MARTIAN HILLSIDE GULLIES: OBSERVATIONS FROM DAO AND HARMAKHIS VALLES

Young hillside gully features on Mars have stirred debate about the potential involvement and relative role of fluids on the surface. Proposed mechanisms include surface runoff fed by subsurface aquifers, discharge of saline ground water or brine, melting of near-surface ice due to warmer conditions in the past, and melting of contemporary snow packs that accumulated due to volatile transport from the poles. Additional hypotheses that do not involve liquid water include liquid carbon dioxide breakout and dry flows of aeolian material. Systematic investigations have found that gullies and gully-related features are preferentially located within latitudinal zones between 30 - 50 degrees north and south; of these bands, the southern hemispheric zone contains more gully features. Most surveys focused on gullies forming along crater walls; however, the Dao and Harmakhis Valles systems provide a unique opportunity to evaluate gully features with respect to latitude, elevation, and slope orientation in a single, distinct geologic setting.

Dao and Harmakhis Valles dissect the east rim of the Hellas basin and collectively extend for a length of ~2400 kilometers, a considerably greater length than for gullied crater rims. These canyons also continuously span nearly 15 degrees of latitude (from ~30 to 45 degrees south, the zone shown to have a significantly higher population of gullies), and they have a 5-kilometer elevation drop from their heads to their termini, which may affect local climate and thus local morphology.

Over 100 Mars Orbiter Camera (MOC) images from Mars Global Surveyor transect the walls of Dao and Harmakhis Valles; numerous gully features are observed. Three distinct morphologies have been classified along their walls: mantled (filled alcoves on the up-slope of the canyon wall), incised (partially to totally filled alcoves with superposed longitudinal depressions), and exposed (complete alcove-gully-apron systems). These morphologies potentially represent an evolutionary sequence of gully formation.

Evaluating this diverse suite of features along the walls of Dao and Harmakhis Valles may help to constrain models of Martian gully formation. Preliminary analysis supports gully formation by melting of a volatile-rich superstrate, potentially from accumulation of snow in certain surface settings.