Open-basin Lakes and the Climate and Surface Environment of Early Mars

Introduction: The existence of probable lakes on Mars has been recognized for more than two decades [1-4]. Lakes can either be open-basins with both inlet valley(s) and an outlet valley [e.g., 5], or they can be closed-basins that lack an outlet [e.g., 6]. Although both open-and closed-basin lakes were likely present on early Mars, open-basin lakes are easier to study, and we can have higher confidence that they once held ponded water. Outlet valleys are recognizable in remotely sensed topography and images because they cross drainage divides and are often significantly in-cised. When an outlet is observed, we can be very confident that a lake must have existed, because overtop-ping of the drainage divide is what allowed the outlet valley to form in the first place. We can also easily measure the minimum lake volume and surface area. In this abstract, we discuss and synthesize observations of open-basin lakes formed by valley networks, and consider their implications for the early Mars climate and surface environment. Lake Population: In 2008, a global catalog of probable open-basin lakes was created [5]; this was later reexamined by [7]. We required that candidate lakes meet two requirements: (1) they must be local topographic lows, outlined by a closed contour within which water would pond, and (2) they must be fed by input valley network(s). This first requirement is met in abundance on Mars largely because of the numerous impact craters on the surface; not every candidate open-basin lake is in an impact crater, but the majority are. The second criterion limits us to lakes formed as a result of, and contem-poraneous with, valley networks. This excludes im-poundments related to outflow channels. Our current catalog (Fig. 1) of candidate open-basin lakes has 229 such features, >90% of which we classify as " confident " identifications [7]. Lakes range in scale over at least five orders of magnitude in area (~2 to 500,000 km 2) and seven orders of magnitude in volume (~0.02 to 200,000 km 3). The largest of these lakes are comparable in size to small seas on Earth. Observational Constraints and Climate Implications: Resurfacing, preservation, and interior deposits: As a population, the candidate open-basins appear to have experienced significant resurfacing after the period of lacustrine activity due to volcanism, glaciation, impacts, and aeolian activity [7]. Post-lacustrine vol-canism appears to be particularly important, with at