Mars Gully Analogs in the Antarctic Dry Valleys: Geological Setting and Processes

Introduction: Malin and Edgett [1,2] initially described a class of young features on Mars that they termed gullies, consisting of an alcove, a channel and a fan. Restricted to middle and high latitude locations, these features were interpreted to have originated through processes related to the presence of liquid water (through groundwater discharge); the potential presence of liquid water on the surface of Mars currently or in the very recent geological past, when liquid water is metastable [3], generated a host of alternative explanations for the gullies [see summary in 4]. Detailed analysis of the conditions under which H2O could flow as a liquid in the current Mars environment shows a range of conditions under which gully-forming activity is possible [3,5]. Recent observations of changes in gullies, interpreted to mean that a few gullies are currently active [6], have intensified this discussion. Terrestrial analogs to martian environments may provide insight into the processes operating on Mars. For example, the nature of perennial saline springs forming channels on Axel Heiberg Island in the Canadian High Arctic has been used to support the argument that martian gullies formed from subsurface groundwater springs [7]. In this analysis we report on the results of ongoing [8-11] field studies in the Antarctic Dry Valleys (ADV), a hyperarid polar desert analog for Mars [11]. Streams and Gullies in the ADV: The Antarctic Dry Valleys can be subdivided into three microenvironments, each of which has distinctive geomorphic characteristics [11]. The majority of the ADV surface is unconsolidated sediment (e.g., colluvium, till) modified by contraction-crack polygons. Ice-cemented permafrost occurs in most places throughout the ADV and is most commonly encountered at depths of 0-50 cm; above the permafrost table, a wet active layer is seen in the warmer coastal microenvironment zone, a dry "active" layer occurs in the intermediate zone, and in the stable upland zone soil temperatures generally fail to rise above 0C, preventing the formation of either wet or dry active layers [11]. Gullies and streams occur in the warmer and intermediate zones, commonly on north-facing slopes, and contain the major geomorphic components (alcove, channel, fan) seen on Mars (Fig. 1-2). Streams and channels vary in width from 1-30 m and can be up to 30 km in length [13]. Lack of pluvial activity and associated distributed runoff means that gully streams have little interaction with the broader landscape [11]. In contrast to the underground aquifer saline spring source in the Arctic Axel Heiberg region [7], streams and gullies in the ADV form from surface top-down melting of snow snow and ice due to enhanced summer solar insolation [11,13] (Figs. 1-2). Water Sources: Specific sources for water feeding streams and gullies are due to the melting of surface and very near-surface snow and ice (Figs. 1-2); no deep subsurface springs (below the permafrost base or within the permafrost) have been reported. Because maximum precipitation in the ADV is less than ~10 cm of snow per year [11], other processes are required to concentrate