Friday, December 7, 2012

Measuring the gravity on the moon: And now gravity field map of the moon, the highest resolution gravity field map of any celestial body


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

"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

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.

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