Open Holes in the Sheepbed Unit of Yellowknife Bay, Gale Crater, Mars

Curiosity investigated the Sheepbed Unit in Gale Crater from Sol 125 to Sol 308. It is locally the lowest stratigraphic unit in Yellowknife Bay and consists of light-toned slabs of mudstone that are typically 0.5 to 1 m across. In a 4 x 2 m area of John Klein at least four open holes were present at the margins of slabs. Photos of the holes were obtained with the mast camera (Mastcam) and hand lens imager (MAHLI). Holes were 5 cm long by about 1 cm wide. One hole observed from directly overhead had an opening the extended downward out of sight at the margin of the slab. Several hypotheses for the formation of the holes are: (1) thermal expansion and contraction of slabs create voids at their edges, which can be deepened by the wind, (2) pre-existing voids were present in the subsurface, and erosion at the edges of the slabs exposed these cavities into which the soil falls; cavities may have formed by dissolution of evaporites or by sublimation of ice, (3) gas venting keeps the holes open. Data from the holes and slabs were used to attempt to evaluate these hypotheses. (i) Holes are close to the margins of slabs and not in the open areas of soil, (ii) winds move the soil but the holes are not filled with soil, which suggests that the holes formed recently, (iii) hydrogen measurements with DAN indicate that there is more water below the slabs than below the soils away from the slabs, (iv) ground temperatures suggest that Yellowknife Bay could be a net source of water although the thermal inertia of the mudstone affects this interpretation, (v) there appears to be no enrichment in volatiles at the margin of the hole analyzed by LIBS, and (vi) the open holes are not similar to wind carved depressions in terrestrial soil or to depressions in soil in frozen ground in the Antarctic Dry Valleys. Multiple diurnal thermal cycles, which approach 100°C, could result in displacements of slabs and open gaps along their margins that do not close due to infilling sand. Heat budget analysis suggests that the sensible heat flux is larger in the areas containing slabs than in the slab-free areas, which is consistent with the venting of water vapor. Holes could allow the surface and shallow subsurface to have access to water reservoirs such as ground ice or brines, while gas venting could keep the holes open. The alternatives have not been resolved at this time.