CONSTRAINTS ON THE CONDITIONS FOR RECURRING SLOPE LINEA ACTIVITY ON MARS

Recurring Slope Lineae (RSL) are seasonally active dark flow features on Mars. RSL form on warm, rocky slopes and in warm seasons, grow incrementally, and fade and re-form annually. These observations are consistent with formation by shallow subsurface flow of liquid water, but the source of water is not understood. The Martian atmosphere contains little water vapor, and some RSL occur in locations where groundwater release is unexpected. We present constraints on the formation processes and conditions from new modeling and observations.

The main loci of known RSL activity are the southern mid-latitudes and Valles Marineris, but some occur in Acidalia Planitia in the northern mid-latitudes. Due to the asymmetry of Martian seasons, the Acidalia observations provide a useful contrast with southern-hemisphere RSL. Ten locations in Acidalia have RSL-like dark features that show incremental growth and/or inter-annual changes, but the number of lineae at a site is typically small compared with sites in the southern hemisphere. While southern RSL mainly grow in the summer, RSL in Acidalia begin growing near the spring equinox. They also show a tendency to repeat flow patterns very precisely from year to year. Models for RSL formation must account for these hemispheric differences.

We investigated the hypothesis that RSL form when water vapor is concentrated as frost in nighttime cold traps and later melted. To melt frost on Mars, the temperature must rise rapidly to reach the melting point before sublimating, and the latent heat loss to vapor must be reduced by high relative humidity or a lowered melting temperature (e.g., due to salts). Modeling the lighting conditions at the times of RSL activity shows that some source areas show swift rises in insolation, but others include only small areas of rapid heating. However, RSL are often associated with late-morning or early-evening shadows where frost might be favored. Persistent frost could stabilize ground ice, but significant volumes of ice are unlikely to exist at depths (a few cm) that regularly reach the melting point. Alternatively, the correlation with shadows could be an artifact of the correlation between RSL and rock. While this modeling has not directly revealed a source of liquid, it provides new constraints on the environmental conditions associated with RSL activity.