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

TLDR

Water ice can accumulate in permanently shaded regions on airless bodies such as Mercury, the Moon, and Ceres, but direct evidence of surface‑exposed ice on the Moon has been lacking. The study aims to detect surface‑exposed water ice on the Moon by analyzing near‑infrared absorption features in Moon Mineralogy Mapper spectra. The authors used indirect lighting in permanently shadowed regions to obtain near‑infrared reflectance spectra from the Moon Mineralogy Mapper, identifying diagnostic water‑ice absorption features. Several thousand M(3) pixels (~280 × 280 m) within 20° of both poles show water‑ice signatures at the optical surface, with only ~3.5 % of cold traps exhibiting ice exposures; spectral modeling indicates some pixels contain ~30 wt % ice mixed with regolith, and the patchy, low‑abundance distribution may be linked to polar wander and impact gardening, confirming that water ice is trapped and occasionally exposed in permanently shadowed lunar regions.

Abstract

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.