Insights from experimental groundwater sapping channels for early Martian hydrological conditions

Terrestrial analogues for morphological features on Mars can be used toestimate periods and nature of liquid surface water, which can be usedto abduce early Martian climate conditions. Channels and box-canyons onMars show morphological element similar to presumed sappinggroundwater-fed channels on Earth which are formed under specifichydrological conditions. However the formative mechanisms of suchchannels, which are key in inferring corresponding climate conditions,are heavily debated even for the terrestrial cases. Here, we conductedexperiments of groundwater sapping systems and similar hydrologicalsystems are created in a laboratory facility to acquire detailedinsights in the hydrological and morphological processes related togroundwater sapping. The experiments were carried out in a 0.4 m by 1.5m and a 1.2 m by 5 m setup. Sapping channels were produced within a fewhours allowing tests with various boundary conditions. Larger grainsizes result in higher hydraulic conductivity but lower sedimentmobility. In order to establish similar morphologies as in reality, theexperiments were scaled to dimensionless sediment mobility by varyingslope and using lightweight plastic sediment. Hydraulic conductivity wasscaled using sediments with different grain sizes and grain-sizedistributions. The parameter space and potential scaling effects werefurther explored by hydrological modelling in Hydrus-2D. The experimentsshow that groundwater sapping induces a combination of mass-movement andfluvial processes which results in distinct areas with characteristicmorphological features, namely a collapsing head wall with slumps, alow-sloping area characterised by debris flows induced by the actualgroundwater sapping and an area further downstream with fluvial activityas the flow reaches velocities above the threshold of motion. Theobserved experimental morphologies correspond qualitatively well withmorphologies found on Mars and Earth and are useful analogues forMartian channels. Ongoing experimental and numerical work relates scaledand Martian morphology to the properties of the sediments and thenecessary and sufficient hydrological boundary conditions.