Lunar south pole boulders and boulder tracks: Implications for crew and rover traverses

Knowledge about the geomechanical properties of the southern circumpolar regolith and its spatial variability is essential for effective and diligent planning of future crew and rover traverses. At this time, the trafficability of the lunar south pole remains untested and could potentially be different from low-latitude regions that have been traversed in the past. Geomechanical properties of those untraversed regions can be estimated using observations of boulder tracks captured from orbit. Eighty-four boulders with tracks in the region poleward from 85 degrees S have been mapped in Lunar Reconnaissance Orbiter's NAC imagery using a deep learning-driven and operator-complemented approach. An additional 13 boulder tracks have been mapped that enter and disappear in Shackleton Crater's permanently shadowed region. A recently developed methodology has been adapted for extreme lighting conditions and used to estimate the bearing capacity of regolith crossed by 16 boulders in sunlit regions in the vicinity of the south pole. Estimated bearing capacity values are similar to those derived for equatorial highland regions of the Moon that were successfully traversed in the past. A qualitative and semi-quantitative comparison of track morphologies across the regions does not show any major differences. Measurements of boulder tracks have then been used to establish a conservative quantitative relation between track depth, local slope angle, and circumpolar bearing capacity. This relation has been used to map the approximate surface strength of the sunlit lunar south polar region as function of topography. The resulting product has been applied to study the estimated sinkage of exploration rovers on slopes, indicating that sinkage-related issues might only start to occur on inclines steeper than 25 degrees while the potential for local, slope independent mechanical strength anomalies remains. The results of this study show no evidence that future south polar surface operations would be affected by insufficient sunlit regolith bearing capacities.