Comparing sublimation and melting of CO2 and H2O ice-cemented sediments into molards: implications for martian surface processes

We performed the first laboratory study on the formation of molards by sublimation processes. On Earth, permafrost molards are cones of loose debris in landslide deposits that can be used as a marker for mountain permafrost retreat (Morino et al., EPSL 2019). They originate from ice-cemented blocks of sediment that are transported downslope within the landslide and melt to form conical mounds over time. Molard candidates have also been found on Mars in the ejecta flows of the one billion-year-old Hale Crater. These show similar morphology and spatial distribution to molards found on Earth (Morino et al., Icarus 2022). In contrast to Earth, these molards likely formed by sublimation, because water is not stable in its liquid form (Harbele et al., JGR 2001). To investigate how molards that formed by sublimation could differ from those formed by melting on Earth we performed experiments at the Open University’s Mars Chamber facility.

We created cylindrical (Ø13 cm) initial frozen blocks of sediment with either H₂O or CO₂ ice. To condense CO₂ gas within the sediment we modified the approach of Kaufmann and Hagermann (Icarus 2017). Because CO₂ has a faster sublimation rate than H₂O, this allowed us to investigate a wider range of sublimation conditions, and reveal processes which may be applicable to comets and/or icy satellites.

We let the initially frozen blocks of sediment degrade on a board in the Mars Chamber while monitoring them with a time-lapse photogrammetry system at a 15 minute interval. This allowed us to quantify the volume transport during the degradation phase. We performed experiments for both ice types at terrestrial and martian pressure for coarse sand, gravel, and JSC-Mars-1a (a Mars regolith simulant). We successfully recreated conical morphologies resembling terrestrial permafrost molards for coarse sand and gravel with CO₂ and H₂O ice under Martian pressure. JSC-Mars-1 fully degrades into conical mounds with CO₂ ice, but only partially degrades for H₂O ice under Martian conditions and does not degrade under terrestrial conditions.

The sublimation gas flux produced by the ice makes the largest difference in morphology between the experiments for the finest sediments. For the JSC-Mars-1a under martian pressure, the CO₂ ice cemented block degrades into a mound that is spread over a wider area than the same block under terrestrial conditions. We infer that the higher the gas production the more likely the grains are to be ejected, rather than just fall. Sublimation is not the dominant degradation process for the H₂O ice cemented JSC-Mars-1a block. All the blocks with coarse sand and ice degrade by sublimation processes. Yet because the grains are barely entrained by the gas flux (even at the highest forcing), the differences are more subtle. The gravel is not influenced by the sublimation gas flux. Our results reveal that sublimation can change the expected morphologies when the gas flux is able to entrain the sediment and has implications for interpreting sublimation pit morphologies on Mars and other planetary bodies where sublimation dominates (Mangold, Geomorphology 2011).