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.

Metastable liquids (including water, brines, and mud) are thought to play an important role in shaping planetary surfaces, from putative sedimentary volcanism and recurring slope lineae on Mars to cryovolcanic activity on icy bodies such as Europa, Enceladus, and Triton (e.g., [1-3]). Under the cold and low-pressure conditions present on many planetary bodies, such liquids become inherently metastable once exposed at the surface, undergoing rapid boiling, evaporative cooling, and freezing (e.g., [4-6]).All these different processes share one important requirement: a liquid must be suddenly exposed to a low-pressure environment before it has time to thermodynamically adjust to the new conditions. Yet most existing experimental setups, whether designed for flows of brines, pure water, or mud, rely on gradual depressurization (e.g., [7-10]). During this process, evaporative cooling progressively pre-conditions the fluid prior to eruption, promoting partial equilibration, early ice or salt-crystal nucleation, and changes in rheology before surface exposure occurs. As a result, the observed flow dynamics may preferentially reflect late-stage, partially equilibrated behaviour rather than the initial response of a metastable liquid to rapid decompression, potentially limiting the applicability of such experiments to planetary surface processes involving abrupt liquid release.Here, we introduce a new experimental approach enabling controlled subsurface release of metastable mud under Mars-relevant low-pressure (~4.5 mbar) and subfreezing (−25 °C) conditions. The experiments used low-viscosity mud stored beneath a frozen crust within a low-pressure chamber. The fluid is stored beneath an ice-sealed reservoir that is subsequently thermally weakened and breached from below using a localized heating element (Fig. 1). Unlike previous surface-release experiments, the presented setup enables subsurface-confined eruptions driven by internally generated boiling pressure. This setup minimizes direct interaction between the mud and the low-pressure environment prior to eruption, allowing the fluid to remain close to its initial thermal state until rupture occurs. The ensuing eruption is therefore driven by in situ boiling and rapid decompression rather than by slow pre-equilibration during chamber evacuation. Fig. 1. Sequence of images showing reservoir opening, fountains, droplets, liquid mud, and flows on panels.The experiments reveal a systematic progression in eruptive behaviour, recognisably analogous to natural sedimentary volcanism. An initial ballistic, Strombolian-like phase is characterised by discrete short-lived bursts ejecting mud droplets from the vent. This activity progressively transitions into a mixed ballistic-effusive regime and ultimately into pulsatory effusive emplacement forming coherent mud flows downslope (Fig. 2). Repeated ballistic and pulsatory activity also promoted proximal accumulation of erupted material and the development of cone-like edifices around the vent. The eruption dynamics are driven by internally generated pressure pulses associated with boiling and phase transitions within the metastable reservoir, rather than by externally imposed pressure gradients. The experiments therefore demonstrate that boiling of metastable mud alone is sufficient to generate cyclic pressurization, pulsatory eruptions, and sustained flow emplacement under Mars-like pressure conditions. Figure 2: Conceptual evolution of eruptive behaviour from ballistic mud ejection to pulsatory effusive flow emplacement under low-pressure conditions. Flow development is controlled by droplet size, transport distance, and limited cooling during ballistic transport (~0.5–4 °C). The relatively small thermal losses during flight allow erupted mud to remain liquid upon deposition despite ambient subfreezing conditions, promoting coalescence of ballistic ejecta into continuous flows (Fig. 3). This suggests that even under Martian gravity and atmospheric pressure, ballistic emplacement may directly contribute to the formation of coherent flow-like deposits. These observations suggest that similar coupling between ballistic emplacement and coherent flow formation may also occur under Martian surface conditions. Figure 3: Formation of mud flows around the eruption site through progressive accumulation and coalescence of ballistic and effusive mud emplacement. Together, these results demonstrate that sedimentary volcanism under low-pressure planetary conditions can operate as a self-sustaining mechanism capable of generating both explosive and effusive behaviour without the need for continuous external pressurization. The presented subsurface-confinement-and-breach approach provides a scalable experimental framework for investigating metastable liquids under rapidly changing low-pressure conditions. Beyond sedimentary volcanism on Mars, the setup is readily adaptable to brines, cryogenic fluids, and cryovolcanic analogues relevant to icy bodies, offering new experimental constraints on eruption dynamics, flow emplacement, and planetary surface evolution. References[1] Fagents (2003), JGR, 108, 5139. [2] Lesage et al. (2021). [3] Brož et al. (2023), Earth Surf. Dyn., 11, 633–661. [4] Hecht (2002), Icarus, 156, 373–386. [5] Bargery et al. (2010), Icarus, 210, 488–506. [6] Brož et al. (2025), EPSL, 668, 119331. [7] Brož et al. (2020a), Nat. Geosci., 13, 403–407. [8] Brož et al. (2023), JGR: Planets, 128, e2023JE007950. [9] Krýza et al. (2025), Commun. Earth Environ., 6, 116. [10] Adler et al. (2025), Commun. Earth Environ., 6, 841.

