An upper limit for ice in Shackleton crater as revealed by LRO Mini‐RF orbital radar

Abstract

[1] Although diverse measurements have indicated H + , OH À , or H 2 O species in the lunar polar regions, pinpointing its location, form, and abundance in specific reservoirs has proven elusive. Here we report on the first orbital radar measurements of Shackleton crater near the lunar south pole. Mini-RF observations indicate a patchy, heterogeneous enhancement in CPR (circular polarization ratio) on the crater walls whose strength decreases with depth toward the crater floor, a result that is most consistent with a roughness effect due to less mature regolith present on the crater wall slopes. However, the results do not rule out a modest ice contribution, and an upper limit of $5-10 wt% H 2 O ice (up to 30 vol.%) present in the uppermost meter of regolith is also consistent with the observations.

Bistatic radar observations of the Moon using Mini-RF on LRO and the Arecibo Observatory

“…The direct detection of water ice within Cabeus crater [Colaprete et al, 2010] and various lines of indirect evidence consistent with water ice at the south pole of the Moon [Feldman et al, 2001;Mitrofanov et al, 2010;Spudis et al, 2010;Thomson et al, 2012;Zuber et al, 2012;Lucey et al, 2014] suggest that Mini-RF bistatic observations of the floor of Cabeus detected the presence of water ice. The measured opposition surge of the imaged portion of Cabeus's floor is narrow and strong, indicative of a coherent backscatter opposition effect (Fig.…”

“…The south pole of the Moon, in general, was a target of the Mini-RF bistatic campaign because of various lines of evidence consistent with the presence of H, OH, water or water ice in the near surface [Feldman et al, 1998;Feldman et al, 2001;Colaprete et al, 2010;Mitrofanov et al, 2010;Gladstone et al, 2012;Spudis et al, 2010;Thomson et al, 2012;Zuber et al, 2012;Lucey et al, 2014;Eke et al, 2015]. Much of the research into water ice at the poles has focused on permanently shadowed regions (PSRs).…”

“…4e) is commonly used in analyses of lunar radar data [e.g., Ghent et al, 2005Ghent et al, , 2008Ghent et al, , 2010Campbell et al, , 2010Campbell et al, , 2012Spudis et al, 2010, Carter et al, 2012;Raney et al, 2012;Thomson et al, 2012;Neish et al, 2011Neish et al, , 2013Neish et al, , 2014Cahill et al, 2014], and is a representation of surface roughness at the wavelength scale of the radar. In other words, surfaces that are smoother at the wavelength scale will have lower CPR values and surfaces that are rougher will have higher CPR values.…”

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) after nine years of operation: A summary

“…The radargrams are resampled to a uniform alongtrack length of % 1000 km. All the orbits are ascending except for orbit 2896 (from Watters et al, 2007). have much higher losses than ice-rich ones (Heggy et al, 2006;Thomson et al, 2012).…”

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

“…However, some multibounce craters in polar areas, which might contain ice deposits as well as rough surface terrain, do have interiors in permanent darkness and are extremely cold, according to Diviner thermal maps [Paige et al, 2010b]. Examples of this latter category include Shackleton [see also the results of Thomson et al, 2012b;Gladstone et al, 2012;Zuber et al, 2012] and a fresh crater on the floor of Rozhdestvensky (Figure 3; Table 3). It is possible for a fresh crater at the poles to contain ice if the timescales of ice accumulation (probably on the order of 10 5 -10 7 years) exceed the likely timescales of geological erosion of its surface ejecta deposits (~10 8 -10 9 years).…”