Recent Gully Activity on Mars: Clues from Late-stage Water Flow in Gully Systems and Channels in the Antarctic Dry Valleys

Introduction: One of the major surprises of the Mars Global Surveyor mission was the discovery of gullies, a class of unusually young features consisting of an alcove, a channel and a fan [1-2]. These features were interpreted to have originated through the flow of liquid water originating from groundwater discharge [1,2], although alternate interpretations have been presented [see 3]. Uncertainty as to the possibility of surface water flow under current martian conditions (below the triple point of H2O) and evidence that conditions might have been more conducive to melting during higher obliquity conditions in the past, led to the idea that these features might be relict. Recent repeat observations of gullies, however, are interpreted to mean that at least some gullies are currently active [4], and this has intensified the discussion concerning their formation mechanisms and age. In order to look for changes since 2000 that might indicate the presence of fluid flow in gully channels, Malin et al. [4] repeatedly imaged thousands of gullys at hundreds of different sites; they ultimately found only two sites at which changes could be documented. These changes included the appearance of light-toned flows that formed at the two locations sometime between the two imaging sequences (8/99-2/04 and 12/01-5/05). On the wall of the crater at the Terra Serenum site a distinct light-toned flow appeared in the channel. On the southern wall of a crater at Centauri Montes, light-toned material flowed down the slope and formed a deposit. The two new deposits have similar apparent brightness, and relatively long, extended digitate distal and marginal branches; they flow down 20-30 slopes, have relatively low relief and divert around obstacles [4]. These characteristics suggest that the material moved slowly, thinned while flowing, and branched easily [4]. Malin et al. [4] interpreted these characteristics to mean that the observed deposits were formed by flow of fluidized material through the gullies to their aprons by release of groundwater from underground aquifers "initiated and fed by the collapse of an ice-impregnated rock dam creating a brief, low-volume debris flow initially charged with liquid but in which ongoing freezing at both the top and bottom surfaces, bed infiltration, and incorporation of slope sediment and debris increases viscosity, which inhibits downslope and runout motion" [4]. Because the light tone has lasted more than a martian year in an environment where water ice is unstable, Malin et al. [4] suggest that the brightness may reflect "replenishment of surface frost by exhalation, elutriation of fine-grained sediment, or precipitation of salts" [4]. Malin et al. [4] conclude that water flowed on the surface of Mars during the last decade and that it was released from an underground aquifer source. Terrestrial analogs to martian environments may provide insight into the processes operating on Mars, and the origin and life cycle of gullies. Here we report on the results of ongoing field studies of gullies in the Antarctic Dry Valleys (ADV), a hyperarid cold polar desert analog for Mars [7]. We address the questions: What is the life cycle of gullies and what processes are responsible for their later stages? How does the ADV insight help to evaluate and understand the evidence [7] for current gully activity on Mars?