Effects of gravity in CO2-sublimation driven granular flows in laboratory experiments

Mass wasting is the downslope movement of rock debris and/or regolith driven by gravity, including falls, slides and flows. It is among the most abundant geomorphological processes in our Solar System contributing to surface evolution on planets, moons, asteroids and comets. On Earth, mass wasting is mostly induced and/or accompanied by liquid water however, on other planetary surfaces, water is at best metastable (i.e., boiling, sublimating and/or freezing). Yet, the distribution of extra-terrestrial mass-wasting features coincides with that of (seasonal) ice and frost. Furthermore, mass wasting frequently occurs well below the angle of repose, suggesting the involvement of fluids or volatiles. While ice sublimation is recognized as a potential mechanism for controlling mass wasting on terrestrial bodies, the effects of gravity remain poorly understood in sublimation-driven mass wasting. This inhibits our ability to identify the effects of gravity and the role of volatiles on the morphology of the deposits, mobility and dynamics of sublimation-driven mass wasting. In new experiments, we attempted to address this critical knowledge gap by generating mass flows driven by CO₂ ice sublimation under extra-terrestrial conditions in a cylindrical low-pressure chamber at Open University (Milton Keynes, United Kingdom). We covered the environmental conditions of a broad range of terrestrial bodies, specifically Mercury, Earth, Mars, Ceres, Vesta, Moon, Comets 67P and 9P. Therefore, a step-wise ambient pressure range of 3 to 1000 mbar was implemented. The mass flows were comprised of dry ice and high-density (∼ 2600 kgm⁻³) or low-density granular material (410 - 1300 kgm⁻³), the latter was utilized to simulate low-gravity bodies. The experiments reveal that the amount of CO₂ gas produced is higher for low ambient pressures, resulting in enhanced pore pressures inside the flow. In turn, the internal particle friction drops, improving the mobility of the mass flows. This effect is more prominent for the low-density mass flows, suggesting that effects of density, i.e., gravity, play an important role in overall fluidization. Additionally, we observe flow behavioural changes at low ambient pressures (≤ 7 mbar). Turbulent CO₂ gas bubbles developed inside the flow, causing the granular material to levitate, in turn, enhancing the flow’s mobility. We hypothesize that these fluidization regimes are developed as a result of CO₂ sublimation and low ambient pressures. This bubbling appearance will be further analysed using Particle Image Velocimetry (PIV) in more detail.