|Dark spots attributed to dry ice along gullies in Lyell crater.|
The commentary by Colin Dundas (Nature Geoscience 9, pp. 10-11, 2016) sets the stage by summarizing the context. Although the gullies look like terrestrial landforms caused by running water, there is a real problem finding the source of the putative water. Groundwater discharge is frequently cited, but this is inconsistent with the occurrence of gullies on sand dunes and isolated peaks. "Whatever the water source, wet models imply the repeated occurrence of thousands of cubic meters of liquid water at each gully, which would have profound implications for both climate and possible biology on Mars." Over the past two decades photographs of the surface have documented channel erosion and the deposition of debris flows in locations where the present climate is too cold for substantial liquid water. However, many (most?) of these locations occur where seasonal CO2 frost occurs, and gully activity occurs mainly in the winter and spring when CO2 frost is observed on the slopes and is available for participating in gully formation.
The model used is one-dimensional and examines the evolution of a column that consists of a regolith underlying a CO2 ice layer and an atmosphere. The atmosphere is in radiative-convective equilibrium and the incident radiation on slopes of varying angles is computed. In the CO2 ice layer and the regolith, heat conduction and radiative transfer through the ice are calculated, as well as diffusion, condensation and sublimation of CO2 and the latent heat exchanges (all described in the Methods section). CO2 is predicted to condense above 50 degrees latitude on flat surfaces and down to ~30 degrees latitude on pole-facing slopes. In such locations, subsurface H2O-ice in equilibrium with the atmospheric water vapor is expected to be present below a layer of dry regolith ranging up to several centimeters thickness. This justified treating the model regolith as a dry porous layer lying above an impermeable, ice-cemented soil.