Where Environments Collide: Aeolian-Fluvial Interactions on Ancient Mars

In deserts on Earth, aeolian (wind-blown) and fluvial (river) (AF) processes display considerable interactions, which have an impact on dune and river trajectories, morphologies, geometries, and distributions. These interactions can lead to water loss to the subsurface and the formation of sabkhas, which are interdune ponds that transform into salt flats. These pools are where primordial continental life on Earth is hypothesized to have emerged, evidenced by microbial mats. Lithified dunes and interwoven inverted river channels, and the discovery of aqueously altered lithified dunes by the Curiosity rover, indicates synergy between AF interactions on ancient Mars. We report the results of the pilot study by the Working group on Aeolian-Fluvial Terrain Interactions (WAFTI), based at the European Space Agency, which examines the effects of these processes in synergy under ancient Martian conditions, using a combination of modelling and geomorphological analysis. Our Martian Aeolian-Fluvial Interactions (MAFI) model is a landscape evolution model based on a coupled implementation of the Caesar-Lisflood fluvial model, and Discrete ECogeomorphic Aeolian Landscape model (DECAL) dunes model. It routes water over a Digital Elevation Model (DEM) and calculates erosion and deposition from fluvial and slope processes changing elevations accordingly. Aeolian material is discretized into slabs on the DEM, and slabs are moved across the space according to a set of simple rules. We conduct simulations of various scenarios to model the interactions between perennial and ephemeral rivers, actively migrating dunes, and different types of terrain (bedrock and unconsolidated sediment). These simulations incorporate factors such as evaporation and water loss to the subsurface. We measure changes in the grainsize distribution of the river sediment both upstream and downstream of a dunefield, and analyse the effects of dune migration on river morphology and pond evolution over hundreds of years of AF interactions. These interactions have a number of salient impacts: meandering inverted channels, the sediment size and distribution of Martian rivers, the formation of interdune pools, and the preservation of Martian valley networks.