Characteristics of Martian Valley Networks and the Implications for past Climates

Introduction: Martian valley networks indicate that at least geologic conditions were different in the past, if not the climate. It is commonly believed that valley networks must be the result of groundwater sapping because their apparent drainage densities are lower than terrestrial runoff channels [1, 2, 3] (Figure 1a). It has also been suggested that valley networks are not uniformly distributed on topography, which would also argue in favor of groundwater sapping [4]. When coupled to the belief that early Mars was cold and dry such observations have led many investigators to suggest that valley networks were fed by geothermal heating of ground ice [4, 5, 6]. Unfortunately, however , plotting the densities of valley networks strictly from imagery data is easily influenced by observational bias and interpretation. To test some of the hypotheses derived from Viking era data, we have used MOLA topographic data and MOC and THEMIS imagery data to reevaluate valley network drainage densities and their related characteristics. Approach: A number of algorithms have been written to allow investigators to extract terrestrial drainage basin information from digital elevation models (DEM's). In particular, the D8 algorithm is widely used (e.g., [7]) and is available in several GIS commercial software packages. At any given pixel flow direction is represented by a single angle taken as the steepest downward slope on the eight triangular facets center at each surrounding pixel. Upslope area is then calculated by proportioning flow between two down-slope pixels according to how close this flow direction is to the downslope pixel. Flow direction is then integrated to determine the most probable flow paths for surface water over the given DEM. Streams of different magnitude are also identified following several conventions [8, 9, 10]. This information can then be used to characterize aspects of the drainage basin. Mars Orbiter Laser Altimeter data collected from ±30° latitude were gridded to ~1 km resolution (Figure 1b). The resulting digital elevation models were then subjected to the D8 algorithm available through both RiverTools and Arc Hydro. Martian DEM's differ from terrestrial DEM's in that there is a great deal of topographic expression from impact craters that postdate the valley network systems. Complicating this is the fact that craters were also forming as the valley networks were developing. It appears that larger cra