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The moon may still be on.
Rumors recorded by seismometers at Apollo's landing sites are probably linked to young faults mapped by NASA's Lunar Reconnaissance Orbiter, scientists said. Eight of these moonquakes took place within a 30-kilometer radius of fault breakouts, steep cliffs on the lunar crust that mark places where one side of the fault has slipped or fallen. If this is true, the discovery suggests that the moon is still tectonically active today, researchers reported on May 13 in Nature Geoscience .
To learn more about this activity, especially where the moon's surface is still moving, could help scientists identify where and where not to land future spacecraft ( SN: 11/24 / 18, p 14 ).
Unlike the Earth, moon tremors are not produced by many large tectonic plates that separate, collide, or slide over each other. Instead, like Mercury and Mars, "the moon is basically a planet to a plate," says Thomas Watters, scientist in planetary sciences at the Smithsonian Institution in Washington, who led ;study.
Even plate objects can be used. have earthquakes ( SN Online: 4/23/19 ). As these objects cool down and the interior contracts, their hard external shell, or lithosphere, compresses and cracks as well. This compression can produce earthquakes. The interior of the moon having cooled, it is thought that its radius has decreased by about 100 meters. But if the moon is still active on the tectonic plane today is a mystery.
In 2010, Watters led a team that examined images of the Lunar Reconnaissance Orbiter, launched in 2009, and identified many sinuous cliffs widely distributed on the surface. Called lobed escarpments, these elements, whose height varies from a few tens to a few hundred meters, represent faults pushed, places where the surface contracts as the moon cools. In the end, the team estimated that these snails were not more than 50 million years old
. He suspected that the defects were much, much younger.
The team then turned to the thousands of lunar tremors detected from 1969 to 1977 by NASA's pbadive seismic experiment, consisting of four seismometers installed by astronauts at the Apollo landing sites. Most of the moonquakes were small and came from inside the moon. But 28 earthquakes were larger and shallower, taking their origin only 200 kilometers from the surface. Even then, some scientists have suspected that the moon tremors could be related to ongoing tectonic activity.
"They had seismic data, but what they did not have was potential sources," says Watters. Now, LRO had provided evidence of abundant faults, "thousands of potential sources".
Sinuous fault
Features such as the curved outcrop of a lobed crack bark, a cliff-like cliff on the moon (white arrows), indicate the surface of the moon is compressing while its interior cool down. A new study reveals that some of these features may be very recent.
But it was difficult to accurately determine the origin of the earthquakes and possibly link them to the faults observed, because the seismometers were grouped relatively close together at the landing sites. The team used a mathematical program to better identify the epicentres of earthquakes, then tried to map them to scarves. Epicentres located more than 30 kilometers from any scarf were considered unbound.
"We found eight at less than 30 kilometers," says Watters. Close matches suggest that the moon continues to actively contract. "It's a data that's only 40 years old," says Watters. "If we detect these landslide events 40 years ago, these flaws are still active." That, he says, must also mean that the moon still has a lot of heat in it.
Always, the motive of the faults. was confusing. An overall contraction of the moon's surface should create a random set of faults. Instead, the faults had a distinct pattern: in the equatorial and mid-latitude regions, they tended to go from north to south. Near the poles, they were oriented east-west.
The only other force powerful enough and close enough to be able to act powerfully on the moon is the Earth. The team therefore examined the timing of earthquakes relative to the position of the moon in its elliptical orbit around the Earth. Scientists discovered, to their surprise, that 18 of the 28 shallow tremor events occurred when the moon was farthest away from Earth, called its climax.
This is counterintuitive, but this conclusion actually reinforces the idea that the Earth is putting extra pressure on the Earth. moon, says Watters. "Stress is the force on a surface unit. When the moon is at its peak, the unit area on which the Earth acts is actually larger. The moon also slows a little as the apogee approaches, leaving more time to accumulate, as a result of changes in the gravitational pull of Earth.
"I would have been surprised that the moon was tectonically active if you asked me 10 years ago," says Amanda Nahm, planetary geologist at Berlin, of Arctic Planetary Science Institute. "The more we learn about these little bodies, the more we realize that they are so much more interesting and dynamic than previously thought," says Nahm, who did not participate in the study. ;study. "The moon is no longer considered" dead "."
Identifying the active faults could be the key to any future project of longer term presence on the moon. "I would not want to be within 30 kilometers of one of those flaws," says Watters. And the reduced gravity could produce significant tremors even of a weak moonquake. "It will not take a lot of shaking to knock you out."
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