Mapping gully-susceptible hillslopes on Mars: implications for their mode of formation and evolution

Martian gullies display spatial patterns according to latitude, topography, and substrate properties that have been used to provide constraints on formation hypotheses. In this study we generate a map of gully-susceptible terrains across the mid-latitudes of Mars by defining the slope, aspect, and thermal inertia conditions that most gullied hillslopes possess. We mapped all terrains in the midlatitudes that possess these distributive conditions. Our map describes the general distribution of gullies well, and we found 331 previously unidentified gullied hillslopes on gully-susceptible terrains. However, the favorable distributive conditions for gullies cover an area 3 times greater than that of gullied hillslopes, and 79.1 % of these terrains do not display gullies. Correlation between gully-lacking terrains and regions with a high concentration of viscous flow features suggest that gully formation is hindered by topographically forced climates in these regions that either inhibit production of liquid water runoff via melting or the condensation of CO₂ on the surface. We found that gully-lacking regions are not constrained to specifically high or low dust index regions and we suggest that gully and dust distribution are co-incidental effects of atmospheric circulation patterns. Lastly, we observed depressions on gully-lacking hillslopes consistent in morphology with buried gully alcoves, and we propose that these features correspond to ancient gully alcoves that have been buried by glacial deposits, which implies that these potential gullies were formed >5–10 Myr ago.