Thermokarst on Mars? Insights from a Survey of Rimless Depressions

Introduction: Circular and elliptical depressions, distinct from impact craters by their steep walls, lack of visible rims or ejecta, and flat floors, have been known to exist on Mars for over thirty years. Since their initial discovery in Chryse Planitia by [1], these features have been identified in Viking imagery at several locations, including Lunae Planum, Utopia Plani-tia, and Ares Valles [2],[3],[4]. More recently, narrow-angle MOC and HiRISE imagery has been employed in the study of the features, resulting in the discovery of several new sites of interest, and permitting the examination of previously identified areas in unprecedented detail [5], [6], [7]. Multiple interpretations concerning the origin of these features have been suggested, including aeolian deflation, periglacial processes such as thermokarst, and various combinations thereof [7] [8]. This study endeavors to discriminate between these competing hypotheses, where possible, by undertaking a geomor-phic investigation of rimless depressions on Mars using a combination of THEMIS VIS/IR and MOC imagery , supplemented by MOLA altimetry. Data and Methods: The region of study, covering the planet from +50º to-60º laitude, was broken up into 396 tiles ten degrees on a side, each of which was then used as the basis for a query of the THEMIS database of images (online at global-data.mars.asu.edu). Each of the matching images was then transformed to a simple cylindrical projection, co-registered with its neighboring images, and cropped to the boundaries of the tile. The resulting mosaics formed the basis for this study, each of which was typically made up of approximately 150 individual THEMIS images. Once all promising locations within a tile had been identified, additional data products were retrieved to facilitate the study of each site in more detail. These included THEMIS visible and MOC imagery (if avaiable), and MOLA altimetry. Results: The global distribution of the features is strongly constrained by latitude, with over forty percent of all occurrences lying between 40º and 50ºN. Equally striking is the depressions' uneven longitudinal distribution within that zone. For example, a relatively high concentration of pits has long been known to exist near Utopia basin, from approximately 70º to 140º longitude. Beyond this region, however, the density of the features is highly variable. The distribution peaks near 45º N, declining steadily with decreasing latitude. The equatorial regions exhibit the lowest densities of the pits. The density of the depressions increases slowly to the south of 15ºS, but never approaches the …