Rethinking Martian Deltas: The Influence of Reduced Gravity on Delta Morphology and Evolution

This study aims to isolate the effect of gravity on delta morphodynamics, a key uncertainty in interpreting Martian deltaic systems. Terrestrial deltas are commonly used as a framework to interpret deltas on Mars, yet the planet’s lower gravity fundamentally alters sediment transport processes and, consequently, delta morphology and evolution. Previous work has demonstrated that reduced gravity enhances net sediment transport for a given discharge and channel geometry, promoting a higher proportion of suspended sediment transport (Braat et al., 2024). However, the implications of these effects for delta morphodynamics have remained largely unexplored.

We conducted physical experiments in the Earth Simulation Laboratory at Utrecht University. Deltas were formed autonomously in a 3 cm-deep flume with a constant water (300 L/h) and sediment supply (2 L/h). Martian gravity was simulated by reducing the sediment particle weight through the use of low-density grains (nutshell particles, ~1350 kg/m³), thereby isolating sediment density as a proxy for gravitational effects. This approach generated higher mobility sediment and a greater fraction of suspended transport, consistent with expectations for Martian conditions. The resulting low-density deltas were compared to reference deltas formed with standard silica sand (~2650 kg/m³).

The experiments show that reduced sediment density leads to deltas with gentler equilibrium slopes and larger surface areas. The lower equilibrium slope requires little aggradation, and most of the sediment supply can be used for progradation. Low-density deltas also develop more pronounced levees, likely due to enhanced suspended sediment transport. These levees, together with minimal gradient advantages across the delta plain, result in reduced system dynamics: channels are more stable, and large-scale avulsions occur at relatively low frequencies. In contrast, normal-density deltas exhibit more frequent channel migration and avulsions. As a result, low-density deltas develop more irregular, multi-lobed planform geometries, whereas normal-density deltas tend to remain semi-circular or half-oval in shape.

These findings demonstrate that gravity alone can exert a first-order control on delta morphodynamics. Morphological characteristics commonly interpreted on Mars as indicators of fine grain sizes, high sediment mobility, or elevated discharges may instead arise from the effects of reduced gravity. Consequently, caution is required when interpreting Martian deltas solely based on terrestrial analogues.