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Link to original content: https://www.ncbi.nlm.nih.gov/pubmed/30126996
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. 2018 Sep 4;115(36):8907-8912.
doi: 10.1073/pnas.1802345115. Epub 2018 Aug 20.

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

Shuai Li  1   2 , Paul G Lucey  3 , Ralph E Milliken  2 , Paul O Hayne  4 , Elizabeth Fisher  2 , Jean-Pierre Williams  5 , Dana M Hurley  6 , Richard C Elphic  7
Affiliations

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

Shuai Li et al. Proc Natl Acad Sci U S A. .

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.

Keywords: Moon mineralogy mapper; lunar polar regions; lunar water ice; near-infrared spectroscopy; permanently shaded regions.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Example M (3) reflectance spectra of ice-bearing pixels (red) and non–ice-bearing pixels (black) plotted with a laboratory spectrum of pure water ice (blue); solid lines are smoothed spectra based on a cubic spline algorithm (33).
Fig. 2.
Fig. 2.
Average spectra and SDs of ice-bearing pixels in the (A and B) northern and (C and D) southern polar regions from 75° to 90° latitude. In A and C we normalized each M (3) spectrum by its mean reflectance for each ice-bearing pixel to accommodate the variation of the intensity of light scattered onto different pixels, which helps to plot the SD of the whole polar region, whereas the average spectra without normalization are shown in B and D.
Fig. 3.
Fig. 3.
(A) Histogram of maximum surface temperatures for ice-bearing pixels (blue) and all shaded pixels from 75° to 90° latitude (black) in the northern polar region and (B) southern polar region. (C) Histogram of LOLA reflectance values for ice-bearing pixels (blue), surfaces with maximum temperature <110 K (green), and all shaded pixels from 75° to 90° (black) in the northern and (D) southern polar regions (E) LAMP “off” and “on” band ratios for ice-bearing pixels (blue), surfaces with maximum temperature <110 K (green), and all shaded pixels (black); only south polar region LAMP data are available. Each dataset is normalized by the total number of pixels in that dataset.
Fig. 4.
Fig. 4.
Distribution of water-ice-bearing pixels (green and cyan dots) overlain on the Diviner annual maximum temperature for the (A) northern- and (B) southern polar regions. Ice detection results are further filtered by maximum temperature (<110 K), LOLA albedo (>0.35) (12), and LAMP off and on band ratio (>1.2, only applicable in the south) (13). Each dot represents an M (3) pixel, ∼280 m × 280 m.
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References

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