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

Pedro_P

▼ Significance
We found direct and definitive evidence for surface-exposed water ice in the lunar polar regions. The abundance and distribution of ice on the Moon are distinct from those on other airless bodies in the inner solar system such as Mercury and Ceres, which may be associated with the unique formation and evolution process of our Moon. These ice deposits might be utilized as an in situ resource in future exploration of the Moon.
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 Res66:3033–3045]. 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.
The small tilt of the rotation axes of Mercury, the Moon, and Ceres with respect to the ecliptic causes topographic depressions in their polar regions, such as impact craters, to be permanently shadowed from sunlight ( 1). As a consequence, surface temperatures in these regions are extremely low (i.e., less than 110 K) ( 1 ⇓– 3) and are limited only by heat flow from the interior and sunlight reflected from adjacent topography ( 4 ⇓– 6). These areas are predicted to act as cold traps that are capable of accumulating volatile compounds over time, supported by observations of water ice at the optical surface of polar shadowed locations on Mercury ( 7 ⇓– 9) and Ceres ( 10, 11). There are a number of strong indications of the presence of water ice in similar cold traps at the lunar poles ( 12 ⇓– 14), but none are unambiguously diagnostic of surface-exposed water ice, and inferred locations of water ice from different methods are not always correlated. Epithermal neutron counts, for instance, can be used to estimate hydrogen in the upper tens of centimeters of the lunar regolith, but such data cannot isolate the uppermost (e.g., optical) surface and cannot discriminate between H2O, OH, or H ( 14). Ratios of reflected UV radiation measured by the Lyman Alpha Mapping Project (LAMP) instrument onboard the Lunar Reconnaissance Orbiter (LRO) have been interpreted to indicate the presence of H2O near the lunar south pole ( 13), but the observed signatures may not be uniquely attributable to water ice because OH may exhibit similar characteristics at UV wavelengths ( 15). High reflectance values at 1,064-nm wavelength have also been observed near the lunar poles by the Lunar Orbiter Laser Altimeter (LOLA) and may be consistent with water ice, but fine particles and lunar regolith with lower degrees of space weathering may also give rise to higher reflectivity at this wavelength, making this interpretation nonunique ( 12, 16).
An advantage of near-infrared (NIR) reflectance spectroscopy is that it provides a direct measurement of molecular vibrations and can thus be used to discriminate H2O ice from OH and H2O in other forms (e.g., liquid, surface adsorbed, or bound in minerals). NIR reflectance data acquired by the Moon Mineralogy Mapper [M (3)] instrument on the Chandrayaan-1 spacecraft provide the highest spatial and spectral resolution NIR data currently available at a global scale, including the polar regions. The wavelength range of M (3) (0.46–2.98 μm) is too limited to properly discriminate OH/H2O species using fundamental vibration modes in the 3-μm region ( 17, 18), and in this study we focus on the detection of diagnostic overtone and combination mode vibrations for H2O ice that occur near 1.3, 1.5, and 2.0 μm. Numerical modeling results suggest NIR spectra representing as little as 5 wt % (intimate mixing) or 2 vol % (linear areal mixing) water ice are expected to exhibit all three of these absorptions ( Methodsand SI Appendix , Fig. S1), although actual detection limits in the M (3) data are dependent on instrumental response and signal-to-noise ratios (SNR).
The cooccurrence of all three absorptions would be evidence for water ice, and M (3) data (optical period 2C) at high latitudes were searched for pixels in shadow whose spectra exhibited these features ( Methodsand SI Appendix , Table S1). The resulting subset of pixels (spectra) was then analyzed using the spectral angle mapping method as a metric to assess the similarity of spectral slopes and absorption features between the M (3) spectra and a laboratory spectrum of pure water frost ( Methods). This step relies on the fact that reflectance spectra of water ice exhibit strong “blue” spectral continuum slopes (reflectance decreases with increasing wavelength) ( SI Appendix , Fig. S1), whereas typical lunar spectra exhibit an opposite “reddening” effect due to nanophase iron produced during space weathering ( 19). A spectral angle less than 30° between each M (3) spectrum and the water frost spectrum was empirically determined to define the final subset of M (3) pixels most consistent with the presence of water ice ( Methods).
Results and Discussion
Reflectance measurements in regions of permanent shadow are enabled by sunlight scattered off crater walls or other nearby topographic highs. The shaded regions studied here lie between directly illuminated areas and areas of deep permanent shadow, and the analysis was limited to pixels for which local solar incidence angle was greater than 90° to ensure the corresponding surface was in shadow. As expected for areas only receiving indirect illumination, the SNR of M (3) pixels in these locations is low, and we evaluated the possibility that our process was selecting noisy spectra that mimic ice spectra by random chance. A random dataset was generated that matched the means and SDs of M (3) data in shaded regions near the lunar poles (75° N/S–90° N/S), but with five times more pixels than M (3) data (∼65 × 106points). The same procedures were performed on the random dataset as on the original M (3) data to detect potential ice absorption features. Only 10−4%of those random points passed our detection criteria, which may indicate the rate of false-positive detections due to random noise. In contrast, ∼0.2% of shaded M (3) data showed positive detections ( SI  Appendix , Fig. S2 C). The average spectrum of positive detections of the random data ( SI Appendix , Fig. S3) is distinct from those observed in the M (3) data ( Fig. 1), indicating that a population of random noise of a size similar to that of the M (3) data is highly unlikely to produce ice-like spectra. (▪ ▪ ▪)

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parivijesh100

Even Indian Chandrayan mission found frozen water in form of ice in the black spotted portion of moon. Thus presence of water is confirmed now.  Next action will to explore life there..