Differences in Fluvial Geomorphology between Earth and Mars

Preserved geomorphological landforms on the surface of Mars indicate the presence of abundant liquid water in the early history of Mars. Many of these geomorphic features were developed by erosion and deposition of sediments by water. It is therefore important to understand how fluvial sediment transport works on Mars and how it is different from Earth. Due to the lower gravity on Mars water flows down slope with less energy, resulting in lower bed shear stresses and flow velocities. Nonetheless, fluvial sediment transport is more efficient. Due to the lower gravity the mobility of the sediment is higher. Larger grains are brought into motion and suspension (Komar, 1980; Burr et al., 2006) and the magnitude of suspended transport is significantly higher (Amy and Dorrell, 2021), as is the total transport flux (Braat et al., 2022). In addition, the settling of sediment is slower, resulting in larger transport distances on Mars compared to Earth. Based on the differences in entrainment due to gravity, different grain size mixtures are transported and settle out in a different manner (Braat et al., 2022). Therefore, the geomorphology and stratigraphy of geomorphic landforms might be different than we expect from Earth observations. In this study, we investigate how fluvial geomorphology differs on Mars through sediment transport calculations on Mars and our terrestrial knowledge and experience.

We use two methods: 1) We use standard hydraulic equations to calculate hydrodynamic conditions based on a slope, channel width and discharge. From these conditions we calculate sediment transport fluxes using multiple sediment transport predictors for both bedload and suspended load. Total load predictors are not suitable for Mars, as they do not account for a variable gravity effect with grain size. 2) We also run numerical hydro-morphodynamic model scenarios to compare the evolution of fluvial geomorphic features with Earth and Mars gravity. We use the software package Delft3D (Lesser et al., 2004), and amended the code to work on Mars.

Simple sediment transport calculations indicate that the sediment fraction at the bedload-suspended load boundary is most affected by gravity. In our examples transport could be up to 6 times higher for this fraction. Overall, the magnitude of the total transport flux on Mars is also bigger, predominantly because of increased suspended transport. As the bedload fraction is the ‘channel-building’ fractions and suspended transport determined channel-floodplain interaction, we hypothesise that floodplain deposition will increase. Additionally, with more sediment entering the floodplain levee accretion will increase, as will cut-off infilling and crevasse splays. We also hypothesise that increased suspension will reduce channel migration, reduce branching, increase the avulsion rate, and create more sinuous, narrow channels (Nicholas, 2013). The preliminary model outcomes confirm our hypothesis that depositional slopes are lower due to longer advections lengths related to lower settling velocities. For example, this will transport more sediment to the delta front and pro-delta, impacting deltas foresets (van der Vegt et al., 2016). Finally, the models agree that geomorphic features develop faster on Mars.