The craters, which we often think of as the "Man on the Moon," were likely caused by a volcanic plume from inside the moon, a new study based on data from NASA's Grail mission shows.
Scientists used to think that the Procellarum region, the moon's largest crater, might have been shaped by an asteroid. There are other, smaller impact basins on the moon, which were caused by asteroids. The Procellarum region is about 1,800 miles wide.
However, a new and highly detailed gravity map of the region shows that the crater it is not circular as previously thought, but polygonal, with sharp angles that could not have been caused by an asteroid. The angles are as wide as 120 degrees, which asteroids are unlikely to make. Scientists proposed the theory that this is an ancient system of rifts that formed a shape due to internal pressure caused by magma from the moon's interior at one point. The scientists posited that the moon's internal heat could have caused the cracks to form, and then the cracks allowed an eruption of magma to flood to the surface, causing dark spots in the crater.
Although this polygonal shape would not have been seen without looking at the gravitational fields, scientists can now recognize it in normal photos of the moon. Mare Frigoris, a dark line on the moon, which we already know about, is the edge of this system of rifts.
The team ran a simulation of the gravity numbers if there had been a volcanic eruption, and found that it matched the numbers that Grail found.
"How such a plume arose remains a mystery. It could be due to radioactive decay of heat-producing elements in the deep interior. Or, conceivably, a very early large impact triggered the plume. But in the latter case, all evidence for such an impact has been completely erased. People who thought that all this volcanism was related to a gigantic impact need to go back and think some more about that," said Maria Zuber, a researcher on the project.
According to the Colorado School of Mines, the newly discovered feature covers roughly 17 percent of the moon's surface, a size equivalent to all of North America, Europe and Asia.
"It's really amazing how big this feature is," said Professor Jeffery Andrews-Hanna.
The Gravity Recovery and Interior Laboratory (Grail) mission consisted of two spacecraft probes launched in September 2011. They orbited the poles of the moon for about a year until their mission ended in December 2012 when the probes intentionally were sent into the moon's surface. During their orbits at an altitude of about 34 miles, they generated a high-resolution gravity field map that would help scientists understand how rocky planets form and evolve. The Grail mission was managed by NASA's Jet Propulsion Laboratory and was part of the Discovery Program.
