Gullies and Thermokarst Landforms in the Central Peak Region of Lyot Crater: Implications for a Late Mars Microclimate.

Detailed morphologic analysis and mapping of gully, channel, and valley systems and the surrounding landscape can reveal much about their formational environments. In our recent mapping of Lyot Crater's gullied central peak region, we have identified intriguing flat-floored depressions connected by channels trending south towards the lowest elevation ( -7100 m) in the N. Hemisphere. These features are similar to beaded streams in terrestrial permafrost terrains experiencing seasonal freeze-thaw processes over several 10² years. Because these systems terminate near -6800 m, we suggest they delineate a paleolake existing during a period of higher obliquity.

Gully formation restricted to the western central peak region suggests a local orographic effect. Here we propose a local hydrological cycle where westerly winds blowing over an ice-covered lake evaporate/sublimate sufficient water vapor to deposit snow at a cold trap 2 km higher on the central peak. Assuming current THEMIS surface T_max( 265 K) on Lyot's floor, snow could accumulate on the central peak over similar areas at a rate of several m/yr of equivalent water with estimated snowfield sublimation rates of 10s of cm/yr (Gulick et. al 1997). Further modeling will be explored.

Could liquid water have flowed in Lyot crater? Haberle et al. (2001) addressed water stability on Mars and found that liquid water need not be stable with respect to evaporation, but only with respect to boiling and freezing. They pointed out, that liquid water is generally not stable on Mars or Earth with respect to evaporation, as the lower limit of liquid water stability is defined by the freezing curve and is independent of ambient pressures. Furthermore, the boiling point is the temperature at which the saturation vapor pressure equals the total external pressure, regardless of the external pressure source. They estimated that the T and P range in which liquid water could exist on Mars is between the triple point of water and 283 K and 12 mbar. Within this range, the total pressure can be supplied by CO₂, and water vapor need not be present to stabilize liquid water against boiling. We are continuing to explore this potential water source. This mechanism if correct would have important astrobiological implications for Mars' late paleoclimatic history.