Gully Formation on Mars: Testing the Snowpack Hypothesis from Analysis of Analogs in the Antarctic Dry Valleys

Introduction: Gullies, a class of unusually young features on Mars consisting of an alcove, a channel and a fan, were initially interpreted to have originated through processes related to the presence of liquid water, primarily through groundwater discharge [1,2]; the current metastability of liquid water on the surface of Mars generated a host of alternative explanations for the gullies [see summary in 4], including H2O flowing as a liquid in the current Mars environment [3,5]. This discussion has been intensified by recent observations interpreted to mean that gullies are currently active [6]. One interpretation of the source for the water thought to have formed the gullies on Mars is snow [7]; in this scenario, gullies "...form by melting of water-rich snow that has been transported from the poles to mid-latitudes during periods of high obliquity..." [7]. Among the main features of the model [7] are: 1) Snow is deposited at mid-latitudes during periods of high obliquity and melted at lower obliquity when mid-latitude temperatures increase; 2) Melting produces liquid water that is stable below an insulating layer of overlying snow; 3) Gullies form on snow-covered slopes (through meltwater erosion or due to meltwater seeping into loose slope materials and destabilizing them); 4) Snow patches remain today, protected from sublimation by a layer of desiccated dust or sediment; indeed melting might be occurring today in currently favorable snowpack locations [7]. Terrestrial analogs to martian environments may provide insight into whether snow might serve as a source of water for gully formation, and if so, what specific environments and processes are involved and whether they are likely to occur on Mars. In this analysis we report on the results of ongoing field studies in the Antarctic Dry Valleys (ADV), a hyperarid, cold polar desert analog for Mars [8]. We describe specific occurrences of annual and perennial snow accumulations serving as sources of water flowing in gullies in the ADV, and field analyses of the anatomy of associated gullies. Streams and Gullies in the ADV: The Antarctic Dry Valleys, a polar desert environment in which sublimation exceeds precipitation, contain streams and gullies in certain microenvironments [11]; many ADV gullies contain alcoves, channels and fans and some flow into ice-covered lakes [12]. Gullies form from surface top-down melting of snow and ice due to enhanced seasonal solar insolation [8,12] and no deep subsurface (below the permafrost) groundwater springs have been reported. Major streams are fed by melting glaciers; meltwater forms at unique positions on some glacier fronts and surfaces related to seasonal insolation intensity and geometry [13] (insolation-induced melting). Melting can be enhanced by the lower albedo of some substrates (albedo-induced melting) [8]. Smaller streams and many gullies are fed by the melting of perennial and seasonal [10] snow pack deposits within the alcoves and channels. Gullies and streams commonly occur on equator-facing slopes. Analysis of ADV satellite images and aerial photographs led us to assess the nature and location of prominent gullies, their distribution as a function of ADV microenvironment, and their similarities to those on Mars [8,14]. On the basis of this, we undertook fieldwork in the ADV in the South Fork of Upper Wright Valley during the 2006-2007 austral summer and studied a series of gully systems along the southern wall of the South Fork of Wright Valley, just south of the Dais (Fig. 1) [9-11]; we report our observations and preliminary results here.