[meteorite-list] NASA's GRAIL Mission Puts a New Face on the Moon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 7 Nov 2013 12:25:26 -0800 (PST)
Message-ID: <201311072025.rA7KPQSf016559_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?release=2013-322

NASA's GRAIL Mission Puts a New Face on the Moon
Jet Propulsion Laboratory
November 07, 2013

Scientists using data from the lunar-orbiting twins of NASA's Gravity
Recovery and Interior Laboratory (GRAIL) mission are gaining new insight
into how the face of the moon received its rugged good looks. A report
on the asymmetric distribution of lunar impact basins is published in
this week's edition of the journal Science.

"Since time immemorial, humanity has looked up and wondered what made
the man in the moon," said Maria Zuber, GRAIL principal investigator from
the Massachusetts Institute of Technology in Cambridge. "We know the dark
splotches are large, lava-filled, impact basins that were created by asteroid
impacts about four billion years ago. GRAIL data indicate that both the
near side and the far side of the moon were bombarded by similarly large
impactors, but they reacted to them much differently."

Understanding lunar impact basins has been hampered by the simple fact
that there is a lack of consensus on their size. Most of the largest impact
basins on the near side of the moon (the moon's face) have been filled
with lava flows, which hide important clues about the shape of the land
that could be used for determining their dimensions. The GRAIL mission
measured the internal structure of the moon in unprecedented detail for
nine months in 2012. With the data, GRAIL scientists have redefined the
sizes of massive impact basins on the moon.

Maps of crustal thickness generated by GRAIL revealed more large impact
basins on the near-side hemisphere of the moon than on the far side. How
could this be if both hemispheres were, as widely believed, on the receiving
end of the same number of impacts?

Scientists have long known that the temperatures of the near-side hemisphere
of the moon were higher than those on the far side: the abundances of
the heat producing elements uranium and thorium are higher on the near
side than the far side, and as a consequence, the vast majority of volcanic
eruptions occurred on the moon's near-side hemisphere.

"Impact simulations indicate that impacts into a hot, thin crust representative
of the early moon's near-side hemisphere would have produced basins with
as much as twice the diameter as similar impacts into cooler crust, which
is indicative of early conditions on the moon's far-side hemisphere,"
notes lead author Katarina Miljkovic of the Institut de Physique du Globe
de Paris.

The new GRAIL research is also helping redefine the concept of the late
heavy bombardment, a proposed spike in the rate of crater creation by
impacts about 4 billion years ago. The late heavy bombardment is based
largely on the ages of large near-side impact basins that are either within,
or adjacent to the dark, lava-filled basins, or lunar maria, named Oceanus
Procellarum and Mare Imbrium. However, the special composition of the
material on and below the surface of the near side implies that the temperatures
beneath this region were not representative of the moon as a whole at
the time of the late heavy bombardment. The difference in the temperature
profiles would have caused scientists to overestimate the magnitude of
the basin-forming impact bombardment. Work by GRAIL scientists supports
the hypothesis that the size distribution of impact basins on the far-side
hemisphere of the moon is a more accurate indicator of the impact history
of the inner solar system than those on the near side.

Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed
Ebb and Flow by schoolchildren in Montana, 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 and
http://grail.nasa.gov

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

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

Sarah McDonnell 617-253-8923
Massachusetts Institute of Technology, Cambridge
s_mcd at mit.edu / cmcall5 at mit.edu

2013-322
Received on Thu 07 Nov 2013 03:25:26 PM PST