Effects of gravity in CO2-sublimation driven granular flows in a simulated experimental environment
Authors: Diamant Sharon , Sylvest Matthew , Conway Susan , Roelofs Lonneke , Emerland Zoe , Patel Manish , Kleinhans Maarten , de Haas Tjalling 
Published: July 2024

IntroductionMass wasting is one of the most prevalent geomorphological processes contributing to surface evolutionacross the Solar system [Moore et al. (1999); Roelofs et al. (2024)]. It is the downslope movement offine material (e.g., regolith) and granular material (e.g., rock debris) under the influence of gravityseen in e.g., falls, slides, avalanches or flows. Identifying theformative processes of mass-wasting can give us insights into the distribution of volatiles across theSolar System which is important for future space missions [Parekh et al. (2021)]. Using terrestrial mass-wasting deposits asanalogues is useful with limitations [Scully et al. (2015)]. In contrast to Earth, on other planetary surfaces, water is at best metastable (i.e., boiling, sublimating and/or freezing).&#160;While sublimation is a plausible surface process on some planetary bodies [Mangold (2011); Roelofset al. (2024)], it cannot be studied on Earth. Here we address this critical gap in our knowledgeof the mobility, morphology and evolution of sublimation-driven mass flows through laboratory-scaleexperiments.Sublimating ice causes gas to flow through granular material, reducing internal friction and enhancing the flow&#8217;s mobility. The velocity of the gas expansion depends on gas density and the material's permeability, regardless of gravity. However,the force needed to lift the sediment particles with a given density is proportional to the gravitationalacceleration. Aspects of the effects of gravity on the morphology, mobility and dynamics of dry andliquid-based mass-wasting have been studied (Kleinhans et al. (2011); Kokelaar et al. (2017)) butremain poorly understood in sublimation-driven mass-wasting.Here we study the role of gravity in sublimation-driven mass-wasting witha set of experimental debris flows in a low-pressure chamber. We analyse the dynamics, morphology,mobility, and fluidization of CO2-sublimation-driven mass granular flows under various atmosphericpressures and sediments with varying densities.MethodsThe laboratory simulations were performed in a&#160;debris flow flume in acylindrical low-pressure Mars chamber of the Space and Planetary Environments Laboratory at theOpen University (Milton Keynes, UK). The debris flow is initiated from a sediment-ice reservoir atop&#160;a chute and runs out on an outflow plain (Figure 1). The downstream part of the chute is instrumented with two relative gas pressuresensors to measure the overpressure (differential gas pressure,i.e. gas flow pressure relative to the ambient pressure), a geophone to record seismic vibrations, a load cell to record the weight of the granular flow and three laser range sensors to capture the flow depth. Moreover, we useda Phantom Miro C110 high-speed camera to capture the movement of the flow and the individualparticles.Figure 1: The laboratory debris flow flume in the Mars chamber at the Space and Planetary Environments Laboratory at the Open University (UK). (a-b) display the chute including the instruments and the sediment-ice reservoir. (c) shows the entire flume looking upslope.To represent atmospheric pressures on different planetary bodies, we applied a range of pressuresbetween 3 to 1000 mbar. The granular flows were simulated by mixing dry CO2 ice with granular materials to create sediment mixtures with different permeabilities and densities.Two groups with angular sediments (quarry sand and walnut shells) and rounded sediments (solid andhollow glass beads) were chosen so that we could also determine the effects of the sediments&#8217; shapeon the granular flows. To assess the effect of lower gravity, each group has a higher-density sedimentused as reference (quarry sand of 2600 kg/m3 and solid glass beads of 2500 kg/m3), and a lower-density sediment (crushed nutshell of 1300 kg/m3 and hollow glass beads of 410 kg/m3). The particle distribution for the angular sediment was similar for all sediment mixtures and fixed to 25% fine (200 -450 &#181;m), 50% medium (450 - 800 &#181;m) and 25% coarse (800 - 1300 &#181;m). The rounded sediment had aparticle size range between 500 - 850 &#181;m. Gravitational acceleration and density have the same effecton the force needed to levitate the flowing sediment. Thus, to simulate low-gravity bodies, we utilizelow-density sediments.Results and discussionOur preliminary results reveal an overall increase in fluidization of the granular flows for decreasingpressures. This is more prominent in the low-density sediments. At 3-10 mbar, we notice afluidization regime similar to bubbling, where the CO2 gas builds up in the flow and escapes followinga small outburst [Van Ommen et al. (2010)].Figure 2: (a) frontal flow velocities, (b) overpressure: the ratio of the maximumdifferential pore pressure (difference between basal and ambient pressure) vs. ambient pressure and(c) runout: the H/L ratio of all the sediments.The enhanced fluidization at low pressures is also apparent in the increased frontal flow velocities,increased overpressure and increased runout length (H/L ratio, where H is 0.43 m and L is runoutlength) (Figure 2). The internal pore pressure increases relative to ambient pressure (Figure 2b) whichdecreases the internal particle friction and enhances the flow&#8217;s velocity (Figure 2c).The levitation effect is observed here as increasing flow depth for decreasing pressures. This confirms the prior hypothesis that, at low ambient pressures, the gas outflux from sublimating CO2 ices increases, which levitates the sediment particles and reduces intergranular friction and particlecollisions [de Haas et al. (2019); Roelofs et al. (2024)].ConclusionExperiments have been performed to study the effects of gravity by mixing CO2 ice and granularmaterial with different permeabilities and densities. Low-density sediments, representing lower gravity,display a longer deposit runout and higher frontal flow velocities and flow depths compared to high-density sediments. At low atmospheric pressures (3-10 mbar), we observe a transition in theflow behaviour known as turbulent bubbling in the low-density sediments. These findings show thatatmospheric conditions and gravity on different planetary bodies lead to changes in the mobility andflow behaviour of granular flows, which may be observable in the deposit dimensions. Further analyseswill focus on PIV data to study the appearance of bubbling at low pressures in more detail and elevationmodels of the deposits from imagery to study their morphology.