Icy worlds such as Europa, Enceladus and Ceres show evidence for subsurface liquids reaching the surface, either as plumes or effusive flows. Regions where this has occurred serve as potential archives of subsurface chemistry and habitability, making them prime targets for future missions. Subsurface fluids on these bodies may range in salinity from dilute to eutectic compositions, with brines approaching eutectic concentrations expected to be more common in the shallow subsurface due to their longevity at low temperatures. Despite their importance, little is understood about how highly saline fluids evolve if exposed to surface conditions.We exposed large quantities (~50 kg) of NaCl and MgSO4 brines at eutectic concentrations to pressures below their triple points and observed their physical behavior and thermal evolution. We found that eutectic brines, if emplaced into low-pressure environments, resist evaporatively driven freezing through the formation of salts at their surface which acts to strongly decrease evaporation rate. Furthermore, instead of evolving towards the eutectic point and thus complete solidification, the salinity and temperature of the brines instead asymptotically approached their hydrate liquidus at a concentration approximately 3% above the eutectic concentration. After 120-300 minutes, both brines approached steady-state whereby salts precipitated at the surface and sank, to be replaced by fresh surficial salts. Our findings indicate that eutectic liquids could be relatively long-lived in low-pressure environments. Furthermore, although emplaced brines at icy worlds may freeze conductively from below, ice formation should not be expected in the upper 10s of cm simulated by these experiments. Instead, we predict the systems should continue to evaporate and precipitate hydrates until dryness, meaning that regions where eutectic brines have been emplaced could be indicated by salt lags rather than salt-bearing ices. Our findings provide a new perspective on surface processes involving the extrusion of high-salinity liquids into low-pressure environments and the possible longevity of liquid water under non-equilibrium scenarios on planetary surfaces.

Nowadays, Mars presents an environment characterized by low atmospheric pressure (similar to 6 mbar), which profoundly alters water-driven surface processes known from Earth. Under these conditions, water rapidly boils, cools, and stabilizes near the triple point, often freezing, which strongly limits its ability to transport sediment. Unlike today, early Mars likely experienced higher atmospheric pressures that allowed liquid water to persist in a metastable, boilable state for longer periods. Here, we investigate how water instability affects sediment transport efficiency, focusing on phase transitions and mm-sized grain movement. In our laboratory experiments, water is released onto a shallow inclined plate, producing a sheet-flow-like regime that mimics downslope sediment transport by thin transient flows. We find that larger calcite grains (2-4 mm) are transported less efficiently under unstable water conditions. Transport efficiency decreases further as pressure drops. However, near the liquid-vapor phase transition temperature (21 degrees C; 25 mbar), we observed asymmetrical and irregular spreading of grains, reflecting altered flow dynamics caused by bubble formation. At the lowest tested pressure (similar to 5 mbar), partial water stabilization occurs because rapid evaporation cools the water, slowing the flow and reducing sediment motion without complete freezing. This behavior contrasts with earlier studies on smaller particles (<2 mm) and suggests a grain-size threshold below which transport patterns reverse. Our results provide constraints on sediment transport by sheet flow under past and present Martian atmospheric pressures, advancing our understanding of sediment dynamics under low-pressure and low-gravity conditions and highlighting the challenge of interpreting Martian surface morphology solely from terrestrial analogs.

