Properties of seasonal ice at Sisyphi Cavi and implications for current modification of Martian gullies

Martian gullies are geologically recent landforms that may form either through liquid-water activity and/or through processes involving CO 2 ice. We investigated the mechanisms responsible for the formation or modification of these features by focusing on the active site of Sisyphi Cavi (68°S, 1°E), located outside the typical latitude range of gully presence. Using CRISM and OMEGA infrared data, we characterized the composition and physical state of seasonal surface ices to test the relevance of H 2 O and CO 2 ice driven mechanisms. Our analysis shows that H 2 O ice is not detected as an independent surface deposit, although it may be present as minor inclusions within the CO 2 ice layer. In particular, during the final phase of CO 2 ice sublimation in late spring, no H 2 O ice signature is observed. In the area, faint spectral signatures of sulfate salts are observed, but their distribution and amount do not suggest any direct link with gully activity. Available observations during early and mid-spring reveal that CO 2 ice is translucent during these times, suggesting that it likely remains in this state through most of the ice season. However, a temporal mismatch between dark spot formation - indicative of CO 2 geysers through translucent ice - and gully modification (respectively occurring late winter to early spring, and mid to late spring) exists. This does not suggest a systematic link between both processes. Overall, the available observations provide no evidence that liquid water contributes to present-day gully activity at Sisyphi Cavi, while offering no support either for the hypothesis that gully modifications are mainly driven by the formation of CO 2 geysers. Gully modifications at Sisyphi Cavi, observed during the late sublimation stages of CO 2 ice, may be rather more appropriately explained by CO 2 -ice-based fluidization or avalanche processes.