Concurrent Orbital Measurements To Identify Potential Landing Sites For Near-Term Landed Polar Volatile Missions
A major objective of commercial and other landed missions to the polar regions of the Moon is the characterization of deposits to support in-situ resource utilization. The primary material of interest is water, H2O, presumably in the form of ice due to the cold surface temperatures although there may also be abundance reservoirs of water in the form of H2O adsorbed onto regolith grains (e.g. 1,2). However, there has not been a mission sent to the Moon optimized for understanding the polar volatiles, either surficial or subsurface and it is a testament to the missions’ and instruments’ designs and operation that they have provided the insights that we have into polar lunar volatiles. Water surface frost/ice has been identified in some but not all of the Permanent Shadowed Regions (PSRs) of the Moon. The measurements are many kilometers per pixel, or even large non-spatially coherent averages, and/or at low signal strength but do clearly show the presence of water ice (e.g 3,4). The spatial distribution of near-surface hydrogen and ice concentrations have been inferred at similar fidelity in association with some but not all PSRs and in some areas occurring outside of PSRs (radar ref, and Lwarence). There are also compelling models for the distribution of near surface ice suggesting impact gardening should have mixed the subsurface ice sufficiently for an originally discontinuous subsurface ice layer to now be assessable as patches via a moderately capable rover even if landing in a dry area (5). However, while these measurements and modeling compellingly demonstrate there are significant reservoirs of volatiles both on the surface and in the near surface of the polar Moon, the spatial scale of the information is at many kilometers resolution horizontally and the certainty of any particularly spot at the scale of lander being volatile rich is low. An exception is Cabaeus crater for which the LCROSS mission definitively found water ice within this extremely cold doubly shadowed crater (6). However, Cabaeus may not be an ideal target for an initial low cost polar volatile explorer because its extreme cold and its floor is outside of direct earth communication. The risk to near-term lunar polar sample volatile mission of finding a ‘dry hole’ instead of volatile deposits at the poles of the Moon can be greatly reduced with remote measurements from orbit designed specifically to characterize these volatiles. High priority measurements to fill in gaps in our knowledge have already been identified by the LEAG Special Action Team Review for further assessing polar volatiles from orbit to enhance robotic and enable human landed missions (7). Relevant measurements include high spatial resolution (10s of meters) mapping of the water at the poles outside of PSRs including its compositional state. Interior PSR measurements could be achieved with both passive and active measurements, a combination could be particularly effective at spatial and compositional variations. Very low altitude neutron measurements would greatly improve the modeling and interpretation of enhanced subsurface hydrogen deposits (e.g. 8) and improved orbital radar measurements could better refine the nature of possible surface ice deposits. There are mission concepts, currently ongoing studies, contributed payloads on international missions, and cubesat missions that are currently selected that all have pertinent measurements of surface and near surface volatiles. I will present an overview of these measurements and how the information they provide would specifically assist in determining the best location to which to send a lander or rover exploring for polar volatiles. References: (1) Poston et al., Icarus, 255, 24-29, 1015; Hibbitts et al., Icarus, 213, 64-72, 2011; (3) Gladstone et al., JGR, 117, doi:10.1029/2011JE003913, 2012; (4) Li et al., (abstract) LPSC 48, #1964, 2505, 2016; (5) Hurley et al., GRL, 10.1029/2012GL051105, 2012. (6) Coloprete et al., Science, 330, 463-468, 2010; (7) LEAG Lunar Human Exploration Strategic Knowledge Gap Special Action Team Review, Sept. 2016; (8) Lawrence et al., Acta Astronautica, 115, 452-462, 2015.