Abstract. JANUS (Jovis, Amorum ac Natorum Undique Scrutator), the high-resolution camera on the JUICE mission (JUpiter ICy moons Explorer), obtained observations of the Earth during and short-after a Lunar and Earth Gravitational Assist maneuver (LEGA) that was run on August 19–20, 2024. We report on the observations of the Earth that were acquired on August 20, 2024 at a closest approach to the surface of 8,408 km, and on September 9, 2024 at a distance of 564,300 km. The close approach observations covered a narrow strip of the Earth, starting in the night-side of the planet and moving over the terminator and day-side. The later observation provided a low spatial resolution portrait of the Earth and the Moon. Here we examine JANUS observations of the Earth as an example of the different science topics that will be addressed in future observations of Jupiter's atmosphere. The Earth night-side images show atmospheric airglow, clouds illuminated by a full Moon, fires in rural areas, lights over the ocean from maritime traffic, city lights, no firm detections of lightning, and two meteor candidates compatible with meteoroids of 1–30 g entering Earth's atmosphere. The day-side images show crepuscular rays under extreme incidence angles, atmospheric gravity waves on elevated cirrus, sun glint on multi-filter images of the tropical Western Pacific, convective storms, internal waves in the ocean and multiple cloud systems at a variety of spatial resolutions. Some images acquired with the panchromatic filter showed islands such as Luzon and the Big Island of Hawaii. We compare spectral trends of the ocean and various cloud systems extracted from JANUS multi-spectral images with spectra from the EnMAP and PRISMA satellites. JANUS images acquired on September 9, 2024 allow to form a multi-spectral view of the Earth at low spatial resolution. The data confirm the expected instrument performance in terms of optical quality and multi-wavelength radiometry precision, and the ensemble of observations contains a wide variety of features that are good analogs to multiple systems in Jupiter's atmosphere.

Purpose. Martian weather forecasting is important for future exploration, but atmospheric behaviour on Mars combines spatial, temporal, vertical, and dust-driven processes in ways that challenge current modelling and forecasting approaches. Existing machine learning studies often reduce this structure to local time series, which limits their ability to capture wider atmospheric dynamics. Methods. This paper introduces a graph-based data engineering framework called MaGMA (Martian Graph-based Multi-horizon Atmospheric Forecasting) that transforms OpenMARS reanalysis fields into structured learning objects for Martian atmospheric forecasting. The framework represents local atmospheric patches as graph nodes and links them through neighbouring regions, successive time steps, longer temporal dependencies, and dynamically similar atmospheric states. It integrates recent atmospheric history, engineered physical descriptors, and vertical atmospheric information to support forecasting across multiple horizons. Results. We evaluate MaGMA across five unseen Martian years, including regular years and a global dust storm year. In regular years, the model achieves overall coefficients of determination of approximately 0.73--0.85, indicating that it captures a substantial proportion of the variation in the target atmospheric variables. For dust-column forecasting, the model outperforms classical and deep temporal baselines in most year--horizon comparisons. In the global dust storm year, dust-column prediction remains strong at shorter horizons, with coefficients of determination above 0.8 for the first two horizons, but broader multivariate performance declines, showing that extreme regimes still challenge generalisation. Conclusion. The study shows that graph-based data engineering can create reusable and diagnostically useful representations for planetary atmospheric forecasting. It also identifies two priorities for future work: improving learning under rare extreme regimes and making better use of vertical atmospheric structure.

