Landscape Evolution Models of Incision on Mars: Implications for the Ancient Climate

Large dendritic valley networks observed on Mars present a paleoclimate paradox. Geologic observations of Noachian units on Mars reveal a global extent of valley networks, which are believed to have been formed through incisions made by flowing water. However, most climate models predict global surface temperatures too far below the freezing point of water to support an active hydrological system. Conflicting observations and models have led to disparate theories for the climate of early Mars. In this work, we surveyed a large region of the cratered southern highlands to identify the location, elevation, and distribution of observed valley heads. These valley head locations were compared to landscape evolution simulations in which the spatial distribution of runoff was varied. The measured valley head distributions were compared to predictions from landscape evolution models for two end-member hypotheses: (a) a warm wet climate that supported spatially distributed precipitation, and (b) surface runoff from ice cap margins, as envisioned by the Late Noachian Icy Highland model (LNIH). The observed elevation distribution in valley heads is consistent with the prediction of precipitation-fed models, and inconsistent with models in which runoff derives exclusively from a single line-source of high-elevation ice-melt. The results support the view that it is unlikely for ice caps to be the sole source of water and are consistent with the hypothesis that precipitation significantly contributed to valley network formation on ancient Mars.