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Lunar true polar wander inferred from polar hydrogen

Abstract

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 spectrometers1,2,3, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures4,5. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury’s poles6. 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 volcanism7,8,9,10,11 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 history7,8,9, 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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Figure 1: Lunar polar hydrogen and predicted ice stability fields.
Figure 2: Antipodal nature of polar hydrogen.
Figure 3: The locations of plausible mass anomalies that produce the required TPW, and the effect of the PKT.
Figure 4: TPW due to the evolution of the PKT.

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Acknowledgements

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.

Author information

Authors and Affiliations

Authors

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.

Corresponding author

Correspondence to M. A. Siegler.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Antipodal symmetry.

a, b, Standard north and south polar maps, respectively, akin to Fig. 1a, b. In each figure we show a hypothetical distribution of antipodally symmetric north (blue) and south (pink) polar ice. In each polar map, we show both the ice present in that hemisphere and the ice on the opposite hemisphere as it would be viewed through the Moon. Despite the antipodal symmetry, the polar ice that would be normally shown in each polar map (blue in a; pink in b) are not separated by 180° as one might expect. c, To test for antipodal symmetry, we rotate the features as viewed through the planet by an angle α. d, Antipodally symmetric features match their antipodal counterparts if rotated by α = 180°.

Extended Data Figure 2 Statistical significance of inter-polar hydrogen.

The statistical significant −log10(P) is shown as a function of the likelihood parameter λ.

Extended Data Figure 3 Full-resolution maps of ice stability depth.

The ice stability depth is defined as the depth at which water ice will sublimate at a rate of 1 mm Gyr−1). af, Full-resolution maps of ice stability depth for areas where water ice is stable at the current orientation (a, b), in the palaeopole orientation (c, d) and in an ‘overlapping’ orientation (e, f); left panels shown the north polar region, right panels show the south. These models define the locations at which isotropically supplied water ice would be stable over geologic time; depth is given as an average of the two models in those locations. Panels ad are the bases for Fig. 1c, d. Models are constrained to ± 300 km in stereographic x, y coordinates. Latitude lines are every 2° poleward of 80°. All other symbols are defined in Fig. 1. Panel b adapted from ref. 5, American Association for the Advancement of Science.

Extended Data Figure 4 Water-ice stability depths for the past and present poles.

af, Water-ice stability depths for the north (a, c, e) and south (b, d, f) polar regions. The model-derived stability depths4,5,6 are shown for the current lunar spin axis (a, b) and the hypothesized palaeo-axis (c, d). Also shown is the optimum admixture of current- and palaeo-axis models that best matches the distributions of polar hydrogen abundance (e, f). Topography measured by the LOLA instrument29 has been superimposed. Latitudinal contours are every 2° poleward of 80°. All other symbols are defined in Fig. 1.

Extended Data Figure 5 Two different modes of planetary reorientation.

ac, Initial spin state (a), changes in obliquity (b) and changes due to TPW (c). In these schematics, we view the reorienting planet in an inertial frame.

Extended Data Figure 6 The Moon’s many published palaeopoles.

A collection of all published lunar palaeopoles, from a combination of palaeomagnetic data (diamonds60,61,63,64, triangles19 and square62), fossil figure estimates (stars20,21) and the epithermal neutron palaeopole reported here (circle). Ellipses around each point indicate 1σ error.

Extended Data Figure 7 Slices through the mass-anomaly parameter-space search.

ad, Coloured contours enclose regions where placing a mass anomaly ΔQ of the indicated size will cause a reorientation of the Moon to within the specified distance (see legend) of the present-day lunar spin pole. Red filled circles denote the epithermal neutron palaeopole. e, Contours enclose regions where mass anomalies ΔQ must be centred to reorient the Moon from the epithermal neutron palaeopole to the present-day spin pole, to within 1°. This figure is the same as in Fig. 3a–c, but in an equirectangular projection; symbols and lines as in Fig. 3a–c.

