Insights into the interaction between defrosting seasonal ices and gully activity from CaSSIS and HiRISE observations in Sisyphi Cavi, Mars

Martian gullies are surface features, typically composed of an alcove, a channel and a depositional apron. They have been extensively studied since their first observations in the 2000s and were initially attributed to the action of liquid water. Later studies highlighted that their activity is spatially and temporally correlated with the seasonal presence of surface CO2 ice, suggesting a link between seasonal frost and gully activity. However, the exact mechanisms leading to gully formation are still under debate. Establishing whether or not gullies are formed by liquid water has important implications for Mars recent climate and habitability. Here, we study the evolution of seasonal frost and its connection to gully activity in Sisyphi Cavi, located in the southern circumpolar region (68°S - 74°S; 345°E − 5°E). The high latitude of this site allows for detailed temporal monitoring, owing to the high frequency imaging by spacecraft in polar orbits, largely unavailable for other martian gullies, hence this site has the potential to reveal new insights on gullies in general. In particular, we used repeat images from MRO HiRISE (Mars Reconnaissance Orbiter, High Resolution Imaging Science Experiment) and TGO CaSSIS (Trace Gas Orbiter, Colour and Stereo Surface Imaging System). Our findings show that the general timing and characteristics of the defrosting patterns in Sisyphi Cavi follow seasonal patterns that are consistent from year to year. The timing of the start of defrosting and its temporal evolution are controlled by the overall orientation of the host hillslope, with equator-facing slopes defrosting before pole-facing ones and the steepest pole-facing slopes defrosting last. Gully alcoves defrost before channels and aprons located on the same hillslope independent of the orientation of the hillslope likely because the aprons have a lower thermal inertia than the alcoves that counteracts the orientation effect, which on pole-facing slopes would mean the alcoves should defrost last. We observe the presence of seasonal and ephemeral dark spots and flows which are interpreted to be a result of sediment deposition on top of the seasonal ice deposits, triggered by basal sublimation. Our observations suggest that the appearance of dark spots and dark flows can be spatially correlated with surface roughness, including presence of boulders, contrasts in material types, irregular lobes/levees and braiding caused by gully activity, and therefore could be used to help detect recently active gullies on Mars in areas with seasonal frost without the need of repeat imaging. Finally, we propose that presently observed gully activity in Sisyphi Cavi is driven by defrosted material flowing on to a frosted apron. We infer that the presence of a frosted apron could be common precursor for this type gully activity. We note that this activity only involves the mobilisation of loose materials and we have not observed any erosion of the wall rock present in Sisyphi Cavi.