Direct evidence of surface exposed water ice in the lunar polar regions

1,2Shuai Li, 1Paul G. Lucey, 2Ralph E. Milliken, 3Paul O. Hayne, 2Elizabeth Fisher, 4Jean-Pierre Williams, 5Dana M. Hurley, 6Richard C. Elphic
Processdings of the National Academy of Sciences of the United States of America (PNAS) 115, 8907-8912 Link to Article [https://doi.org/10.1073/pnas.1802345115]
1Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822
2Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912
3Department of Astrophysical & Planetary Sciences, University of Colorado Boulder, Boulder, CO 80309
4Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095
5Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723
6Ames Research Center, NASA, Mountain View, CA 94035

Water ice may be allowed to accumulate in permanently shaded regions on airless bodies in the inner solar system such as Mercury, the Moon, and Ceres [Watson K, et al. (1961) J Geophys Res 66:3033–3045]. Unlike Mercury and Ceres, direct evidence for water ice exposed at the lunar surface has remained elusive. We utilize indirect lighting in regions of permanent shadow to report the detection of diagnostic near-infrared absorption features of water ice in reflectance spectra acquired by the Moon Mineralogy Mapper [M (3)] instrument. Several thousand M (3) pixels (∼280 × 280 m) with signatures of water ice at the optical surface (depth of less than a few millimeters) are identified within 20° latitude of both poles, including locations where independent measurements have suggested that water ice may be present. Most ice locations detected in M (3) data also exhibit lunar orbiter laser altimeter reflectance values and Lyman Alpha Mapping Project instrument UV ratio values consistent with the presence of water ice and also exhibit annual maximum temperatures below 110 K. However, only ∼3.5% of cold traps exhibit ice exposures. Spectral modeling shows that some ice-bearing pixels may contain ∼30 wt % ice that is intimately mixed with dry regolith. The patchy distribution and low abundance of lunar surface-exposed water ice might be associated with the true polar wander and impact gardening. The observation of spectral features of H2O confirms that water ice is trapped and accumulates in permanently shadowed regions of the Moon, and in some locations, it is exposed at the modern optical surface.

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