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Science: GRAIL Mission Reveals Moon’s Crust in Detail
An artist’s depiction of the twin spacecraft Ebb and Flow that comprise NASA's GRAIL mission.
Image credit: NASA/JPL-Caltech/MIT
New results from the GRAIL mission offer a high-resolution picture of the lunar crust, filling in many details of the moon’s surface and the rock right below.
The GRAIL mission’s principal investigator Maria Zuber of MIT and her colleagues have produced a map of the moon’s gravity field, which reveals areas of different density in the lunar crust and should help researchers understand the history of the moon.
The initial results of the mission appear in a set of three papers published at the Science Express Web site on 5 December to coincide with a related presentation at the meeting of the American Geophysical Union.
The Gravity Recovery and Interior Laboratory mission—GRAIL for short—has been measuring and mapping the moon’s gravity field since early 2012. Accomplishing this goal has been a challenge because the moon rotates in synchrony with the Earth, so the same moon hemisphere is always facing the planet.
The GRAIL mission deals with this problem using a pair of spacecraft, each about the size of a washing machine, that co-orbit the moon. The spacecraft track each other and measure changes in the distance between them as they are perturbed by the gravitational attraction of topographic features and changes in mass below the moon’s surface.
“This is the highest-resolution gravity model for any planet including Earth,” Zuber said. A two-satellite mission called GRACE is also analyzing Earth’s gravity field, but the satellites are farther from the Earth’s surface due to its thicker atmosphere.
The papers reveal several new geologic features not previously revealed by gravity mapping.
“We not only resolved craters but also parts of craters, rings of basins, central peaks, and ejecta blankets,” Zuber said. Another key finding is that the density of the moon’s upper crust is less than what was thought previously and probably more porous. The crust appears to be about 35-40 kilometers (21 to 24 miles) thick, the researchers report.
The lunar crust is also cut by widespread sheets of cooled magma, known as dikes, according to the GRAIL researchers. These may have formed during a period of expansion early in the moon’s history.
5 December 2012