Volcanology of Arnus Vallis, Mars

Arnus Vallis (AV) is a >300-km-long sinuous, rille located on the northeastern flank of the Syrtis Major volcano on Mars. Observational evidence presented here suggests that AV formed as an open lava channel that was at least partly incised into the pre-existing terrain. The lava source area consists of a sub-circular pit at the southwestern end of a 7-km-long straight section of channel. AV trends down slope from this source with an average bottom slope of 0.26% or 0.14°. Width varies from ∼1 km at the source to ∼0.6 km near the distal end, with a mean of 0.76 km. Depth decreases from ∼180 m at the source to ∼25 m near the distal end. The AV terminus is obscured by a large impact crater. We suggest that the material that flowed in AV must have been a relatively high temperature, low viscosity lava dynamically and perhaps compositionally similar to terrestrial komatiite or some lunar basalt lavas. If correct, this finding has implications for the mode of construction of Syrtis Major.