Constraining early mars glacial conditions from paleodischarge estimates of intracrater inverted channels

Inverted fluvial channels are globally distributed across Mars. Many inverted channels are interpreted as the depositional counterparts to large, regionally integrated valley networks. These features indicate the presence of flowing liquid water in the early (Noachian) period of Mars geologic history. We previously identified inverted fluvial channels within the Noachian-aged crater “B” in Terra Sabaea. Unlike the larger inverted channels associated with valley networks, the drainage that formed these inverted channels appears to be derived from within the crater itself, i.e. a closed-source drainage basin (CSDB). Remnant geomorphic features indicative of cold-based glacial flow led us to propose that the inverted channels in crater B formed through top-down glacial melting in a Late Noachian Icy Highlands climate scenario. The flow rate through glacial meltwater channels is directly related to the melting rate of the source glacier; thus, the estimation of water fluxes is critical to understanding the climatic conditions under which such features may have formed. We follow recently described paleohydrologic reconstruction methods for Mars to estimate the discharge of several inverted channel segments within the CSDB crater B. Discharge is proportional to both channel depth and width. Using the width of an entire inverted channel can result in significant overestimations of paleodischarge if the inverted channel preserves a multi-thread channel belt as opposed to a single-thread channel. Channel depth can be estimated as a proportion of caprock thickness. Direct measurements of caprocks from orbit are less reliable than measurements of total ridge height, so a caprock to ridge height (T/z) ratio is required to accurately determine caprock thickness on Mars. We use several inverted channel segments to derive independent estimates of channel width by assuming that these specifically represent single-thread tributaries where ridge width is close to the original channel width. We derive a best-fit T/z of ~0.92 and a range of paleodischarges from ~100-7000 m3/s. These estimates will enable us to better characterize rates of glacial melting and will provide a key constraint for further early Mars climate modeling.