How the martian residual south polar cap develops quasi-circular and heart-shaped pits, troughs, and moats

The martian Residual South Polar Cap (RSPC) is a 1-10 m thick deposit of permanent CO_2 ice perched on the much larger H_2O ice cap. The CO_2 ice is dissected into mesas by erosional landforms that can be broadly classified as (i) quasi-circular pits, (ii) heart-shaped pits, (iii) linear troughs, and (iv) moats. We use HiRISE (25-50 cm/px) images taken at a cadence of days to months to track meter-scale changes in the RSPC in order to investigate the mechanisms that lead to the development of these four distinct morphologies. For the first time, we report the development of dark fans on the sides of the CO_2 mesas and the fracturing and deterioration of the initially smooth upper surface of CO_2 mesas. We interpret these features as indicating the sublimation and subsequent escape of CO_2 from the interiors of mesas, which undermines structural support of mesa tops, causing them to collapse. The collapse of mesa tops, along with uneven deposition of CO_2 ice, creates steep scarps that erode during the summer due to preferential sunlight absorption. During the winter, CO_2 deposition acts to smooth topography, creating gently sloping ramps. We propose that the interplay between the steep scarps and gentle slopes leads to either quasi-circular pits, heart-shaped pits, linear troughs, or moats, depending on local conditions.