The Effect of Varying Annual Snow Accumulation on Gully Formation in Anarctica: Comparisons between ‘wet’ and ‘dry’ Seasons and Implications for Gully Formation on Mars

Introduction: The discovery of gullies on Mars attracted significant attention because of the apparent role of liquid water in their formation and because of their very youthful nature [1,2]. They were initially interpreted to be the result of groundwater discharge [1-3], though further considerations regarding the current metastability of liquid water on the surface of Mars generated alternative explanations, including atmospherically deposited sources of water [4-7]. Recent modeling work has demonstrated that snowfall [8] and the melting of such deposits [4,5,9] is possible in the regions where gullies are located under periods of higher obliquity. This has provided support for the snowmelt model of gully formation [4-6,9]. The utilization of terrestrial analogs provides an essential natural laboratory for testing the snowmelt model as a viable means of gully formation. In this analysis we report on results obtained from field studies within the McMurdo Dry Valleys of Antarctica (ADV), a hyper-arid polar desert that has long been held as one of the most Mars-like of terrestrial analogs [10-12] (Fig. 1). Within this environment, precipitation only falls as snow and so the availability of liquid water is restricted to meltwater generated by the solar heating of ice and snow deposits. In order to best constrain the Martian conditions we concentrated our efforts within the most elevated (and hence driest) portion of the intermediate microclimate zone [12] situated within the South Fork (SF) region of upper Wright Valley. This region marks the most inland extent of active fluvial features within the ADV and is most analogous to Mars during Amazonian periods of high obliquity in terms of precipitation and max T values [12] (Fig.1). Unlike other gullied regions of the ADV, there are no significant adjacent glacial systems to supply meltwater, and so the most significant ice deposits are snowbanks that more closely resemble the deposits predicted to form on Mars during its recent history [4-6,9]. We found that there is significant inter-annual variability in the abundance of snow for gully generation. However, even in years with minimal snowfall, windblown snow can be concentrated in suitable geomorphic traps. 2006-07: The 'Wet' Year. The majority of the larger gullies studied in SF in 2006-7 exhibited fluvial activity associated with snowmelt from within two geomorphic traps: Alcoves. The northern edge of the Asgard Mountains in SF contains a number of alcoves along