December 6, 2012

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Purple Holes And Red Peaks

image by NASA/JPL-Caltech/IPGP

A windfall of first results was presented yesterday by Maria Zuber and team members of the Grail spacecraft currently orbiting the Moon. Grail's tandem spacecraft, Ebb and Flow, yield the most precise measurements ever made of lunar gravity. Combining the new gravity data with the unexcelled topography from LRO allows the calculation of crustal thickness, as shown in this map by Wieczorek and other Grail scientists. Red marks the thickest crust, up to about 60 km, and dark blue to purple is thinnest. Remarkably, the crust in parts of Mare Crisium and Mare Moscoviense is only 1 km thick or less. On Earth the thinnest crust under ocean floors is about 10 km thick and attempts to drill through it to reach the Moho, the crust-mantle boundary, failed. In Crisium, a crater 5 km wide could penetrate the crust and reach the lunar mantle; such a crater would be an important sample return target. The average lunar crustal thickness is found to be about 34 km, with the thickest being about 60 km in the farside highlands, of course some of that is due to piles of ejecta from the South Pole-Aitken basin. The purple stars represent areas that data from the Japanese Kaguya spacecraft indicated to be olivine-rich. Olivine is a mineral that is thought to be associated with the mantle, and the distribution of rare olivine around basins is consistent with their excavation of mantle material (which otherwise is not indicated by geochemistry). The excess of olivine around Crisium and Moscoviense is said to be expected because of their very shallow crusts, but of course the basin-forming impact is why the crust is thin.
Chuck Wood

Related Links
More than a dozen different maps are shown in the Grail image gallery.
The Moho Song

Yesterday's LPOD: No I Won't

Tomorrow's LPOD: Spider Webs


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