Examining Formation Mechanisms of Martian Gullies Using Mars Reconnaissance Orbiter Context Imagery

Introduction: Current temperature and pressure conditions on Mars suggest that liquid water is unstable at the surface with respect to boiling and freezing. However, gully features, generally consisting of alcoves , channels, and debris aprons and existing in a band at mid latitudes mainly in the southern hemisphere with a possible preference for pole-facing orien-tations [1, 2], suggest liquid agents, water perhaps being the most plausible. These observations are consistent with models by [3] that suggest liquid water is possible at some subsurface depths. Alternative proposed mechanisms suggest that gullies have been carved by release of liquid water or CO 2 reservoirs [4, 5, 6], melting of ice-rich snow pack [7], obliquity-induced melting derived from near surface ground ice [8], melting of permafrost ice by localized geothermal activity [9], or by dry or granular flows [10, 11]. Mars Orbiter Camera (MOC) and HiRISE have provided opportunities to understand gully features and formation mechanisms on Mars; however, both MOC and HiRISE typically provide partial views of the crater in which gullies tend to be found. Mars Reconnaissance Orbiter (MRO) Context (CTX) images provide wider views, and gullies studied in crater-wide contexts may show spatial patterns occurring at local scales. Our study, using CTX images, will characterize gullies by morphometry on the basis of apparent maturity and level of complexity of fluvial features, from gullies showing apparent dry flow features to highly incised, sinuous and debris flow-like features. We will determine the spatial distribution of gully classes and assess possible mechanisms of gully formation by evaluating formation hypotheses for each gully class. Methods: We use CTX image strips (30 km with a resolution of 6 m per pixel) to classify gully features that occur within craters. This pilot study focuses on images acquired from the first six months of the primary mission (P01-P06, November 2006 through April 2007) in Mars Chart (MC) 29 (30-60º S, 180-240º W). Although additional Mars Charts from the same latitude band will eventually be added, we limit this abstract to MC 29. Ninety-three CTX images in the study region have craters with gullies. A crater site contains one or more clusters of gullies. Representative gullies within morphologically distinct clusters were sampled. Data collected for each cluster include gully characteristics selected to test several formation hypotheses by determining whether