article image

Lunar probes reveal mystery of the Moon's gravity

Jenny Winder, News Writer
Jun 3, 2013, 7:00 UTC

Sen—NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission has discovered the origin of the Moon's uneven gravity.

GRAIL's twin spacecraft were launched in September 2011 and entered lunar orbit on December 3ist 2011 and January 1st 2012 respectively.

The probes studied the internal structure and composition of the Moon for nine months. They pinpointed the locations of large, dense regions called mass concentrations, or mascons, which are characterized by strong gravitational pull that affects the operations of lunar-orbiting spacecraft.

The spacecraft, named Ebb and Flow, were intentionally crashed into a mountain near the Moon's north pole in December 2012 after completing their study. GRAIL's findings mean spacecraft on missions to other celestial bodies will be able to navigate with greater precision in the future.

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, a GRAIL co-investigator at Purdue University in 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."

Lunar mascons were first discovered in 1968. Researchers generally agree 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.

GRAIL's Gravity Map of the Moon. Image credit: NASA/JPL-Caltech/MIT/GSFC.

On a map of the Moon's gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-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 bulls-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 knowledge of planetary geology beyond that of Earth and our moon.

"Mascons also have been identified in association with impact basins on Mars and Mercury," said GRAIL principal investigator 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."