NASA's Grail Mission Solves Mystery of Moon's Surface Gravity

May 30, 2013

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov

DC Agle
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-9011
agle@jpl.nasa.gov

Elizabeth Gardner
Purdue University, West Lafayette, Ind.
765/494-2081
ekgardner@purdue.edu

Jennifer Chu
Massachusetts Institute of Technology, Cambridge, Mass.
Phone: 617-715-4531
j_chu@mit.edu


RELEASE: 13-164

NASA'S GRAIL MISSION SOLVES MYSTERY OF MOON'S SURFACE GRAVITY

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 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.
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 (JPL) 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 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 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."

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 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


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