An updated global survey of alluvial fans on Mars: Distinguishing alluvial fans from other fan-shaped features through morphologic characterization

Alluvial fans formed on Mars during the late Hesperian to early Amazonian in a climatic transition from a time of regular surface water activity to the current dry and cold climate. The depositional environments recorded by alluvial fans represent the final era of potential habitability on the Martian surface. Alluvial fan deposits can often look similar to deltaic or fluvial systems in visible remote sensing images but are formed in depositional and climatic environments that are distinct from those that lead to other radial depositional features. In order to better constrain regional and global depositional environments and further inform the analysis of the sedimentary history of Mars, we expanded the global catalog of alluvial fans on Mars by adding newly identified features and characterizing all new and previously identified features based on morphologic parameters. Using planform shape, radius, average radial slope, and radial profile shape, we classified features as alluvial fans, possible alluvial fans, and non-alluvial radial features (NARFs), based on how closely they fit the morphologic definition of alluvial fans. Other radial-shaped depositional features form in more water-rich depositional environments whereas alluvial fans form by subaerial deposition in arid to hyper arid environments from sporadic high-energy flow events interspersed with long dry periods of inactivity. By identifying new features and distinguishing alluvial fans from other fan-shaped features in the global catalog, we have isolated distinct depositional environments on a local scale and updated the global perspective of the distribution and variety of aqueous depositional environments during the early history of Mars. Newly identified alluvial fans are found as far as 78°S latitude as well as in numerous locations within the Hesperian to Noachian transition unit along the dichotomy and occur both in craters and along non-crater topographic features, expanding the global range of alluvial fan environments into regions that previously have been overlooked.