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• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

Understanding when, where, and how frequently liquid water was stable on Mars since the Late Noachian/Early Hesperian (3.2-3.9 Ga) is important for understanding the evolution of Mars' climate and hydrology. Some relatively young features on Mars require multiple wetting events to form, whereas others are consistent with single wetting events. Small and rare exit breach craters or "pollywogs" are craters between 0.5 and 15 km in diameter with valleys leading away from the lowest point on their rims but no visible inlet valleys. These craters must have been filled with water to the point of overspill to form the observed valleys. The two possible water sources are precipitation and groundwater. In this paper we use measurements from Digital Elevation Models (DEMs) of 18 pollywog craters (21 outlet valleys) and a fixed channel width 0-D breach erosion model to determine whether pollywog exit breach valleys are consistent with a single crater overspill event, or if their formation requires multiple overspill events. Our model, which we compare to a selection of dam breaching events on Earth, predicts runaway erosion for two pollywog exit breaches. No runaway erosion is observed. We discuss potential explanations for this mismatch between the data and our model. We show that the majority of pollywog craters on Mars are consistent with formation during a single crater overspill event, incorporating a work around for the long-standing problem of unknown grainsize into our approach. Three pollywog craters require either multiple events or sustained water supply to drive erosion. We discuss potential source mechanisms for crater-filling water and conclude that pollywogs either formed in a single erosion event, driven by groundwater discharge, or through many small erosion events, driven by draining of small meltwater lakes formed on crater-filling bodies of ice. • Small exit breach craters record wet events on Hesperian/Amazonian Mars. • We use a fixed channel width 0-D breach erosion model to predict valley incision in one overspill event. • Pollywogs either formed during a single, groundwater-driven crater overspill event, or smaller repeated climate-driven draining events. [ABSTRACT FROM AUTHOR]