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Observations of Late Noachian-Early Hesperian-aged Martian surfaces reveal valley networks, lakes, degraded craters, and putative oceanic shorelines, often interpreted to require a persistent "warm and wet" climate, characterized by mean annual temperature >273 K and abundant rainfall. We simulate this "warm and wet" climate (global mean annual temperature similar to 275 K) with a 3-D global climate model to determine whether these features could have formed in this climate through rainfall activity. We find that rainfall is limited in abundance and areal distribution, precipitation is dominated by snowfall, and highlands temperatures are <273 K for the majority of the year. We conclude that, in this simulated climate scenario, (1) Late Noachian-Early Hesperian valley networks and lakes could not have formed through rainfall-related erosion, (2) crater degradation by rainsplash and runoff is not predicted, (3) global clay formation through long-lived rainfall, fluvial activity, and warm temperatures is unlikely, and (4) the presence of a rainfall- and overland flow-fed northern ocean is improbable.

Plain Language Summary Observations and analyses of Martian surface features, including fluvial and lacustrine features, imply that liquid water was abundant similar to 3.7 Ga. The characteristics of these features has led researchers to conclude that the early climate was likely to have been "warm and wet", characterized by abundant rainfall and surface runoff. Here we implement a three-dimensional climate model and simulate the conditions of a "warm and wet" climate scenario, with globally averaged surface temperature similar to 275 K, just above the melting point of water, to determine whether these surface features could have actually formed in this climate scenario through rainfall-related activity. Contrary to previous predictions, we find that rainfall is extremely limited in this climate scenario, precipitation is dominated by snowfall, and temperatures are below freezing for the majority of the year in regions where the fluvial and lacustrine features are abundant. We suggest that snow accumulation, melting, and surface runoff may offer a more plausible explanation for the formation of these features.