CO2 gas fluidization possible origin of Martian polar gullies

Martian gully landforms, indicative of rapid flow of sediment down steep slopes, have been cited as evidence of the action of nearsurface liquid water in Mars' recent past. Gullies in polar regions cannot involve pure liquid water because ambient temperatures are too low. Here, we show that polar gullies could be initiated by fluidization of sediment over a subliming seasonal deposit of CO2 frost, as has been proposed previously. For gullies in sand, the gas speed and CO2 sublimation rate required for fluidization is calculated with the semiempirical Ergun relation (as validated in industrial applications). For gullies in dust, the gas speed and sublimation rate required for fluidization are estimated from laboratory experiments with comparable materials. To determine if those rates can be achieved, we compute diurnal and seasonal surface (and subsurface) temperatures through a Martian year (including CO2 condensation and sublimation) using the MARSTHERM onedimensional finite difference thermal model. Models were run without sediment, and with layers of sand or dust 10-4-10-1 m thick deposited over a seasonal layer of CO2 frost. The simulations show that, in the spring, sufficient heat reaches the CO2 frost, underlying the surface sediment layer, to fluidize them. This result confirms that Martian gullies may have diverse origins and that polar and mid latitude gullies can be initiated on steep slopes by the fluidization of loose sediment on a sublimating seasonal deposit of CO2 gas.