From Earth, the series of dark blotches on the surface of the moon resemble a man's face. They're not actually the face of a man in the moon, of course; they're a roughly circular basin of volcanic terrain. The largest of the blotches, Oceanus Procellarum, was long thought to have been formed by the impact of an asteroid collision with the moon billions of years ago. But recently, a paper was published that challenges this theory based on data collected from NASA's GRAIL (Gravity Recovery and Interior Laboratory), two probes that orbited the moon from January to December 2012.
As the pair of probes passed over areas of higher or lower density, one or the other would speed up or slow down in response to the Moon's gravitational pull, causing the distance between the two probes to grow or shrink. Researchers from MIT, the Colorado School of Mines, and other institutions were able to analyze the variable distances between the probes to create a density map of the moon's surface, including the area around the Procellarum. They expected to find a smooth circular or elliptical rim, as would be formed by an impact—but instead, they found an angular border comprised of rifts buried beneath dark volcanic plains.
"The rectangular pattern of gravity anomalies was completely unexpected," Jeff Andrews-Hanna, a GRAIL co-investigator and the lead author of the paper, said in a NASA statement. "Using the gradients in the gravity data to reveal the rectangular pattern of anomalies, we can now clearly and completely see structures that were only hinted at by surface observations."
The new hypothesis, which was upheld in testing by the researchers, suggests that a large flood of molten lava rose towards the moon's surface in the Procellarum region from the lunar interior. The drastic temperature difference between the magma and the moon's crusts created a series of fractures that served as pathways for future eruptions to send lava out on to the lunar surface, creating the volcanic plain we see today.