Examining the role of water in Martian lowland gully formation

A variety of processes have been proposed to form Martian gullies with little consensus reached. Detailed mapping combined with morphological and morphometric studies for gully and channel systems on Earth can distinguish these processes from one another, as well as measure the degree to which gullies are formed by liquid water. These parameters include tributary stream orders and magnitudes; drainage densities; channel profile concavities; gully cross-sectional profiles; sinuosity; gully, apex, and apron slopes; and gully/apron volumes. We applied these morphometrics, detailed gully mapping, and morphologic studies to four gullied craters in the Northern Lowlands of Mars. Morphometric parameters indicate that their morphology is most consistent with a fluvial origin, especially in craters at 63 N, 53 N and 48 N latitude. The gullies in these craters have many first-order tributaries contributing to high tributary stream orders up to 4 and magnitudes ranging from 10 to 65, drainage densities ranging from 8 to 60 km/km², and concave channel profiles with concavity indices between 0.102 and 0.185. These gullies also form on shallow slopes between 8 and 20 degrees, below the angle of kinetic friction (21 degrees), which indicates fluid emplacement of materials. Gullies in a crater located at 60 N latitude and on the west wall of the 53 N Crater are more poorly developed, with lower tributary integration reflected in the stream magnitude and stream order values (ranging from 2 to 3 and 28 respectively), straight channels, and lower concavity values <0.1 indicating a smaller contribution of water to their formation. Morphological characteristics within the gully systems analyzed in this study, such as multiple apron formation, cross-cutting features, and subsequent deposition indicate that gully formation was episodic. Reduced channel sinuosity and smaller deposits imply that gully activity experienced less water availability over time as conditions within the craters became less suitable for liquid water. A source of water that is compatible with the observed morphologies would be basal melt from dusty snowpacks. These snowpacks might have been deposited at higher obliquity or formed from the cold-trapping of steam freed from the subsurface by impact and the resulting post-impact hydrothermal system.