Earth’s moon is shrinking. Here’s what scientists say this could mean

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An area of ​​the moon that is at the center of a new international space race because it may contain water ice could be less hospitable than once thought, new research shows.

Interest in the moon’s south pole increased last year when India’s Chandrayaan-3 mission made the first successful soft landing in the area, just days after the Russian Luna-25 spacecraft crashed en route to attempt the same. NASA has chosen the region as a landing site for its Artemis III mission, which could mark the return of astronauts to the moon as early as 2026, and China also has plans to create future habitats there.

But now a NASA-funded study is raising alarm bells: As the moon’s core gradually cools and shrinks, its surface develops folds — like a grape shriveling into a raisin — that cause “moonquakes” that can last for hours, as well as landslides. . Like the rest of the natural satellite’s surface, the region of the South Pole that is the subject of so much interest is susceptible to these seismic phenomena, potentially threatening future human settlers and equipment.

“This is not to alarm anyone and certainly not to discourage exploration of that part of the moon’s south pole,” said the study’s lead author, Thomas R. Watters, a senior scientist emeritus at the National Center for Earth. Air and Space Museum. and Planetary Studies, “but to warn that the moon is not this benign place where nothing happens.”

Finding the source of moonquakes

The moon has shrunk in circumference by about 45 meters over the past few million years – a significant amount in geological terms, but too small, researchers say, to cause any ripple effect on Earth or tidal cycles.

On the moon’s surface, however, it’s a different story. Despite what its appearance suggests, the moon still has a warm interior, making it seismically active.

“There is an outer core that is molten and cooling,” Watters said. “As the moon cools, the moon shrinks, its interior volume changes, and the crust has to adapt to that change – it’s a global contraction, to which tidal forces on Earth also contribute.”

Because the moon’s surface is brittle, this pulling causes cracks, which geologists call fractures. “The moon is seen as a geologically dead object where nothing has happened for billions of years, but that couldn’t be further from the truth,” says Watters. “These mistakes are very young and things happen. We even detected landslides that occurred during the time the Lunar Reconnaissance Orbiter was in orbit around the moon.”

NASA’s Lunar Reconnaissance Orbiter, or LRO, was launched in 2009 and maps the moon’s surface with several instruments. In the new study, published Jan. 25 in The Planetary Science Journal, Watters and his colleagues used data collected by LRO to link a powerful moonquake — detected with instruments left behind by Apollo astronauts more than 50 years ago to a series of errors in the moon. South Pole.

“We knew from the Apollo seismic experiment, which consisted of four seismometers operating for about seven years, that there were shallow moonquakes, but we didn’t really know what the source was,” Watters added. “We also knew that the largest of the shallow moonquakes detected by the Apollo seismometers was near the south pole. It became something of a detective story to try to figure out what the source was, and it turns out that these young mistakes are the best suspects.

The strongest recorded earthquake was the equivalent of magnitude 5.0. On Earth that would be considered moderate, but the moon’s lower gravity would make it worse, Watters said.

“On Earth you have a much stronger force of gravity holding you to the surface. On the moon it’s much smaller, so even a little bit of ground acceleration will potentially knock you off your feet as you walk along,” he said. “That kind of shaking can really throw things around in a low-G environment.”

Moonquakes: Short-Term Versus Long-Term Implications

The study’s findings will not impact the Artemis III landing region selection process, and that is due to the size and duration of the mission, said study co-author and NASA planetary scientist Renee Weber.

“This is because it is difficult to accurately estimate how often a specific region experiences a moonquake, and as with earthquakes, we cannot predict moonquakes,” Weber said. “Strong shallow moonquakes are uncommon and pose a low risk to short-term missions on the lunar surface.”

NASA has identified 13 Artemis III landing sites near the moon’s south pole, she added, using criteria such as the ability to land safely in the region, the potential to meet science objectives, the availability of launch windows and conditions such as terrain, communications and lighting. . As part of the mission, two astronauts will live and work on the lunar surface for about a week.

However, Weber said that for a long-term human presence on the moon, the site selection process could indeed take into account geographic features such as proximity to tectonic features and terrain.

Like flashlights in the moon

Moonquakes could indeed be a problem for future manned landing missions, said Yosio Nakamura, professor emeritus of geophysics at the University of Texas at Austin, who was among the researchers who first looked at the data collected by Apollo’s seismic stations.

However, Nakamura, who was not involved in the research, disagrees about the cause of the earthquakes, saying Apollo data shows the phenomena originate tens of kilometers below the surface.

“We still don’t know what causes shallow moonquakes, but it’s not the sliding fault near the surface,” he said. “Regardless of the cause of these earthquakes, it is true that they pose a potential threat to future landing missions, and we need more data on them.”

Regardless of the underlying cause, the potential danger that moonquakes pose to astronauts will be limited by the fact that — at least in the near future — humans will be on the moon for short periods of time, a few days at most, Allen Husker said. , a research professor of geophysics at the California Institute of Technology, who was also not involved in the study.

“It is very unlikely that a major moonquake will occur while they are there. However, it is good to know that these seismic sources (which cause the earthquakes) exist. They could be an opportunity to better study the moon like we do on Earth with earthquakes,” Husker said. “By the time there is an actual moon base, we should have a much better idea of ​​the actual seismic hazard on future missions.”

That sentiment is echoed by Jeffrey Andrews-Hanna, an associate professor of planetary science at the University of Arizona, who also did not participate in the work. “Moonquakes are an incredible tool for doing science,” he said in an email. “They are like flashlights in the interior of the moon, illuminating its structure so we can see them. Studying moonquakes at the South Pole will tell us more about the moon’s internal structure and its current activity.”

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