A NEW CONCEPTUAL MODEL FOR SLOPE-STREAK FORMATION ON MARS

New orbital observations indicate involvement of a liquid phase in the formation of ubiquitous elongated albedo-features known as ‘slope streaks', presently forming on dust-mantled slopes in the tropical latitudes of Mars. The common interpretation for slope streaks is dry mass wasting in the form of ‘dust avalanches', although competing hypotheses involving liquids have also been postulated primarily on the basis of morphologic resemblance between slope streaks and terrestrial liquid-flow or seepage analogs. In this study we tested the consistency of these hypotheses with slope-streak morphologic characteristics derived from HiRISE (High Resolution Imaging Experiment) images, and their spectral characteristics as derived from HRSC (High Resolution Stereo Camera) and THEMIS (Thermal Emission Imaging System) data. Morphologic evidence revealed in ~30 cm/pixel HiRISE images, such as light-toned substrates below thin dust mantles on which dark slope streaks occur, demonstrate conclusively that the slope streaks analyzed were not formed by an erosional process, such as a dust avalanche. Furthermore, spectral data extracted from HRSC and THEMIS images, indicate that dark slope streaks originating from a ‘point source' were more consistent with the liquid-flow or seepage hypotheses because they were spectrally distinct from nearby bedrock outcrops and unaffected dusty slopes and could not be explained as spectral mixtures between such scene elements. However, the spectral characteristics of the dark slope-streaks also preclude persistent wetting or freezing of the dust mantle as the cause for their observed albedo. We therefore interpret the slope streaks analyzed as the result of residue precipitated from a short-term interstitial seepage of a liquid phase through the pour space of the thin dust mantle. Important elements of this model, e.g., the source of the liquids, their composition and the nature of the residue, remain the subject of ongoing experimental research. The conceptual diagenetic model we present is consistent with existing observational data for slope streaks but differs from previously suggested seepage models in that it requires small amounts of liquids and does not require their long-term stability, or that of ice, on low-latitude surfaces of Mars under present-day conditions.