Estimating subglacial water discharges needed to form Amazonian-aged mid-latitude eskers on Mars

Abstract

Eskers are sinuous sedimentary ridges formed in meltwater-filled subglacial tunnels. They are widespread in formerly glaciated landscapes on Earth. A small but growing number of late Amazonian-aged (~110-330 Ma) candidate eskers have been identified in Mars’ mid-latitudes in association with extant buried glaciers. These eskers are thought to have formed during periods when mid-latitude glaciation on Mars was more extensive than at present, due to variations in planetary spin-axis obliquity. The basal melting required for esker formation seems likely to have required elevated local or regional geothermal heating.A recent study using current terrestrial theories for subglacial water flow adapted for Mars suggests that, if water was present beneath Martian ice masses, lower gravity favours the formation of efficient, tunnel-based drainage, as opposed to water flow through a distributed system of small cavities linked by water-filled orifices which is favoured for terrestrial ice masses. Tunnel-based drainage systems are more efficient, leading to lower water pressures and gradients, and slower water velocity.  Our previous experiments with a Mars-adapted model of esker sedimentation also suggest that, once a subglacial tunnel has formed, sediment deposition occurs more readily on Mars than Earth, as the lower gravity, and consequent lower water pressure and velocity, allows more rapid deposition.These factors suggest that if subglacial water and mobilised sediment are present beneath Martian ice masses, esker formation is more likely on Mars than Earth as subglacial tunnels would be more widespread, and sediment deposition within them more rapid. However, this leads to questions regarding the likely source(s) of esker-forming sediment, and the water volumes needed to erode it. Initial calculations with a Mars-adapted model for erosion by subglacial water suggest that for a particle size typical of Martian sandy regolith (150 mm), erosion requires water velocities > 0.1 ms-1. Calculated erosion rates vary from 5x10-10 ms-1 to 3.5x 10-7 ms-1 for water velocities between 0.1 ms-1 and 1 ms-1, and are higher than for equivalent terrestrial channels, largely because the critical shear stress needed to mobilise sediment is lower due to Mars’ gravity. This suggests that sediment will be readily mobilised beneath wet Martian ice masses, making the supply of water the critical limiting factor. Thus, this study will use an ice flow model to reconstruct a more extensive glacier over a candidate esker in the Phlegra Montes of Mars’ northern mid latitudes. Geothermal heat will be varied, along with other glaciological and climatic parameters, to investigate the possible extent of warm-based ice in the region, and to estimate the extent and volume of subglacial meltwater. The modelled meltwater will then be input into the Mars-adapted subglacial water erosion model to explore the impact of water availability and sediment characteristics on the possible extent of sediment erosion. Modelled sediment supply will then be compared with the sediment volume within the candidate esker, reconstructed from a 1 m/pixel digital elevation model, to help constrain the regional glaciological, sedimentological, climatological and geothermal conditions needed for esker formation.