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The moon might look like a wasteland and dead, but there is much more activity on this gray rock, 4.5 billion years old, than we attribute to it. In a study published this week in Nature Geoscience, a team of researchers showed how the moon shrank, becoming smaller and smaller inside its interior, continuing to cool, shrink and leave behind geophysical faults. It is in this perspective that scientists are now able to measure the degree of activity of the interior of the moon and to see the persistence of moonshares, even today.
"The story really begins with Apollo," says Thomas Watters, lead author of the research paper and senior scientist at the Smithsonian Center's Center for Earth and Planetary Studies in Washington, DC. NASA astronauts have installed five seismic data collection stations on the moon. during Apollo 11, 12, 14, 15 and 16. From 1969 to 1977, the stations recorded four types of seismic activity on the moon, from magnitude 2 to 5: earthquakes due to thermal expansion, earthquakes due to to meteorites, deep moonquakes caused by tidal stress created by the orbit around the Earth and shallow moonquakes that have no perceptible cause.
It was the superficial moon tremors that most intrigued Watters and his team, and they were prompted to look for some relationship with another dataset that summed up his own set of mysteries: huge fault scarves resembling to cliffs ground and dotted the lunar surface.
Since the launch of the Lunar Reconnaissance Orbiter in 2009, we have finally seen how widespread these features are in the world and what they look like in more detail, allowing Watters and his team to map distributions, models and the orientations of the population of faults. And they noticed that these models indirectly indicated the source of escarpments: the overall contraction.
Most scientists believe that when the moon's interior cooled, the crust began to shrink like a raisin, which caused crustal material to rise in several places and caused leaks. The problem is that this process should result in a very not specific foul scarf pattern. "That's not exactly what we saw," says Watters. "They were very organized in a specific way. Which meant that something was going on somewhere else.
The team took Apollo seismic data for 28 lunar earthquakes recorded from 1969 to 1977 and superimposed location data on LRO imaging of scarves. Through computer modeling, Watters and his team discovered that eight of the earthquakes were produced from true tectonic activity occurring below the surface along the faults. The modeling suggests that the epicentres were within 30 km of the faults themselves, which means that the slip on the faults probably caused the earthquakes.
This means that the escarpments are indeed hotspots for geological stress and are signs that the moon is shrinking again today. This reinforces previous suggestions that the moon's interior is still active and is cooling down after billions of years.
This mechanism also explained why the morphologies of the scarves seemed so young (tens of millions of years), always seemed crunchy and had not been eroded over time by the impacts of meteorites and other disturbances – the shrinkage and the moon quake was permanent. process, and it is influenced by the thousands of scarves we have found so far.
"For me, the most interesting and enigmatic suggestions of the study are the notion that a silicate body or rock body the size of the moon has managed to conserve its inner heat for 4.51 billion years, "says Watters. Conventional wisdom has always suggested that a small body will quickly lose its heat and become quite inactive. "The moon just did not follow that path."
"It's starting to make sense now," says Nathan Williams, a postdoctoral fellow at NASA's Jet Propulsion Laboratory in Pasadena, California, who recently wrote a separate study published in Icarus examine the characteristics of the lunar surface created by the narrowing of the moon. "We are pulling together the pieces to look at this process as a whole and it seems that everything has decreased, even quite recently. And it looks like he's still active thanks to the moon tremors. "
In addition to helping us better understand the evolution of rocky celestial bodies in the solar system, the results raise practical considerations as we prepare to return to the moon in the next decade. "We do not want to build in stressful places" and present a stormy tectonic activity, says lunar scientist Clive Neal of the University of Notre Dame, who was not involved in the study. There are still many questions about how to accurately pinpoint the epicenter of these activities, but, like many others, Neal insists on the need to know where we intend to locate the infrastructure for a permanent position. You do not want your lunar base to fall on itself.
I hope the results of this paper illustrate the need for a modern lunar geophysical network, Watters said. "Many countries around the world want to go to the moon and stay there. This is important for our long-term goals on the moon. "
Not everyone is convinced that fault escarpments account for this type of superficial moon tremors. "It's just one of those hypotheses that are trying to explain what really causes the shallow moonquakes," says Yosio Nakamura, emeritus professor of geophysics at the University of Texas at Austin. "I appreciate their efforts, but I am not convinced that this hypothesis is valid. I may be wrong, of course, but the evidence presented here is problematic, "including the depth of the shallow hypocentres of the earthquake and the temporal distribution of earthquakes. "We have to wait to have more real data and additional observations to find out what actually causes these enigmatic seismic events," he said.
We may have these observations very soon, if NASA can achieve its goal of 2024 of returning astronauts to the lunar surface. We will certainly have many things to study upon our return, and it seems that the moon is not as lifeless as we thought.
"I think it's generally accepted that the moon is a dead and boring place," says Williams. "And that's not quite true. We went to the moon and did some very good scientific research, but there is still a lot we do not know. The moon is narrowing – we did not really understand it until recently. It's huge. It is a much more active and interesting place than we thought. "
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