The moon as observed in visible light (left), topography (center, where red is high and blue is low), and the GRAIL gravity gradients (right). The Procellarum region is a broad region of low topography covered in dark mare basalt. The gravity gradients reveal a giant rectangular pattern of structures surrounding the region.
New data obtained by NASA's GRAIL mission reveals that the Procellarum region on the near side of the moon - a giant basin often referred to as the "man in 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. One hypothesis suggested that it was formed by a massive impact, in which case it would have been the largest impact basin on the moon. Subsequent asteroid collisions overprinted the region with smaller - although still large - basins. Now researchers from MIT, the Colorado School of Mines, and other institutions have created a high-resolution map of the Procellarum, and found that its border is 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. Maria Zuber, the E.A. Griswold Professor of Geophysics and also MIT's vice president for research, says that as cracks occurred, they formed a "plumbing system" in the moon's crust through which magma could meander to the surface.
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