lunar earthquakes, according to a new study.
Researchers have revealed the previously unknown form of seismic activity on the Moon for the first time through an analysis of Apollo-era data using modern algorithms.
The massive temperature changes occurring on the Moon may cause man-made structures to expand and contract in a way that produces these vibrations, the report suggests. The lunar surface is an extreme environment, fluctuating between minus 208 degrees Fahrenheit (minus 133 degrees Celsius) in the dark and 250 degrees Fahrenheit (121 degrees Celsius) in direct sunlight, according to a press release on the study.
In fact, the entire surface of the Moon expands and contracts with cold and heat, notes the study published September 5 in the Journal of Geophysical Research: Planets. Yet scientists were able to use a form of artificial intelligence to gain such an in-depth understanding of Apollo-era data that they could spot slight tremors emitted by an Apollo 17 lunar lander located just a few hundred meters away from instruments that recorded earthquakes. according to a synopsis of the study conducted by researchers from institutions including the California Institute of Technology and NASA. (NASA provided funding for the study.)
The analysis offers new insights into how the Moon responds to its surroundings and what may influence its seismic activities. The rumbles were not dangerous and would likely have been imperceptible to humans standing on the lunar surface.
Understanding earthquakes could be essential for future exploration, experts say, should NASA and its partners build a permanent outpost on the lunar surface — a goal of Artemis, the agency’s lunar exploration program.
“How robust are our structures and what other risks do we need to mitigate against?” Dr. Angela Marusiak, assistant research professor at the University of Arizona’s Lunar and Planetary Laboratory, talked about the questions this type of data analysis can help answer. Marusiak was not directly involved in the study, although she had contact with the authors as a fellow expert in lunar seismology.
Marusiak noted that each Apollo mission carried instruments to detect lunar earthquakes. But the Apollo 17 mission, launched in 1972, was notable because it left behind a series of seismometers capable of detecting thermal earthquakes – or tremors induced by the drastic heating and cooling of the moon’s surface.
“Thousands of these signals were recorded during an 8-month span from 1976 to 1977 on four seismometers used during the Apollo 17 lunar seismic profiling experiment, but poor data quality makes analysis difficult,” they wrote researchers. “We have developed algorithms to precisely determine the arrival times of the waves, measure the strength of the seismic signal and find the direction of the earthquake source.”
Scientists revisited the data for the first time in decades. The new analysis allowed the research team to conclude that a certain type of lunar earthquake – called an impulsive thermal lunar earthquake – did not come from natural sources but rather from the heating and cooling of nearby spacecraft.
“Every lunar morning, when the sun hits the lander, it begins to pop out,” study co-author Allen Husker, a research professor of geophysics at Caltech, said in a statement. “Every five or six minutes (there was) another one, for a period of five to seven Earth hours. They were incredibly regular and repetitive.”
According to the study, these tremors differed from another type of lunar earthquake, called an emergent thermal moonquake, which is likely caused by the ground’s natural reaction to exposure to sunlight.
More seismic activity
Researchers hope that future lunar missions will offer an even more holistic picture of the phenomenon.
In addition to thermal earthquakes, the Moon is also known to exhibit deep and shallow tremors, as well as activity believed to be caused by falling meteorites.
It is important to note a key difference between the Moon and Earth: on the lunar surface there are no moving tectonic plates that could cause catastrophic events. But the Moon has an active inner life, and like Earth, some types of seismic events can occur at any time or place on the lunar surface, Marusiak said.
Marusiak was excited about the mission of India’s lunar lander, Chandrayaan-3, which included a seismometer. The Indian Space Research Organization has already confirmed that the instrument was able to detect a lunar earthquake. (ISRO researchers have not yet released extensive recording data or proposed a suggested cause for the event.)
The Chandrayaan-3 instrument, which recorded activity near the lunar south pole for the first time, was put on hold in early September. Researchers will attempt to wake the spacecraft for further data collection on September 22, when Chandrayaan’s landing site re-enters sunlight.
“I hope that with the Artemis program, seismometers will continue to be included because they are really vital to understanding what’s going on, not only at the surface, but also deeper into the regolith (soil),” Marusiak said.
But scientists are excited that in-depth analysis of Apollo-era data with modern technology could produce fascinating new results.
“It’s important to know as much as possible from existing data so we can design experiments and missions to answer the right questions,” Husker said. “The Moon is the only planetary body other than Earth to have had more than one seismometer at a time. It gives us the unique opportunity to study another body in depth.”
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