Episodic Mass Wasting and Exposure of High Reflectance Material Within the Shackleton Crater Permanently Shadowed Region

The Shackleton crater permanently shadowed region (PSR) exhibits high 1,064 nm reflectance and S-band radar circular polarization ratios (CPR), which previous studies interpreted as indicating the presence of water-ice or frost. In the absence of photogeologic evidence, alternative hypotheses offered for the reflective properties of the Shackleton crater PSR (mass wasting, presence of pure anorthosite, and interior blockiness) were not confirmed. ShadowCam has acquired high signal-to-noise ratio stereo images of Shackleton crater; we use these images, derived digital terrain models, and M3 data to show that episodic mass wasting is active inside Shackleton, arising from the instability of steep wall slopes. We conclude that the relatively high reflectance within the Shackleton PSR results from continued exposure of plagioclase-rich material and not from water-ice or frost. In addition, mass wasting also excavates small blocks that collect on the wall and floor, mainly responsible for the enhanced radar CPR signature.