Elysium Planitia lava flows: Crater count chronology and geological implications

Mars Global Surveyor (MGS) and Viking images allow analysis of chronological and geological relations among lava flows in southern Elysium Planitia, based on crater populations. MGS and Viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces. There is no evidence of substantial dust mantling on the young flows, and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves. The youngest flows have some of the lowest crater densities that we have seen on Mars, some <1% of lunar mare values, at crater diameters below about 180 to 500 m. We infer that thin, scattered volcanic flows, some tens of meters thick, have been emplaced atop stratigraphic units within the last 100 Myr, and possibly within the last 10 Myr. Volcanism is thus a continuing process in the recent geologic history of Mars, and this must constrain geophysical models of the planet.