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| [Image credit: SA/JPL-Caltech/ IPGP] |
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.