Colorado School of Mines/MIT/JPL/GSFC/NASA
The largest plain on the nearside of the moon is not the residual mark of an asteroid impact. Instead, the vast flatland — called Oceanus Procellarum — probably formed as tectonic-like activity stretched and thinned the moon's crust billions of years ago.
Researchers came to that conclusion after looking at gravity maps of the moon from the GRAIL mission. The maps revealed a more rectangular subsurface rim around Oceanus Procellarum (the Ocean of Storms) rather than a circular one, which would be a stronger signature of an asteroid impact, scientists suggest October 1 in Nature.