Formation of Ares Vallis (Mars) by effusions of low-viscosity lava within multiple regions of chaotic terrain

Abstract

Ares Vallis is one of the largest outflow channels on Mars, extending northward >1500 km from the highlands of Margaritifer Terra into the Chryse impact basin. This outflow system developed primarily during the Hesperian and Amazonian as a result of voluminous effusions from the subsurface that took place within Iani Chaos, Aram Chaos, Margaritifer Chaos, and Hydaspis Chaos. Though Ares Vallis is widely interpreted as a product of catastrophic outbursts from aquifers, its basic attributes do not appear to support aqueous origins of any kind: aquifer outburst mechanisms lack meaningful solar system analogs, clear examples of fluvial or diluvial sedimentary deposits are apparently absent at Ares Vallis, and there is no mineralogical evidence along component channels and within terminal basins for extensive aqueous alteration or for deposition of thick evaporite units. Instead, as is the case at hundreds of ancient channels of the inner solar system, the nature of Ares Vallis is aligned with dry volcanic origins. Such origins are broadly consistent with the system's relatively pristine mineralogy, the widespread mantling of component channels by lava flows, and the apparent presence of voluminous mare-style flood lavas within terminal basins. With assumed lava viscosities of 1 Pa s and temperatures of 1350 °C, discharge rates of ~97 × 106 m3/s are estimated at Ares Vallis for 100-m-deep flows along channel reaches with widths of 25 km and slopes of only 0.2°. Mechanical and thermal incision are respectively estimated to be ~12.2 m/day and 2.3 m/day for 100-m-deep flows on these slopes. Formation of the main Ares Vallis channel and associated regions of chaos is likely to have required a minimum effused lava volume of ~1.8 × 106 km3, and still greater lava volumes would have been necessary to form associated channel reaches including those located south of Margaritifer Chaos.