P43A-2106
Hidden in the Neutrons: Physical Evidence for Lunar True Polar Wander

Thursday, 17 December 2015
Poster Hall (Moscone South)
James Tuttle Keane, Lunar & Planetary Laboratory, Tucson, AZ, United States, Matthew A Siegler, Planetary Science Institute Tucson, Tucson, AZ, United States, Richard S Miller, University of Alabama in Huntsville, Huntsville, AL, United States, Matthieu Laneuville, Tokyo Institute of Technology, Tokyo, Japan, David A Paige, University of California Los Angeles, Los Angeles, CA, United States, Isamu Matsuyama, University of Arizona, Tucson, AZ, United States, David J Lawrence, Johns Hopkins University, Baltimore, MD, United States, Arlin Crotts, Columbia University of New York, Palisades, NY, United States and Michael Poston, California Institute of Technology, Pasadena, CA, United States
Abstract:
Airless bodies like the Moon are time capsules of planetary and solar system evolution. Lunar polar ices, in particular, record a history of volatile delivery, orbital dynamics, and solar system chemistry. However, despite two decades of orbital geochemistry measurements, the observed abundances and spatial distribution of lunar polar volatiles (likely water ice, as inferred by epithermal neutron deficits) remain unexplained. The observed deposits do not correlate with measured surface temperatures or thermal models of ice stability and are notably asymmetric about the lunar poles, with the peak abundance offset from the present-day pole by 5°. Here we show, for the first time, that polar volatile deposits at the North and South pole are antipodal, displaced equally from each each pole along opposite longitudes. These off-polar volatiles likely represent fossilized cold-traps, formed when the moon had a different spin pole. Reorientation of the Moon from this paleopole to the present pole (i.e. true polar wander) altered the locations of cold-traps and resulted in the asymmetric, but antipodal, polar hydrogen distribution. Since true polar wander results from changes in the distribution of mass within a planet, the direction and magnitude of this wander can be used to constrain the evolution of the lunar interior. We find a causal link between this paleopole and the unique thermal evolution of the nearside Procellarum KREEP Terrane (PKT). Radiogenic heating within this province not only resulted major mare volcanism, but also altered the Moon's moments of inertia. We use a combination of analytical, and numerical 3-D thermochemical convection models to show that the evolution of the PKT naturally produces the correct direction and magnitude of polar wander (albeit early in lunar history, when the PKT was most active). This work provides a self-consistent explanation for the spatial distribution of lunar polar volatiles and opens a deeper connection to the evolution of the lunar interior. Our hypothesis will be readily testable with forthcoming lunar missions, including high-resolution orbital geochemistry instruments, in-situ and returned sample analysis, and geophysical networks.
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