NASA's GRAIL Mission Solves Mystery of Moon's Surface Gravity
Jet Propulsion Laboratory
May 30, 2013
PASADENA, Calif. -- 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, or
mascons, which are characterized by strong gravitational pull. Mascons
lurk beneath the lunar surface and cannot be seen by normal optical
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. The
findings are published in the May 30 edition of the journal Science.
"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 West Lafayette, Ind., and lead author of the paper.
"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 in Pasadena, Calif. 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.
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
"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 well beyond that of Earth and our nearest
"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
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.
JPL, a division of the California Institute of Technology in Pasadena,
Calif. managed GRAIL for NASA's Science Mission Directorate in
Washington. The mission was part of the Discovery Program managed at
NASA's Marshall Space Flight Center in Huntsville, Ala. NASA's Goddard
Space Flight Center, in Greenbelt, Md., manages the Lunar Reconnaissance
Orbiter. Operations of the spacecraft's laser altimeter, which provided
supporting data used in this investigation, is led by the Massachusetts
Institute of Technology in Cambridge. Lockheed Martin Space Systems in
Denver built GRAIL.
For more information about GRAIL, visit http://www.nasa.gov/grail
DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
Dwayne Brown 202-358-1726
Elizabeth Gardner 765-494-2081
Purdue University, West Lafayette, Ind.firstname.lastname@example.org
Jennifer Chu 617-715-4531
Massachusetts Institute of Technology, Cambridge, Mass.email@example.com