Concentrating Ice in Polar Deserts: Lessons for Mars from Punctuated Gully Incision in the Mcmurdo Dry Valleys

Introduction: The latitude dependent distribution [1-3] and non-uniform orientations [4-8] of young gullies on Mars suggest that their formation and modification are controlled by the activity of volatiles at or near the surface. Given the relative abundance of gullies and Mars' current thermal environment, CO 2 and H 2 O are the most likely candidates for contributing to gully evolution. Contemporary modification of gullies in the southern hemisphere is temporally correlated with the removal of CO 2 frost [9-12], but morphologic properties [13-14], slope values [7], depths of incision [15], and thermal modeling [16-18] of gullies indicate that their formation may be controlled by the action of liquid water in recent high-obliquity excursions, when water ice was capable of accumulating in the mid-latitudes [19-21]. If water is a contributor to gully evolution, then understanding fluvial processes in polar desert environments on Earth is essential for formulating hypotheses for fluvial activity on Mars. The McMurdo Dry Valleys (MDV) of Antarctica host a spectrum of water related landforms [22] that do not form elsewhere on Earth but are morphologically similar to Late Amazonian features on Mars [23]. Here, we outline the geometric and environmental conditions necessary for achieving net erosion of gully channels in the MDV and evaluate whether similar processes may explain their counterparts on Mars. Erosion of Gullies in the MDV: Gullies generally form on relatively warm equator-facing slopes in the MDV [23-24] and exhibit similar alcove/channel/fan morphologies to gullies on Mars [1]. In the South Fork of Upper Wright Valley, which represents the most inland area/zone to experience fluvial erosion [23], alcoves are cut into bedrock cliffs while channel incision occurs along the valley walls below within the uppermost ~20 cm layer of colluvium. This superposes a pervasive ice-cement layer (which is an aquiclude during gully activity [25]). Alcoves serve as traps for windblown snow in the winter, then as inhibitors to rapid sublimation via topographic shielding in the summer [26]. This, in a region that experiences < 5 cm of snowfall annually [27], provides a method for net accumulation of ice in gully source regions. Despite mean annual temperatures in Wright Valley of-19.3°C [28], peak surface temperatures in gully alcoves surpass the melting point on cloudless days in austral spring and summer. Due to the narrow geometry of South Fork, melting occurs in gully alcoves first