GRAIL mission solves mystery of Moon’s surface gravity
The spacecraft pinpointed the locations of large, dense regions called mass concentrations, which are characterized by strong gravitational pull.
NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission has uncovered the origin of massive invisible regions that make the Moon's gravity uneven, a phenomenon that affects the operations of lunar-orbiting spacecraft.
Because of GRAIL's findings, spacecraft on missions to other celestial bodies can navigate with greater precision in the future. GRAIL's twin spacecraft studied the internal structure and composition of the Moon in unprecedented detail for nine months. They pinpointed the locations of large, dense regions called mass concentrations (mascons), which are characterized by strong gravitational pull. Mascons lurk beneath the lunar surface and cannot be seen by normal optical cameras.
GRAIL scientists found the mascons by combining the gravity data from GRAIL with sophisticated computer models of large asteroid impacts and known detail about the geologic evolution of the impact craters.
"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient Moon when its interior was much hotter than it is now," said Jay Melosh from Purdue University in West Lafayette, Indiana. "We believe the data from GRAIL show how the Moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies that we recognize as mascons."
The origin of lunar mascons has been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. Researchers generally agree that mascons resulted from ancient impacts billions of years ago. It was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust.
On a map of the Moon's gravity field, a mascon appears in a target pattern. The bull’s-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bull’s-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse, and cooling following an impact. The increase in density and gravitational pull at a mascon's bull’s-eye is caused by lunar material melted from the heat of a long-ago asteroid impact.
"Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts," Melosh said. "Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits."
This new understanding of lunar mascons also is expected to influence planetary geology well beyond that of Earth and our nearest celestial neighbor.
"Mascons also have been identified in association with impact basins on Mars and Mercury," said Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "Understanding them on the Moon tells us how the largest impacts modified early planetary crusts."
Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow, operated in a nearly circular orbit near the poles of the Moon at an altitude of about 34 miles (55 kilometers) 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.