South Pole Hydrogen Distribution for Present Lunar Conditions: Implications for Past Impacts

It has been known since the Lunar Prospector mission that the poles of the Moon evidently harbor enhanced concentrations of hydrogen [1,2]. The physical and chemical form of the hydrogen has been much debated. Using imagery from Clementine it was possible to roughly estimate permanently-shadowed regions (PSRs), and to perform image reconstructions of the Lunar Prospector epithermal neutron flux maps [3,4]. The hydrogen concentrations resulting from these reconstructions were consistent with a few weight percent water ice in selected locations. With the LCROSS impact, we now know that hydrogen in the form of ice does exist in lunar polar cold traps [5]. Armed with this information, and new data from LRO/Diviner, we can examine whether the pre-sent-day distribution of hydrogen in the form of water ice is consistent with a past large impact that delivered a large mass of volatiles to the lunar surface. These volatiles, mixed with solid impact ejecta, would then be lost from locations having high mean temperatures but would otherwise remain trapped in locations with sufficiently low mean annual temperatures [6]. The time scales for loss would depend on the location-dependent temperatures as well as impact history.