Insights from experimental groundwater sapping channels for early Martian hydrological conditions

Terrestrial analogues for morphological features on Mars can be used to estimate periods and nature of liquid surface water, which can be used to abduce early Martian climate conditions. Channels and box-canyons on Mars show morphological element similar to presumed sapping groundwater-fed channels on Earth which are formed under specific hydrological conditions. However the formative mechanisms of such channels, which are key in inferring corresponding climate conditions, are heavily debated even for the terrestrial cases. Here, we conducted experiments of groundwater sapping systems and similar hydrological systems are created in a laboratory facility to acquire detailed insights in the hydrological and morphological processes related to groundwater sapping. The experiments were carried out in a 0.4 m by 1.5 m and a 1.2 m by 5 m setup. Sapping channels were produced within a few hours allowing tests with various boundary conditions. Larger grain sizes result in higher hydraulic conductivity but lower sediment mobility. In order to establish similar morphologies as in reality, the experiments were scaled to dimensionless sediment mobility by varying slope and using lightweight plastic sediment. Hydraulic conductivity was scaled using sediments with different grain sizes and grain-size distributions. The parameter space and potential scaling effects were further explored by hydrological modelling in Hydrus-2D. The experiments show that groundwater sapping induces a combination of mass-movement and fluvial processes which results in distinct areas with characteristic morphological features, namely a collapsing head wall with slumps, a low-sloping area characterised by debris flows induced by the actual groundwater sapping and an area further downstream with fluvial activity as the flow reaches velocities above the threshold of motion. The observed experimental morphologies correspond qualitatively well with morphologies found on Mars and Earth and are useful analogues for Martian channels. Ongoing experimental and numerical work relates scaled and Martian morphology to the properties of the sediments and the necessary and sufficient hydrological boundary conditions.