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

Although diverse measurements have indicated H⁺, OH^-, or H₂O 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% H₂O ice (up to 30 vol.%) present in the uppermost meter of regolith is also consistent with the observations.