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

Abstract

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.

Key Points

• Developed landscape evolution models comparing valley network formation from precipitation (warm wet) versus ice melt (icy cold) sources

• In icy cold models, valley heads originate around the ice stability line, whereas warm wet models have heads distributed among all elevations

• Valleys on Mars have heads originating across many elevations, indicating a past climate warm enough for widespread precipitation

Plain Language Summary

Our study addresses a longstanding mystery about the climate of ancient Mars. Observations of large valley networks suggest formation by flowing water. However, most climate models cannot sustain temperatures above freezing. To understand this contradiction, we modeled the two leading theories for valley formation from precipitation (a warm wet climate) or temporarily melted ice from the edge of an ice cap (an icy cold climate). We found that the main difference between these scenarios was the location of the origin of the valleys that formed. In a warm wet setting, valleys start at many different elevations. In the icy cold scenario, valleys start only near the elevation where ice melted. We then examined a region of Mars with many large valley networks, focusing on the location and elevation of valley heads. Our findings showed that the distribution of valley heads matches predictions for a climate that includes precipitation rather than just runoff from melting ice caps. This suggests that precipitation played a significant role in forming these valleys, indicating that ancient Mars likely had a climate warm enough to support rain. These results help us better understand Mars's past climate and the planet's potential to have supported life.