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.
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.
