AbstractsEarth & Environmental Science

Studies of gully development on Mars have proposed that sources of erosive agents and formation mechanisms are attributed to subsurface liquid reservoir expulsions in which reservoir liquids emanate from a slope surface and initiate gully erosion, meltwater formation at or near the surface, or non-liquid, dry flows. These mechanisms are tested using orientations, distributions, dimensions, and morphology of gullies and the surrounding terrain. Observations are gathered from southern hemisphere craters occurring within 30-65º S latitude and 120-180º W, 300-0º W, 190-240º W, using images taken from the Context camera on board the Mars Reconnaissance Orbiter spacecraft, between 2006 and 2007. Gully clusters are distributed between 29º to 58º S latitude, preferentially on poleward slope orientations between 30º to 40º S, and southwest and western orientations between 40º to 60º S, although gullies are found on all slope faces within all latitude bands. The morphological analysis reveals that most gullies have incised channels (89% of all gully clusters) and elongated alcoves (94%), consistent with erosion by a liquid, indicating that dry flow processes are rare. The occurrence of gullies on all slope orientations, the common occurrences of bedrock-like alcove surface textures, and of alcove base depths at depths greater than 250 m suggest that a shallow liquid water aquifer mechanism associated with a dry, overburden soil is not well favored. Gully formation on all slope orientations and the wider range of alcove base depths observed also indicate that the deep aquifer formation mechanism, and gullies formed by the expulsion of shallow aquifers triggered by localized geothermal heating, are not strongly favored. Formation by near-surface ground ice meltwater and liquids attributed to reservoirs of CO2 and liquid brine are possible, though the results are inconclusive due to limited diagnostic criteria. Formation by snowpack meltwater is the most plausible. High occurrences of gully features forming in snowpack-like mantling material, on all slope orientations, and on slopes of crater central peaks and pits support this conclusion. Snowpack meltwater-driven gullies are proposed indicators of high obliquity periods when water-rich snow accumulates in the mid-latitudes from water transported from the poles, and of subsequent low obliquity periods as the average mid-latitude temperatures rose to permit the formation of gully-carving meltwater. While gully clusters associated with the snowpack meltwater formation hypothesis are best supported by MRO Context imagery, the range of features observed indicates that no one mechanism is responsible for the formation of all gullies. The morphologic diversity of gully clusters suggests that gullies may have formed by more than one type of mechanism under diverse environmental conditions.