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Lunar true polar wander inferred from polar hydrogen

Siegler, M. A. and Miller, R. S. and Keane, J. T. and Laneuville, M. and Paige, D. A. and Matsuyama, I. and Lawrence, D. J. and Crotts, A. and Poston, M. J. (2016) Lunar true polar wander inferred from polar hydrogen. Nature, 531 (7595). pp. 480-484. ISSN 0028-0836.

[img] Video (AVI) (Video 1: Visualizations of lunar true polar wander due to the thermal evolution of the Moon, for our nominal PKT thermal model ("W") and a rigid lithosphere (C=0)) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 1: Antipodal symmetry) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 2: Statistical significance of inter-polar hydrogen) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 3: Full-resolution maps of ice stability depth) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 4: Water-ice stability depths for the past and present poles) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 5: Two different modes of planetary reorientation) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 6: The Moon’s many published palaeopoles) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 7: Slices through the mass-anomaly parameter-space search) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 8: Simple physical models for reorienting the Moon, and the effect of lunar impact basins) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 9: Mass anomalies and TPW of the Moon due the thermal evolution of the PKT) - Supplemental Material
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[img] Image (JPEG) (Extended Data Figure 10: Predicted TPW paths due to the thermal evolution of the PKT for a range of models and compensation states) - Supplemental Material
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The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury’s poles. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System.

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Keane, J. T.0000-0002-4803-5793
Poston, M. J.0000-0001-5113-1017
Additional Information:© 2016 Macmillan Publishers Limited. Received 13 July 2015; Accepted 21 January 2016; Published online 23 March 2016. This project was supported in part (M.A.S., R.S.M. and D.J.L.) by NASA’s SSERVI VORTICES node, the Lunar Reconnaissance Orbiter (M.A.S. and D.A.P.) and the NASA Lunar Advanced Science and Exploration Research (LASER) programme (J.T.K. and I.M.). We thank L. L. Hood for sharing insight on palaeomagnetic data. We dedicate this project to Arlin Crotts, who passed shortly after the completion of this paper. These authors contributed equally to this work. M. A. Siegler, R. S. Miller & J. T. Keane Author Contributions: M.A.S. posed the fundamental true polar wander question (with M.J.P.), identified contributors needed to address key elements of the hypothesis, and provided the near-surface thermal and ice stability models from models developed by D.A.P. R.S.M. developed and performed the analyses of neutron datasets, including the determination of hydrogen abundance and spatial distribution, as well as the multi-dataset correlation, statistical and admixture analyses. J.T.K. performed all true polar wander analyses, identified the PKT as the plausible cause of the reorientation and produced all figures. M.L. provided critical input, including results from his previously published thermal evolution model of the lunar interior. D.A.P., I.M. and D.J.L. contributed expertise related to the interpretation of thermal, gravity and orbital geochemistry science. M.J.P. and A.C. contributed expertise regarding long-term volatile stability models. M.A.S., R.S.M. and J.T.K. were the primary authors, with M.L., D.A.P., I.M., D.J.L., M.J.P. and A.C. providing additional contributions and assisting with the review of the paper. The authors declare no competing financial interests.
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Issue or Number:7595
Record Number:CaltechAUTHORS:20160328-102629698
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65699
Deposited By: Tony Diaz
Deposited On:28 Mar 2016 23:11
Last Modified:22 Apr 2020 17:43

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