Scientists Solve Decades-Old Lunar Mystery

NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

For ages, people have looked up at the craters in the Procellarum region of the moon and imagined different shapes within the features; a man's face in particular. But where did these features come from? Hotly debated, the the prevailing theory has been that the "man in the moon" was formed by an impact event with a massive asteroid. However, a team using NASA's Gravity Recovery and Interior Laboratory (GRAIL) has discovered that the trademark lunar topography is actually due to a massive ancient plume of magma. The research was led by Maria Zuber of MIT and the results were published in Nature with Jeff Andrews-Hanna as the first author.

Zuber's team analyzed data obtained by two probes during the GRAIL mission in 2012. Variations in the density of the lunar surface caused very slight dips in gravity, which caused one probe to slow down and the second one to slightly catch up. When the trailing probe passed over the same area, it also slowed down. These various differences in distance between the probes were recorded, charting where variations in the gravitational field were. The probes mapped the entire lunar surface using this inchworm-like method. 

The Procellarum region is a not quite circular area almost as wide as the United States. Topography readings made it a fairly plausible assumption that the moon was struck by a massive asteroid, with smaller collisions occurring later, deforming the large crater's circular shape. However, gravity field readings have indicated that the crater isn't circular at all; it has sharp 120-degree angles that rule out an asteroid collision. 

“A lot of things in science are really complicated, but I’ve always loved to answer simple questions,” Zuber said in a press release. “How many people have looked up at the moon and wondered what produced the pattern we see — let me tell you, I’ve wanted to solve that one!”

Instead, Zuber and her team believe that a large plume of magma built up beneath the surface of the moon. This pressure caused the surface to crack and the magma to cool at the surface, forming the sharp angles that are seen around the basin. These giant tension cracks then formed a network, which Zuber referred to as a "plumbing system" for magma within the moon, providing access to the surface. The surface magma filled in the basins and created darker regions. These darker regions are also more dense, contributing to the alteration in gravity. What is less clear, however, is how the magma did this in the first place.

“How such a plume arose remains a mystery,” Zuber continued. “It could be due to radioactive decay of heat-producing elements in the deep interior. Or, conceivably, a very early large impact triggered the plume. But in the latter case, all evidence for such an impact has been completely erased. People who thought that all this volcanism was related to a gigantic impact need to go back and think some more about that.”

In order to thoroughly investigate the origin of this magma plume, the researchers believe it might require a specialized mission by a probe able to take seismic and heat readings from far below the lunar surface. If such a mission did occur, it would likely further supplement, and not replace, the data obtained by the GRAIL mission.

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