QUANTIFYING VALLEY NETWORK MORPHOLOGY ON MARS USING THE MARSSIM LANDFORM EVOLUTION MODEL

The valley networks of the Martian Noachian highlands have been cited as evidence for enhanced fluvial activity on Mars in the ancient past, but debates persist as to exactly what mechanisms led to their formation. Morphological similarities to terrestrial drainage basins seem to indicate that precipitation-based runoff was responsible; other theories suggest the valleys were formed by groundwater sapping due to hydrothermal heating. Studies have shown that Martian valley networks have different scaling laws than those on Earth: Hack’s exponent ranges from ~0.5-0.6 on Earth but from ~0.3-1.3 on Mars. The concavity of longitudinal stream profiles is typically concave up on Earth, but many valley networks on Mars are concave down or linear. Possible causes for this discrepancy may include the relative immaturity of Martian valleys and thus a closer correlation between stream profile and regional terrain, the effects of impact cratering, or a lack of tectonic uplift, but no definitive conclusion has been reached.

The MARSSIM landform evolution model has the ability to simulate erosional processes on both idealized and actual terrains. Topographic data is converted into DEM files for use in the model, which can be manipulated by changing input variables relating to the rate and mode of erosion. Model runs can then be analyzed using hydrologic functions in ArcMap to determine Hack’s exponent and other scaling parameters. Our study thus far has focused on an idealized topography characterized by a perfectly smooth N-S gradient. In this way we have been able to make a thorough inventory of the model’s specific input-output relationships without the complicating factors of real terrain. These initial model runs have yielded consistently higher Hack’s exponents under terrestrial conditions than are seen on Earth, and ones that are closer to the observed values for Mars. This may suggest that the addition of uplift on Earth (something not simulated as yet) is a causative factor in the scaling law discrepancy. Forthcoming simulations using DEMs from real Martian terrains will seek to find a combination of input parameters that create simulated valleys closest in morphology to the ones actually observed. We hope to determine the degree to which any given variable might have affected the formation of valley networks on Mars.