A Survey of Condensed Volatiles within Lunar South Polar Cold Traps using LRO-LAMP

Abstract:

Lunar south polar cold traps are dynamic areas of volatile sequestering, impact gardening, photodesorption, and sublimation. Volatiles with stability temperatures low enough to persist as ices within cold traps include H₂O, CO₂, and NH₃, among others. Identification and characterization of these condensed volatiles is essential for understanding volatile transport and solar system processes, and building an inventory of resources available for future crewed missions. Although cold traps within Permanently Shadowed Regions (PSRs) prove difficult to observe, the Lyman Alpha Mapping Project (LAMP), an EUV-FUV imaging spectrograph on-board the Lunar Reconnaissance Orbiter (LRO), is well suited to observe surface volatiles at the lunar poles. LAMP performs nighttime measurements of PSRs by collecting surface-reflected light from UV-bright stars and other diffuse illumination sources, such as hydrogen atoms resonantly scattering solar photons at Lyman-alpha. By accumulating counts from the start of the science phase of the mission in mid-September 2009 through the end of September 2016, we produce brightness and albedo maps of the lunar south pole at 2 km resolution. In our detailed spectral analysis we've improved the filtering of regions where scattered sunlight influences the spectra. The resulting datasets provide cold trap albedo spectra for five major south polar craters: Faustini, Shoemaker, Haworth, Cabeus, and Amundsen. Spectra for an additional unnamed cold trap region, neighbor to Haworth and Shoemaker, is also investigated. We trend the Off-band (175 nm - 190 nm) to On-band (148 nm - 162 nm) albedo ratio (the UV wavelength ranges with strong and weak H₂O ice reflection, respectively) with temperature and identify high Off-band/On-band ratios for regions within these cold traps, characteristic of condensed volatile populations. This study incorporates ~5 more years’ worth of data, more than doubling the data used in similar previous LAMP PSR studies. Results show high Off-band/On-band LAMP albedo ratios within cold traps and redder spectral slopes >160 nm than non-cold trap regions, indicative of surface volatiles. Temperature regions associated with these populations are consistent with the stability temperatures of pure H₂O, CO₂, and NH₃ ices, all of which may be explained by high Off-band/On-band ratio.