Updated global map of Martian valley networks and implications for climate and hydrologic processes

Martian valley networks have long been viewed as some of the best evidence of prolonged surface water on Mars. Analysis from Viking data showed that valleys were primarily contained on the ancient crust, drainage networks were immature, and large undissected regions occurred in between individual systems. These observations led many to propose that they formed primarily by groundwater processes, and limited or no climate change from the present state was required. Using more recent and higher-resolution data sets, including visible, infrared, and topographic data, we have manually remapped valleys on a global scale. More than eight times as many valleys have been identified on the surface, and calculated drainage densities are, on average, higher by a factor of 2. Further, regions previously thought to be undissected have significant incision by fluvial processes. Most of these systems seem to have formed around the Noachian-Hesperian boundary (∼3.8-3.6 Ga). Minor valley formation continued through the Hesperian and into the Early Amazonian epoch, approximately 2.8 Ga ago. The new data reveal characteristics of sustained precipitation and surface runoff including inner braided channels, terraces, multiple periods of formation, complex network morphology, and correlation with other fluviosedimentary features and chloride salts on Mars. Groundwater processes or a transient steam atmosphere generated by impacts played, at most, a minor role. These new results imply that Mars had a long-lived period or periods of clement conditions toward the end of the Noachian epoch that supported a hydrologic cycle and potentially a biosphere.