Extended Data Figure 8 Simple physical models for reorienting the Moon, and the effect of lunar impact basins.

a, A schematic of our spherical cap model, showing a spherical cap on the surface of the Moon (green circle), centred on a particular latitude and longitude (arrow). b, The mass anomaly ΔQ for a spherical cap as a function of cap size and cap surface density (or cap thickness, assuming a density of ρ = 2,550 kg m−3). c, A schematic of our mantle-spanning interior anomaly, with a spherical mass anomaly (green circle), centred on a particular latitude and longitude (arrow), grazing the core (dark grey circle) and the surface. d, The mass anomaly ΔQ for the spherical mantle anomaly as a function of the density contrast of the anomaly (black line). Over-plotted (green shading and orange lines) is the ΔQ required if the PKT is responsible (ΔQ = −0.45; Fig. 3c). e, North polar projection of all possible palaeopole positions based on a mass anomaly placed at either the PKT or South Pole–Aitken basin (SPA). PKT paths always pass through the neutron palaeopole, whereas SPA paths are nearly orthogonal to this path. f, Mass anomalies ΔQ of the largest lunar impact basins derived from inverse fitting of the present-day lunar gravity field, following the method outlined in ref. 21. The only impact basin with a large enough mass anomaly is the South Pole–Aitken basin, which is not properly located to drive the observed TPW (Fig. 3b). The only major impact basin that is properly located is Moscoviense, which has a negligible mass anomaly (and with the wrong sign). Error bars are 1σ uncertainties from the inverse solution (see ref. 21).

Extended Data Figure 9 Mass anomalies and TPW of the Moon due the thermal evolution of the PKT.

ae, Results for model W, in which KREEP is mixed within the crust. fj, Results for model B, in which KREEP is mixed beneath the crust. a, f, Temperature cross-sections of the lunar mantle, as a function of time. The dark circle is the lunar core. White regions are partially molten. b, c, g, h, The mass anomaly Q (b, g) and ΔQ (c, h) associated with the thermal anomalies shown in a and f, respectively, as functions of time for various assumed compensation states (coloured lines). C = 0 corresponds to a rigid lithosphere; C = 1 corresponds to a strengthless lithosphere. The region required to explain the epithermal neutron palaeopole is highlighted in green. d, i, The distance between PKT and the instantaneous spin pole as a function of time. e, j, The co-latitude of the instantaneous spin pole as a function of time. In this plot, the co-latitude is defined as positive if the northern spin pole is on the near side and negative if it is on the far side. The co-latitude of the epithermal neutron palaeopole is highlighted in green.

Extended Data Figure 10 Predicted TPW paths due to the thermal evolution of the PKT for a range of models and compensation states.

af, Model W, in which KREEP is mixed within the crust; gl, model B, in which KREEP is mixed beneath the crust; mo, model W, with a time-varying compensation state. In general, these TPW paths are consistent with the epithermal neutron pole forming early (4 ± 0.5 Gyr ago), as long as the lithosphere is partially rigid. If the lithosphere is weak or strengthless (df, k, l), the topographic uplift from the PKT thermal anomaly dominates and the TPW track never passes through the epithermal neutron pole. m, An example where the lithosphere starts strengthless (fluid; C = 1) and becomes perfectly rigid (C = 0) by the present day. n, o, Examples where the lithosphere starts partially rigid and becomes weaker with time. Although these cases may not be geophysically feasible, it is interesting as it confines the TPW paths to within the observed hydrogen distribution and reduces the age of the epithermal neutron palaeopole. Error bars indicate 1σ uncertainty in the palaeopole position due to the rotational ambiguity of the PKT thermal models; the error bars are often smaller than the plotted pole positions. Contours and symbols as in Fig. 2a.

Supplementary information

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)

Top left: The Moon as it would appear if viewed from the Earth during the 4.5 Gyr of TPW evolution. Red axes denote the locations of the instantaneous maximum and minimum principal axes of inertia. The maximum principal axis (pointing upward here) controls the Moon's spin axis. The minimum principal axis of inertia (viewed edge-on here) sets the orientation of the Moon with respect to the Earth. The blue axes denote the Moon's present-day maximum and minimum principal axes of inertia. Dashed lines mark intervals of 30° latitude and longitude. Pink lines denote PKT border anomalies, and green contours enclose 3.5ppm Thorium abundance; both of these, along with the readily visible maria in the LRO/WAC morphology base-map, highlight the PKT. Top middle and top right: These views show the Moon as it would appear from above the North and South poles respectively, and highlight the connection between the lunar spin axis and the epithermal neutron distribution, shown in cyan. In this model, the spin pole passes through this distribution between 0 and 1.2 Gyr. Bottom: A temperature cross-section through the PKT thermal evolution model ("W"), with the PKT on the right. Colors are as in Fig. 3B. (AVI 26873 kb)

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Siegler, M., Miller, R., Keane, J. et al. Lunar true polar wander inferred from polar hydrogen. Nature 531, 480–484 (2016). https://doi.org/10.1038/nature17166

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