Wednesday, December 5, 2012

JPL releases most detailed map of lunar gravity

From JPL's newly released GRAIL Map of Moon's Crust, a still frame centered near 240° East, for comparison with the LROC WAC orthographic projection at the end of the previous post [NASA/JPL/MIT]
The GRAIL twin spacecraft, "Ebb" and "Flo," in close orbit around 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) investigators, 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.

The gravity field map reveals an abundance of features never before seen in detail, tectonic structures, volcanic landforms, basin rings, central peaks and numerous simple craters. Data also show the Moon's gravity field is unlike that of any known rocky planet.
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, more than any other celestial body we know, the Moon wears its gravity field on its sleeve," said GRAIL Principal Investigator Maria Zuber of the Massachusetts Institute of Technology. "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."

Zuber says 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 is now more precisely measured, on the Moon.

The probes revealed the bulk density of the moon's highland crust is substantially lower than generally assumed.

From JPL's newly released GRAIL Gravity Map of the Moon [NASA/JPL/MIT].
This low-bulk crustal density agrees well with data obtained during the late Apollo "J" missions, that samples returned by astronauts are indicative of global processes.

"With our new crustal bulk density determination, we find the average thickness of the moon's crust is between 34 and 43 kilometers, about 10 to 20 kilometers thinner than previously thought," said Mark Wieczorek, GRAIL co-investigator at 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. Orbiting over areas of greater or lesser gravity related to visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft changed 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. "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 August 30 and concludes December 17. As End of Mission nears, the spacecraft will operate at lower orbital altitude.

When launched in September 2011, the probes were named GRAIL A and B. They were renamed Ebb and Flow last January by elementary students in Bozeman, Montana, following a nationwide contest. Ebb and Flow were placed in near-polar, near-circular orbit at an altitude of approximately 55 kilometers, December 31, 2011 and January 1, 2012, respectively.

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 . For more information about the mission, visit HERE .

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