Rethinking the origin of the Moon's “Ocean of Storms”
Sen—Scientists studying data from NASA's lunar Gravity Recovery and Interior Laboratory (GRAIL), mission, have found evidence that the Moon's Oceanus Procellarum region is the result of the formation of ancient rift valleys.
Oceanus Procellarum—the Ocean of Storms—is a rectangular region, roughly 1,600 miles (2,600 kilometres) across, characterized by low elevations, unique composition, and numerous ancient volcanic plains. Previous theories suggested it was formed by an asteroid impact.
"The nearside of the moon has been studied for centuries, and yet continues to offer up surprises for scientists with the right tools," said Maria Zuber, principal investigator of NASA's GRAIL mission, from the Massachusetts Institute of Technology. "We interpret the gravity anomalies discovered by GRAIL as part of the lunar magma plumbing system—the conduits that fed lava to the surface during ancient volcanic eruptions."
The rifts are buried beneath dark volcanic plains on the nearside of the Moon and have been detected only in the gravity data provided by GRAIL. The lava-flooded rift valleys may at one time have resembled rift zones on Earth, Mars and Venus.
Another theory arising from recent data suggests this region formed as a result of churning deep in the interior of the Moon that led to a high concentration of heat-producing radioactive elements in the crust and mantle of this region. Scientists studied the gradients in gravity data from GRAIL, which revealed a rectangular shape in resulting gravitational anomalies.
Earth's moon as observed in visible light (left), topography (centre: red is high and blue is low), and the GRAIL gravity gradients (right). Image credit: NASA/GSFC/JPL/Colorado School of Mines/MIT
"The rectangular pattern of gravity anomalies was completely unexpected," said Jeff Andrews-Hanna, a GRAIL co-investigator at the Colorado School of Mines and lead author of the paper. "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 rectangular pattern contradicts the theory that Procellarum is an ancient impact basin, since such an impact would create a circular basin. Instead, the new research suggests processes beneath the moon’s surface dominated the evolution of this region.
The study also found a similarity between the rectangular pattern of structures on the moon, and those surrounding the south polar region of Saturn’s icy moon Enceladus. Both patterns appear to be related to volcanic and tectonic processes.
"Our gravity data are opening up a new chapter of lunar history, during which the moon was a more dynamic place than suggested by the cratered landscape that is visible to the naked eye," said Andrews-Hanna. "More work is needed to understand the cause of this newfound pattern of gravity anomalies, and the implications for the history of the moon."
Launched September 2011, the twin GRAIL probes, named Ebb and Flow, operated in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometres) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface. GRAIL’s prime and extended science missions generated the highest resolution gravity field map of any celestial body.