Subsurface migration of H₂O at lunar cold traps

Abstract

[1] Permanently shaded areas near the poles of the Moon and Mercury may harbor water ice. We develop a physical model for migration of water molecules in the regolith and discover two pathways that can lead to accumulation of H 2 O in the subsurface. A small fraction of water molecules delivered, either continuously or abruptly, to permanently cold areas diffuses into the regolith and can remain there longer than on the surface. Higher temperatures lead to deeper burial. At constant temperature, this diffusive migration produces less than one molecular layer of volatile H 2 O on grains, because it is driven by differences in surface concentrations. The water is therefore expected to be in adsorbed form, and the amount stored in this fashion could be at most a few hundred ppm of H 2 O. A second pathway is pumping by diurnal temperature oscillations from a transient ice cover that may have formed during a large comet impact. It can lead to high ground ice densities, but the ground ice layer lasts not long beyond the disappearance of the ice cover. Both types of subsurface charging mechanism work best for temperatures typical of permanently shaded areas with sunlit surfaces in their field of view.

Evidence for the sequestration of hydrogen-bearing volatiles towards the Moon’s southern pole-facing slopes

“…That position corresponds to an upper-limit temperature of 135 K and the LAVI rate = 25%. The result is consistent with upper-bound temperature estimates found by Schorghofer and Taylor (2007). Fig.…”

“…Theoretical studies suggest that the extreme end-member cryogenic temperatures <100 K in the polar PSR's may be not provide the optimal thermal conditions for the entrainment of hydrogen-bearing volatiles (Siegler et al, 2011;Schorghofer and Taylor, 2007). LEND found that the epithermal neutron flux was significantly suppressed in some polar regions outside PSR's, inferring the presence of hydrogen-bearing materials in regions that regularly experience some level of solar irradiation (Mitrofanov et al, 2012).…”

The Putative Cerean Exosphere

“…Due to the colder temperatures, surface residence times can be long even in sunlit areas near the poles. Residence times are based on vapor pressure relations (Schorghofer & Taylor 2007). For example, at 200K the model residence time is 2ms, and at 110K it is almost one cerean year.…”

Lunar exospheric argon modeling

“…Therefore, a transport and storage model is required in order to study the behavior of argon, whose lateral migration likely resembles other important volatiles, like water, which have been modeled in recent years (e.g. Hodges, 2002;Crider and Vondrak, 2002;Schorghofer and Taylor, 2007;Hurley, 2010;Farrell et al, 2013).…”