Flow dynamics and behavioural characteristics of sublimation-driven granular flows under laboratory conditions

Throughout our Solar System, erosional processes reshape the surfaces of terrestrial and icy bodies, ranging from planets and moons to asteroids and comets. One such process is mass wasting, which transports loose material downslope driven by gravity, forming slides, avalanches or flows depending on conditions. Over the past decades, the role of volatiles in their formation has been debated. Our understanding of extraterrestrial mass wasting relies heavily on Earth analogues; however, these are mostly influenced by liquid water, which is not stable on other planetary surfaces. Yet, numerous extraterrestrial landforms indicative of mass wasting occur on planetary surfaces with (seasonal) ice or frost and on slopes too gentle for dry material to move unaided.
Ice sublimation is a potentially plausible mechanism for driving extra-terrestrial mass wasting, whereby solid volatiles directly transition into vapour. This can initiate flow and reduce friction between sediment particles. However, because of the lack of terrestrial analogues and the complexity of producing a usable numerical model, the mechanics of sublimation on sediment mobilisation, particle dynamics and flow behaviour remain unclear. Here, we investigate the roles of volatiles and environmental conditions on the mobility and dynamics of sublimation-driven mass wasting and the morphology of their deposits.
Over the past two years, we created flows driven by sublimating CO₂ using flume set-ups in two low-pressure chambers at the Open University (Milton Keynes, United Kingdom) and Aarhus University (Aarhus, Denmark). Ambient pressure was varied stepwise from 0.1 to 1000 mbar to cover the
environmental conditions of a broad range of terrestrial and icy bodies. The mass flows consisted of dry ice mixed with either high-density (∼ 2600 kgm⁻³) or low-density granular material (410 - 1300 kgm⁻³), the latter was utilised to simulate reduced gravity. The results show that reduced ambient pressures increase the volume flux of gas, thereby enhancing the fluidisation, flow mobility and runout length, particularly for low-density flows. This suggests that terrestrial bodies with lower surface gravity have more mobile sublimation-driven flows. The behaviour of the mass flows varied noticeably with ambient pressure, showing transitions through different fluidisation regimes, each marked by distinct features. At high pressures (> 20 mbar), we observe steady flows. In the 20 - 1 mbar range, the flows start to exhibit bubbles, surges and outbursts. Below 1 mbar, turbulent behaviour emerges with a diffuse particle suspension flowing above a dense layer. These behavioural regimes are similar to the regimes observed in fluidised bed experiments and have been recognised in snow avalanches and pyroclastic density currents on Earth. Currently, we are analysing internal particle dynamics and velocities for these regimes using particle tracking software. Our research shows that sublimation can be an effective driver for mass wasting on terrestrial bodies with low ambient pressures, low gravity and the presence of volatiles other than water, and might operate in distinct fluidisation regimes.