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Searching for water: Comparisons or LRO LAMP and Mini-RF datasets within Polar Anomalous Craters

Angela Stickle
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Abstract Text: 

The permanently shadowed regions (PSRs) at the lunar poles are hypothesized to be excellent traps for volatiles due to their constant darkness and extremely cold temperatures. Remote sensing data can provide clues as to the nature of these traps, and various data sets suggest the presence, or provide evidence, of trace amounts of ice or bound OH on the Moon. Some PSRs show signs of enhanced hydrogen concentrations possibly located within the craters. One such area, the PSRs within Cabeus crater, was the target of the LCROSS mission that identified the presence of water ice in the ejecta plume following impact, providing “ground truth” to some of the remotely sensed data. Though water has been observed using multiple data sets and various wavelengths, more work is necessary in order to fully understand the distribution and nature of volatiles on the Moon. The suite of instruments onboard LRO provide opportunities to examine signatures of volatiles across different wavelengths to better understand where water ice is located on the Moon and its behavior. Here, we compare data from two instruments onboard LRO: the Miniature Radio Frequency (Mini-RF) radar and the Lyman Alpha Mapping Project (LAMP) far ultraviolet (FUV) imaging spectrograph, to examine the possibility of volatiles being trapped in a set of specific PSRs: the anomalous craters identified in Spudis et al (2013). These craters exhibit high CPR only in their dark, cold (less than 100K) interiors, and scattering models suggest that deposits are consistent with the presence of water ice. Alternative hypotheses have also been suggested, though, including radar incidence angle and slope effects.

Water ice has a diagnostic spectral reflectance signature in the FUV. Areas containing water frost will have a lower Lyman-α albedo, and frost can be identified by examining the ratio of spectra values above and below the 160-180 nm absorption edge. Gladstone et al. (2012) reported that LAMP observed the Lyman-α albedo in PSRs to be lower than surrounding areas, regions which roughly correspond to areas of coldest Diviner temperature. The best explanation for this difference is that the porosity is higher than other areas and that there is 1–2% water frost in the upper micron of the regolith. If the anomalous, high radar backscatter is due to water ice in the craters identified by Spudis et al., we might expect similar behavior when looking at them in LAMP data. Preliminary studies show no clear correlation between LAMP data and Mini-RF data in these regions. For most of the examined craters, LAMP does not identify the inside of the craters as being water-ice rich compared to the outside, though select craters exhibit features consistent with water frost inside the crater. These craters may be regions where ice exists from the surface to some depth which is being sampled by the different wavelengths of Mini-RF and LAMP.

References: Gladstone, R. et al. (2012) J. Geophys. Res. 117(E12), E00H04; Spudis, P. et al. (2013) J. Geophys. Res. 118(10), 2016-2029.

G.W. Patterson, J.T.S Cahill, D.B.J. Bussey
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Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."