David Lawrence, a scientist at the Department of Energy's Los Alamos National Laboratory, described some of the latest results in a presentation today at the Lunar and Planetary Science Conference in Houston. These new results include elemental distribution maps with more than twice the resolution of previous Lunar Prospector results.
"These new data are quite spectacular," Lawrence said. "We're picking out all the thorium hot spots and finding they are associated with certain craters. We are also deriving the first estimates of absolute thorium abundances, which represents the ground truth for modeling lunar evolution."
Thorium is an important element for study because it is a constituent of KREEP, the acronym for potassium (symbol "K"), rare Earth elements and phosphorous. KREEP is the last material to solidify from a geologic melt.
The moon once was hot and molten and as it cooled minerals crystallized and sank to form the core, if they were heavy, or floated upward to form the crust, if they were light. The elements in KREEP do not readily incorporate into minerals and so the mixture remains molten the longest. These elements, then, are signatures of the original material under the moon's crust, and their presence on the surface indicates some process -- volcanic events or impacts strong enough to punch through the crust -- must have dredged them up from the interior.
Thorium emits gamma rays -- a high-energy form of light -- of a distinct energy. A Los Alamos team built Lunar Prospector's gamma ray spectrometer, which counts and measures gamma rays as Lunar Prospector swings around the moon. Lawrence and his colleagues use this information to create maps showing the abundance of thorium in different locations.
"Because thorium is a tracer for KREEP-rich material, these data provide fundamental information regarding the locations and importance of geologic formations that are rich in KREEP-bearing materials," Lawrence said.
And they show that the moon's evolution was a complicated story.
Thorium emissions neatly trace out the outer rim of Mare Imbrium, one of the distinctive features on the moon's Earthward-facing side. Lawrence said this signal "provides a telltale sign of deposition by ejecta. This indicates that around Mare Imbrium the dredge-up process, at least in part, was related to an impact."
By contrast, the South-Pole Aitkin basin shows much less thorium-rich material though it is the largest known impact crater in the solar system. The impact presumably was large enough to poke through the lunar crust to the moon's mantle material, but it did not encounter as much thorium-rich material.
In addition to the gamma ray spectrometer, the Los Alamos team built Lunar Prospector's neutron spectrometer and alpha particle spectrometer. Data from the neutron spectrometer point to large quantities of water ice at the moon's poles. The alpha particle data could show signals of outgassing and hence geologic activity, but analysis has not yet yielded definitive evidence of such events.
Alan Binder of the Lunar Research Institute in Gilroy, Calif., proposed and leads the Lunar Prospector mission. NASA's Ames Research Center in Moffett Field, Calif., controls mission operations.
NOTE TO REPORTERS: Dr. Lawrence can receive messages during the Lunar and Planetary Science Conference at (281) 488-0220 or by e-mail at djlawrence@lanl.gov.
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