How Many Overspill Events Does it Take to Erode a Pollywog? Insights from Breach Erosion Modelling

Abstract:

Understanding the availability of surface liquid water on Mars over the past 3.5 Ga can inform us about how its climate and hydrology have evolved over geologic time. From features like low latitude alluvial fans and fresh shallow valleys we know that liquid water continued to be present on Mars’ surface after the intense fluvial erosion of the valley network forming period. 0.5-15 km craters with outlet valleys but no visible inlets (“pollywogs”) are a recently identified indicator of surface liquid water during the Hesperian/Amazonian. In order for outlet valleys to form, these craters must have filled with water to the point of overspill, fed by either precipitation or groundwater outflow. We use Digital Elevation Models (DEMs) to measure craters and outlet valleys for 18 pollywog craters (21 outlet channels). We use these measurements as inputs to a 0-D breach erosion model that couples lake drainage and sediment transport to determine which pollywogs are consistent with formation in a single overspill event, and which require multiple events or continuous water supply. Our model predicts runaway erosion (i.e. erosion to the base of the crater rim) for two pollywogs, which is not observed, but can potentially be explained by ice filled craters. Three pollywogs require either sustained water flux into the crater or multiple overspilling events to form. We find that no existing Hesperian climate models can provide the precipitation required to fill these two pollywogs and erode the observed valleys in a single wet event.