A Round Plain On A Square Hole

A map showing the rifts beneath the Oceanus Procellarum.

One of the most prominent features on the Moon, a dark, flat plain, 1,800 miles across—the Oceanus Procellarum—has long been held to be the work of bombardment, mainly because of its roundish shape and enormous size. But a look beneath the lunar surface changes that notion.

Researchers have found underneath it, a series of long, narrow rift valleys (a crack in the crust, often with steep sides and a flat floor) in the shape of a rough square, which suggests that it isn’t the result of an asteroid strike, but copious volcanic activity.

Around the time of the Moon’s birth, rivers of magma began to stream downhill, “toward the interior of the polygon.” It flowed so wide, and for so long that it flooded an area, one-fourth the size of the U.S.

h/t: SLATE


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