Linear dune gullies are a unique surface feature on Mars of which no Earth analogue exists. They have long and constrained channels, lack an alcove, and often end in circular depressions called pits. Satellite observations have shown that the activity of linear dune gullies is related to the presence of CO2 ice, which led to the hypothesis that these landforms are carved by blocks of CO2 ice. Here, we experimentally test this hypothesis under Martian atmospheric conditions and establish a better physical understanding of how this process works. We conducted >100 experiments under Martian atmosphere in which we released CO2-ice blocks on a sloping sand bed. Our experiments show that CO2-ice blocks move downslope by two different modes of transport, both driven by large gas fluxes produced by CO2-ice sublimation under Martian atmosphere. In general, on steep coarse-grained slopes, the blocks slide down, carving narrow and shallow elongated depressions. On gentle fine-grained slopes, the blocks burrow themselves into the sand and slowly carve deep elongated depressions with high and narrow levees, ending in circular pits. These two modes of transport of the CO2-ice blocks can explain the different morphologies of linear dune gullies on different locations, with some gullies having very defined levees and some not, as well as the transition in morphology of certain linear dune gullies from steep upper slopes to gentle lower slopes that we observe on Mars. These experiments allow us to reconstruct how linear dune gullies formed based on their morphology, and infer the necessary climatic and topographic conditions allowing these gullies to form: 1) CO2 ice needs to be preserved at the top of the dune until the beginning of spring, 2) the rest of the dune needs to defrost and heat up to induce sublimation of the CO2 ice once it has started to move downslope, and 3) the dunes need to be made of material that is fine enough to be propelled away from the block and form levees. The first two conditions should limit the formation of linear gullies to south-facing concave slopes at the beginning of spring, which is also where and when activity in these gullies is observed.

The dust ejected by cometary nuclei encodes valuable information on the formation and evolution of the early Solar System. Multiple short-period comets have been studied in situ, but several perihelion passages considerably modified their pristine condition. Comet Interceptor is the first space mission selected by the European Space Agency to study a pristine dynamically new comet in situ. During a fast flyby through the comet coma, hypervelocity impacts with dust particles will represent not only an important source of information, but also a serious hazard to the spacecraft and its payload. Here we discuss the assessment tests performed on the dust shield of the Dust Impact Sensor and Counter instrument (DISC), part of the Comet Interceptor payload, which will be directly exposed to the cometary dust flux. Using a Light-Gas Gun, we shot mm-sized particles at similar to 5 km/s, transferring momenta and kinetic energies representative of those foreseen for the mission. The impact effects on the DISC breadboard were compared to theoretical predictions by a ballistic limit equation for hypervelocity impacts. We find that, with a simple improvement in the dust shield design, DISC is compatible with the expected cometary environment.

Because of its rotation period of 243 days, Venus is considered a slowly rotating planet. However, its persistent superrotating atmospheric jets, which increase in speed from surface to cloud tops, effectively set a faster rotation speed than the surface rotation. Using the Venus Planetary Climate Model and wind measurements taken by the Pioneer Venus entry probes, we show that the Rossby radius of deformation of the atmosphere varies with height. The atmosphere falls into three circulation regimes: (1) from the surface to 20 km, the Rossby radius of deformation exceeds the planetary radius and no Rossby waves form; (2) from 20 to 50 km, the tropical Rossby radius becomes smaller than the planetary radius, and a circulation regime characterized by a superrotating equatorial jet and mid-latitude Rossby gyres appears; (3) from 50 to 70 km, the extratropical Rossby radius becomes smaller than the planetary radius, the jet develops mid-latitude maxima, and the Rossby gyres shift to high latitudes. Studies of exoplanetary circulation regimes as a function of rotation period have repeatedly shown a similar progression. While observing the circulations of exoplanets to confirm these predictions is not currently possible, the presence of different circulation regimes on Venus and their dependence on altitude could be tested by observing campaigns. Such evidence would be the first observational support for the theory connecting differences in planetary rotation periods to circulation regime transitions and would ground predictions of exoplanet circulations in a validated framework.

Martian gullies are alcove-channel-fan systems that are undistinguishable from debris-flow systems on Earth. Therefore, they have long been hypothesized to be formed by the action of liquid water and brines. However, over the past decade, growing evidence of widespread, extensive, and particularly, seasonal activity in these gully systems, has shifted the formation hypothesis of these landforms away from water-driven processes. The correlation between the spatial and temporal distribution of CO2 frost on the Martian surface and the formation of new lobes, the movement of meter-scale boulders, and the cutting of new channels has led to a new hypothesis: debris flows on Mars are driven by the seasonal sublimation of dry ice (CO2 ice). However, the lack of direct observations of these flows hinders our understanding of the exact conditions that lead to these granular flows, their dynamics, and erosional capacity, which hinders our understanding of the formation of these gullies over the last five million years.Over the last three years, we have conducted three experimental campaigns in two environmental chambers (at the Open University, UK, and Aarhus University, Denmark) with different flume set-ups at varying scales to explore the feasibility of the CO2-driven granular flow hypothesis. We have quantified the CO2-driven granular flow dynamics under Martian atmospheric conditions, the physical limits under which these flows can occur, and have determined their erosional capacity. From these results, we conclude that CO2-driven granular flows can occur on Mars under specific environmental conditions and that the sublimation of very small amounts of CO2 ice (

