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The progressive degradation and loss of impact craters throughout the Noachian Period on Mars indicate prolonged erosion, rather than only a brief spike around the Noachian/Hesperian transition. The purpose of this study is to determine which suite(s) of geomorphic processes and rates best reproduce the relict Noachian landscape. We modeled the evolution of two study areas in the Martian highlands, Noachis Terra and Terra Cimmeria, to constrain the long-term erosional processes and rates that prevailed ~4.0-3.7Ga, before the valley networks formed. Our model runs indicate a very low erosion rate and inefficient runoff production during the Middle and Late Noachian Epochs. The terrestrial equivalent of 2 and 1.5Myr worth of erosional work under arid to semiarid conditions with inefficient runoff production was required in order to replicate the landscape at Noachis Terra and Terra Cimmeria, respectively, over ~300Myr. Low areas in the landscape filled with sediments, while mass wasting and fluvial erosion were focused on crater rims and interior walls. The surface characteristics that best reproduce the observed landscape are a bedrock weathering rate around 0.0005-0.001m/year with regolith that is 100 times more erodible than bedrock, suggesting that impact comminution and aqueous weathering produced abundant transportable material on Noachian Mars. The simulation results suggest that the Noachian paleoclimate was more arid than around the Noachian/Hesperian transition, but it supported occasional precipitation (snow or rain) that weathered surface materials and transported sediments.

Plain Language Summary Evidence shows that Mars was once wetter with liquid water flowing long enough on the surface to form valley networks. Previous studies indicate that the climate responsible for valley formation was short lived, although older craters are much more degraded than can be explained by short-term climate change. A landscape evolution model was used to recreate the landforms we see at two locations on Mars, Noachis Terra and Terra Cimmeria, to estimate the possible climatic conditions responsible for shaping the topography of early Mars before the formation of valley networks (~4.0-3.7Ga). We controlled parameters such as aridity, the weathering rate of the surface material, and how much of the precipitation contributes to surface runoff. Our model runs indicate conditions similar to those of arid to semiarid environments on Earth with a very low erosion rate and inefficient runoff during the Middle and Late Noachian Epochs. Our results also show that crater impacts and aqueous weathering produced abundant loose materials that are transportable by surface flow. A major portion of early Mars was more arid than around the time of intense valley formation, but it supported occasional precipitation (snow or rain) that weathered and transported surface sediments.