[meteorite-list] NASA Twin Spacecraft Create Most Accurate Gravity Map of Moon (GRAIL)

[Ron Baalke]

Dec. 5, 2012

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

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

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

RELEASE: 12-417

NASA TWIN SPACECRAFT CREATE MOST ACCURATE GRAVITY MAP OF MOON

WASHINGTON -- Twin NASA probes orbiting the moon have generated the 
highest resolution gravity field map of any celestial body. 

The new map, created by the Gravity Recovery and Interior Laboratory 
(GRAIL) mission, is allowing scientists to learn about the moon's 
internal structure and composition in unprecedented detail. Data from 
the two washing machine-sized spacecraft also will provide a better 
understanding of how Earth and other rocky planets in the solar 
system formed and evolved. 

The gravity field map reveals an abundance of features never before 
seen in detail, such as tectonic structures, volcanic landforms, 
basin rings, crater central peaks, and numerous simple, bowl-shaped 
craters. Data also show the moon's gravity field is unlike that of 
any terrestrial planet in our solar system. 

These are the first scientific results from the prime phase of the 
mission, and they are published in three papers in the journal 
Science. 

"What this map tells us is that more than any other celestial body we 
know of, the moon wears its gravity field on its sleeve," said GRAIL 
principal investigator Maria Zuber of the Massachusetts Institute of 
Technology in Cambridge. "When we see a notable change in the gravity 
field, we can sync up this change with surface topography features 
such as craters, rilles or mountains." 

According to Zuber, the moon's gravity field preserves the record of 
impact bombardment that characterized all terrestrial planetary 
bodies and reveals evidence for fracturing of the interior extending 
to the deep crust and possibly the mantle. This impact record is 
preserved, and now precisely measured, on the moon. 

The probes revealed the bulk density of the moon's highland crust is 
substantially lower than generally assumed. This low bulk crustal 
density agrees well with data obtained during the final Apollo lunar 
missions in early 1970s, indicating that local samples returned by 
astronauts are indicative of global processes. 

"With our new crustal bulk density determination, we find that the 
average thickness of the moon's crust is between 21 and 27 miles (34 
and 43 kilometers), which is about 6 to 12 miles (10 to 20 
kilometers) thinner than previously thought." said GRAIL 
co-investigator Mark Wieczorek of the Institut de Physique du Globe 
de Paris. "With this crustal thickness, the bulk composition of the 
moon is similar to that of Earth. This supports models where the moon 
is derived from Earth materials that were ejected during a giant 
impact event early in solar system history." 

The map was created by the spacecraft transmitting radio signals to 
define precisely the distance between them as they orbit the moon in 
formation. As they fly over areas of greater and lesser gravity 
caused by both visible features, such as mountains and craters, and 
masses hidden beneath the lunar surface, the distance between the two 
spacecraft will change slightly. 

"We used gradients of the gravity field in order to highlight smaller 
and narrower structures than could be seen in previous datasets," 
said Jeff Andrews-Hanna, a GRAIL guest scientist with the Colorado 
School of Mines in Golden. "This data revealed a population of long, 
linear, gravity anomalies, with lengths of hundreds of kilometers, 
crisscrossing the surface. These linear gravity anomalies indicate 
the presence of dikes, or long, thin, vertical bodies of solidified 
magma in the subsurface. The dikes are among the oldest features on 
the moon, and understanding them will tell us about its early 
history." 

While results from the primary science mission are just beginning to 
be released, the collection of gravity science by the lunar twins 
continues. GRAIL's extended mission science phase began Aug. 30 and 
will conclude Dec. 17. As the end of mission nears, the spacecraft 
will operate at lower orbital altitudes above the moon. 

When launched in September 2011, the probes were named GRAIL A and B. 
They were renamed Ebb and Flow in January by elementary students in 
Bozeman, Mont., in a nationwide contest. Ebb and Flow were placed in 
a near-polar, near-circular orbit at an altitude of approximately 34 
miles (55 kilometers) on Dec. 31, 2011, and Jan. 1, 2012. 

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the 
mission for NASA's Science Mission Directorate in Washington. GRAIL 
is part of the Discovery Program managed at NASA's Marshall Space 
Flight Center in Huntsville, Ala. Lockheed Martin Space Systems of 
Denver built the spacecraft. 

To view the lunar gravity map, visit: 

http://bit.ly/grailtour 

For more information about the mission, visit: 

http://www.nasa.gov/grail 

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