'Man in the moon' basin possibly created by volcano, not asteroid

In a new study, scientists have found that it may have been a volcanic plume, and not an asteroid, that created the moon's largest 'man in the moon' basin.

According to the new data obtained by NASA's GRAIL mission, the Procellarum region on the near side of the moon likely arose not from a massive asteroid strike, but from a large plume of magma deep within the moon's interior.

The Procellarum region is a roughly circular, volcanic terrain some 1,800 miles in diameter, nearly as wide as the United States. Researchers from MIT, the Colorado School of Mines, and other institutions created a high-resolution map of the Procellarum, and found that its border was not circular, but polygonal, composed of sharp angles that could not have been created by a massive asteroid.

Instead, researchers believe that the angular outline was produced by giant tension cracks in the moon's crust as it cooled around an upwelling plume of hot material from the deep interior.

The team mapped the Procellarum region using data obtained by GRAIL - twin probes that orbited the moon from January to December 2012. Researchers measured the distance between the probes as they chased each other around the moon. As the leading probe passed over a region of lower density, it briefly slowed, caught by the region's gravitational pull. As the probes circled the moon, they moved in accordion fashion, the distance between them stretching and contracting in response to varying gravitational attraction due to the mass variations in the lunar interior.

From the variable distance between the probes, Maria Zuber and her team determined the strength of gravity across the moon's surface, creating a highly detailed map, which they then used to determine where the lunar crust thickens and thins.

From this mapping, the researchers observed that the rim of the Procellarum region is composed of edges that abut at 120-degree angles. As asteroid impacts tended to produce circular or elliptical craters, Zuber said that the Procellarum's angular shape could not have been caused by an impact.

Instead, the team explored an alternative scenario: Some time after the moon formed and cooled, a large plume of molten material rose from the lunar interior, around where the Procellarum region is today. The steep difference in temperature between the magma plume and the surrounding crust caused the surface to contract over time, creating a pattern of fractures that provided a conduit for molten material to rise to the surface.

To test the hypothesis, the researchers modeled the region's gravitational signal if it were to contain volcanic intrusions, magma that seeped up to just beneath the moon's surface and, over time, cooled and crystallized. The resulting simulation matched the gravity signal recorded by GRAIL, supporting the idea that the Procellarum was caused by a magma plume, and not an asteroid.

The study is published in the journal Nature.