Sharon Diamant

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 theenvironmental 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.

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.

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). 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’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 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 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’ 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 µm), 50% medium (450 - 800 µm) and 25% coarse (800 - 1300 µm). The rounded sediment had aparticle size range between 500 - 850 µm. 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’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...

The November 2019 eclipse of γ Per was a rare opportunity to seek evidence for a chromosphere of the G8 giant, hitherto suspected but not detected. Twenty-nine years after the only other observed eclipse, we aim to find chromospheric absorption in the strong Ca II H&K lines, and to determine its column densities and scale height. Using the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE) in Guanajuato (central Mexico) before, during and after the 8 days of total eclipse, we obtained good S/N spectra of the G8 giant alone and composite spectra of the partial phases, near eclipse and far from eclipse. In the near UV of the Ca II H&K and H_ϵ lines, the G giant spectrum that was adequately scaled was subtracted from the composite spectra in partial phases, near and far from eclipse, to obtain the A3 companion spectra with and without traces of chromospheric absorption. In addition, we used PHOENIX full non-local thermodynamic equilibrium model atmospheres on the blue A star spectrum, iSpec spectral analysis of the red G giant spectrum, and evolution tracks to study both components of γ Per. For the first time, we present evidence for this rare type of a not very extended G giant chromosphere, reaching out about half of an A-star radius (~1.5 Gm) with a scale height of only 0.17 Gm. By its location in the Hertzsprung-Russell diagram, the γ Per G8 giant is very close to the onset of more extended chromospheres. Furthermore, we show that this giant has a rather inactive chromosphere, and a recent 5 ksec XMM pointing reveals only a very faint, low-energy corona. While the γ Per primary has a mass of ~3.6 M_⊙, and its A3 companion has one of ~2.4 M_⊙, the latter is too cool (8400 ± 300 K), which is too evolved on the main sequence to be the same age as the primary. The high eccentricity of the 5329.08 days long-period orbit may therefore be reminiscent of a rare capture event. Using the eclipse method, we resolve a pivotal case of a G giant chromosphere, which seems to represent a low-gravity analogue of the inactive Sun. A systematic change of giant chromospheric extent by Hertzsprung-Russell diagram position is confirmed. Compared to the solar chromosphere, the density scale height increases with gravity by ∝ ɡ^−1.5.

Laboratory results of the optical properties of vapor-deposited water ice, specifically the refractive index and extinction coefficient, are available mainly for a selective set of wavelengths and a limited number of deposition temperatures. Experimental limitations are the main reason for the lack of broadband data, which is unfortunate as these quantities are needed to interpret and predict astronomical and planetary observations. The goal of this work is to address these lacking data, using an experimental broadband method that is capable of rapidly providing reliable water ice data across the entire UV–visible range. This approach combines the simultaneous use of a monochromatic HeNe laser and a broadband Xe-arc lamp to record interference fringes of water ice during deposition at astronomically relevant ice temperatures. The ice thickness is typically more than 20 μm. Analyzing the period and intensity patterns combining both the monochromatic and broadband interference patterns allows the determination of the wavelength-dependent refractive index and extinction coefficient. We present accurate refractive index and extinction coefficient graphs for wavelengths between 250 and 750 nm and ices deposited between 30 and 160 K. From our data, we find a possible structural change in the ice in the 110–130 K region that has not been reported before. We also discuss that the data presented in this paper can be used to interpret astronomical observations of icy surfaces.

We report multiepoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo forward-modeling method, we achieve median precisions of 0.5 and 0.9 km s⁻¹ for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin-disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously unresolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 and 12 yr, respectively. We find a kinematic age of 3.5 ± 0.3 Gyr for local T dwarfs, consistent with nearby late M dwarfs (4.1 ± 0.3 Gyr). Removal of thick-disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2 ± 0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4–L6 subtypes, likely reflecting the terminus of the stellar main sequence. We provide a compilation of precise radial velocities for 172 late M, L, and T dwarfs within ∼20 pc of the Sun.