A Reappraisal of Subtropical Subsurface Water Ice Stability on Mars

Massive reservoirs of subsurface water ice in equilibrium with atmospheric water vapor are found poleward of 45 degrees latitude on Mars. The absence of CO2 frost on steep pole-facing slopes and simulations of atmospheric-soil water exchanges suggested that water ice could be stable underneath these slopes down to 25 degrees latitude. We revisit these arguments with a new slope microclimate model. Our model shows that below 30 degrees latitude, slopes are warmer than previously estimated as the air above is heated by warm surrounding plains. This additional heat prevents the formation of surface CO2 frost and subsurface water ice for most slopes. Our model suggests the presence of subsurface water ice beneath pole-facing slopes down to 30 degrees latitude, and possibly 25 degrees latitude on sparse steep dusty slopes. While unstable ice deposits might be present, our results suggest that water ice is rarer than previously thought in the +/- 30 degrees latitude range considered for human exploration. The presence of water ice near the equator is a key issue for future human exploration of Mars. In the current climate, this ice cannot exist near the equator but could be stable at accessible depths below pole-facing slopes down to latitudes of 25 degrees, that is, close enough to the equator for a crewed mission. Here, we study the possible presence of this subsurface ice with a new model that simulates the microclimates associated with slopes on Mars. Our results show that, contrary to the arguments put forward in the literature, the slopes close to the equator (20 degrees-30 degrees) may in fact be too warm to allow subsurface water ice to be stable, and that the observations that suggested the presence of ice under these slopes can be explained otherwise by our model. Thus, the widespread presence of water ice under these slopes at subtropical latitudes is not demonstrated. However, our model cannot rule out the presence of ancient ice reservoirs, that would be slowly sublimating today. We use a new model of steep slope microclimates to explore the stability of subsurface water ice on Mars at latitudes lower than 30 degrees Our model shows that warm plains and large-scale atmospheric dynamics heat these slopes, preventing ice from being stableSubsurface ice is predicted to be present down to 30 degrees of latitude, possibly down to 25 degrees but for sparse slopes with favorable conditions