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

Introduction: Epithermal neutrons provide a robust measure of hydrogen abundance and have been used on many planetary bodies to measure hydrogen enhancements [e.g. 1,2,3,4]. Lunar polar epithermal neutrons display a strong, statistically significant, offpolar enhancement that is not clearly explainable by the current thermal environment. One plausible explanation is that these enhancements represent locations of past water ice stability [5]. Here we suggest the polar hydrogen distributions are consistent with an hypothesis incorporating a record of lunar true polar wander (TPW). Specifically, regions near the maxima of the hydrogen distributions may be evidence of a “paleo-pole”. Of particular note is that the location and wander direction from the paleo-pole are consistent with theoretical expectations of TPW caused by a large mass anomaly associated with the Procellerum KREEP Terrain (PKT), an outcome expected from the postulated thermal anomaly in the PKT mantle [e.g. 6]. Background: Suppression of the epithermal neutron leakage flux provides a measurements of hydrogen to depths of approximately a meter and remains the key evidence of subsurface ice and hydrated minerology on many solar system bodies. The distribution of hydrogen at the lunar poles (see Figure 1) has been a longstanding enigma [1,7,8,9], and a satisfactory explanation for the observed spatial distribution has remained elusive. Recent deposition of volatiles from comets/asteroids or solar wind-related processes is expected to be controlled by temperature. Yet the inter-polar hydrogen spatial distributions are more closely correlated to each other than they are to expectations based on thermal models of the respective poles [10]. This inter-polar hydrogen correlation is especially strong when the statistical significance of observed neutron features [9,11] is taken into account. Correlation: Spatial relationships between multiple datasets have been quantified using a 2D correlation analysis (see Figure 2). The only statistically significant inter-polar correlation is between the water equivalent hydrogen (WEH) abundance distributions. These were derived from epithermal measurements [9,11] and show a strong correlation (max significance at an offset of ~168±13 equivalent to 9.3σ) supporting a near-antipodal relationship. Inter-polar correlations between temperature related parameters, including predicted ice locations [10], are not significant. The peaks in hydrogen enhancement are located at roughly (84.9N, 147.9E) and (-84.2S, 309.4E). These locations are coincident with the 44 km diameter Rozhdestvenskiy U in the North and the 100 km Cabeus crater in the South. Although both of these craters are perminantly shadowed regions (PSRs), neither are particularly unique in their thermal environment (colder craters exist), and they are not unique in their ability to store water ice. If these craters exisited while the Moon was in a former orientation, they might also be expected to have retained Hydrogen more effectively due to their then polar location.