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

[Ron Baalke]

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