Constraints on Environmental Conditions on Mars during Periods of Alluvial Fan Formation: Results from Landform Evolution Modeling

Abstract:

As depositional systems forming within enclosed crater basins, the Late Noachian and Hesperian –aged [1] alluvial fans on Mars (including the Peace Vallis fan in Gale crater) may be representative of the last vestiges of widespread fluvial activity on the planet’s surface, an era during which the climate transitioned from a wetter early Mars to the cold and dry planet we observe today. We have constructed a landform evolution model that combines sediment transport with channel avulsion to study the evolution of a fan-forming channel network over timescales of decades to hundreds of thousands of years. We aim to address two related questions: (1) what were the characteristics of water discharge (flow magnitude and duration); and (2) what are the associated implications for the responsible climatic environment (e.g. amount and frequency of precipitation sourcing the fans).

The model uses a cellular network with a grid spacing set equal to the channel width. Two end-members of sediment are transported through the channel network: gravel bedload and fine grained material that is deposited overbank as a function of distance and elevation difference from an active channel. Overbank deposition creates channel levees, which must be overtopped for the channel to undergo an avulsion. By recording the relative amounts of bedload and overbank deposition, the 3-D stratigraphy is recorded as the fan is constructed.

Using measures such as channel width, relative proportions of channel versus overbank deposited sediment, and frequency of channel branching, output is statistically compared with digital elevation models that we been produced from high-resolution CTX and HiRISE stereo pairs. Our modeling suggests that the fans formed from many flow events over many thousands of years, in agreement with estimations based on geomorphological observations by [2]. We are continuing to refine the model to test for varying patterns of precipitation, duricrusts, and limits on sediment supply in the source basin catchments.

[1] Grant, J.A., Wilson, S.A., 2011. Late alluvial fan formation in southern Margaritifer Terra, Mars. Geophys. Res. Lett. 38

[3] Morgan, A.M., et al., 2014. Sedimentology and climatic environment of alluvial Fans in the martian Saheki crater and a comparison with terrestrial fans in the Atacama Desert. Icarus 229