THE UNCERTAIN NATURE OF POLAR LUNAR REGOLITH

Lunar polar regions are receiving considerable attention because they might contain sizeable quantities of H2O, which could be useful for lunar development and space commerce [1]. Plans to use those resources are limited by our ignorance of the nature of polar regions. Major uncertainties are outlined here. All can be addressed by missions to permanently shadowed polar regions on the Moon. Regoltih characteristics: The typical lunar regolith has a mean grain size of ∼100 μm, with ∼10% of the material smaller than 10 μm [2]. However, the polar regions are in the most ancient lunar highlands, which have been subjected to the most intense bombardment for more than 4 billion years. Hartmann [3] suggests that the upper hundreds of meters have been reworked so extensively that it resembles the typical lunar regolith. Since the heavy bombardment ceased about 3.8 billion years ago, the upper several meters of the Moon have been modified by micrometeorite impacts. That regolith may be much finer grained than typical regolith as it developed on hundreds of meters of fine-grained material. If so, we cannot predict with confidence the physical properties of the regolith (porosity, thermal conducitivity, shear and bearing strength, angle of repose, tribology). A finer grain size provides a much larger surface area for a given mass of regolith, which could enhance adsorption of H2O and other volatiles and their reaction with regolith grains. Characteristics of the H2O deposits: There is clear evidence for enrichment in H in lunar polar regions [4], but what form is it in? Models depict the observed enrichments in hydrogen as being due to solar wind hydrogen, water ice deposited by H2O released from soil grains that have been bombarded with solar wind hydrogen, and more complex ices released by impacting comets. These deposits could form thin films around regolith grains (adsorbed chemically or physically [5]), partiallyto completelyfilled pore spaces, or form layers of ice (in the case of comet impacts). Each of these scenarios leads to potentially different physical and geotechnical properties of the regolith, and different properties of the resource. To show the complexities, consider a cometary source for H2O. In this case the H2O would be accompanied by CO, CO2, CH4 and other gases. If the H2O precipitated as one of the many forms of ice, it could be relatively pure because of the low solubility of gases it in, but might be associated with deposits of less stable carbon gases. If the H2O precipitated as amorphous ice, the amorphous ice might contain dissolved CO and other gases. When heated during exploration or extraction, the amorphous ice would crystallize, releasing the trapped gases and possibly producing jets of dust and gas, as happens as comets are heated [e.g., 6]. Moreover, the crystallization is exothermic, possibly leading to a runaway effect, release of CO2 from its frozen form, loss of the resource, and possibly damage to extraction equipment. At the very least it prevents us from knowing how to design equipment for surface mobility or to extract volatiles from polar regolith. Finally, whatever its state, we do not know how the H2O resource is distributed with depth or laterally.