The Solar Wind Plasma Environment along the Floor of the Moon's Shackleton Crater: An Affect on Floor Brightening?

Recent reports [Zuber et al, 2012; Lucey et al., 2014] indicate that the lunar permanently-shadowed south polar crater, Shackleton, has an anomalously bright floor as observed in 1064 nm LIDAR reflections. It has been suggested that unusual space weathering or possibly adsorbed water may be the cause of that brightening. In this paper, we examine the solar wind plasma inflow into the crater using LRO LOLA topography and plasma expansion formalism, and show that plasma flux to the surface is far lower than in topside regions directly exposed to the solar wind flow. The lack of plasma along the crater floor is expected to reduce space weathering of the regolith. Ironically, the lack of plasma also reduces sputtering allowing any adsorbed water/frost to persist for longer times relative to topside regions. We will thus consider how the reduced plasma inflow affects water and other volatiles that reside on the crater floor.