Dust devils are atmospheric vortices that loft dust from the ground, typically at the hottest times of the day and year on Mars. They contribute dust to the atmosphere and so indirectly affect the global atmospheric circulation. Their size, shape and velocity can provide indications of the weather on Mars. Hence, tracking their occurrence in time and space provides useful data for understanding Mars' current climate. They are also of relevance to landed missions as they can clean solar panel surfaces, extending mission lifetimes. Despite the numerous observations and surveys of dust devils to date, the global extent, distribution and occurrence of dust devils is not yet consistently constrained, mainly due to the relatively limited spatial and temporal scope of manual analyses. To provide the most comprehensive global catalogue of active martian dust devils to date, we applied a RetinaNet convolutional neural network to existing remote sensing images of Mars to identify the distinctive signature of the light-toned lofted dust cloud and dark shadow formed by active dust devils on Mars. The algorithm used similar to 6 m/pixel Context Camera (CTX) images from Mars Years 28-36, scanning through a total of 132 359 images. False positives were manually removed with the help of the Zooniverse platform, resulting in 13 409 detections. This survey presents the most spatially and temporally exhaustive global catalogue of dust devils to date. We confirm many trends revealed in disparate previous studies. For example, approximately half of the detections are concentrated in the Amazonis Planitia monitoring site - a hotspot identified from previous imaging campaigns. In addition, we confirm that orbital observations are not well-suited for detecting dust devils at landing sites, despite the ubiquitous detection of vortices with in-situ data. Our study reveals previously understudied hotspots where dust devil lofted clouds can be seen from orbit, most notably southern Hellas Planitia where only dust devil tracks had been previously extensively reported. Importantly, our results reveal latitudinal clusters of dust devils, and in particular large dust devils, at around 60 degrees N and 60 degrees S during local summer solstice, which had only been hinted at by previous work. This concentration is at a much higher latitude than previous modelling suggests, indicating that dust devil generation on Mars is controlled by more factors than are currently accounted for.

Extensive fields of sub-kilometre- to kilometer-scale edifices have been discovered on Mars and the process of subsurface sediment mobilization has been proposed as their formation mechanism. However, as igneous volcanism might form similarly looking features, it is currently unknown how they formed. Previously it was shown that when low viscosity muds would be exposed over cold sandy surfaces under the reduced martian atmospheric pressure, such muds would behave in similar fashion as Pahoehoe lavas on Earth. This shows how difficult it can be to distinguish mud volcanoes from igneous volcanoes based on morphology alone.However, the composition of the propagating mud as well as of the substrate might be crucial to the overall dynamics and the finite pattern of developed flow features on Mars. On the Red planet, a wide range of substrates is expected to be present globally; covering a transition from dry and warm unconsolidated regolith to permafrost with a higher content of ice. Therefore, to get a better understanding of the behavior of muds exposed to reduced atmospheric pressure and the resulting shapes of putative martian mud volcanoes, we performed a set of experiments, in which we studied the effect of warm, pre-cooled or continuously frozen substrates on general flow properties. We also considered different granular materials, transitional compositions or their spatial sequencing, using mainly silica sand, flour or pure water ice. All tested scenarios showed a significantly contrasting style in mud spreading over the various surfaces. The streaming style and finite morphology of the flows differed from fast, flat spreadings, with the levitation component of transport, to slow and narrow flows with a characteristic ropy pattern. The most important observed feature was an alternation of melting and recrystallization of the ice substrate, caused by interplay between the latent heat release and consumption in between the mud and substrate. Importance of ice in the substrate was also shown through rapidly extended boiling potential and prolonged flow ability of mud, probably due to combination of phase transitions in mud-permafrost and mechanical properties of the substrate itself. These findings are interesting for an evaluation of mud behavior in various environments occurring on Mars or other bodies within the Solar system where the sedimentary volcanism or cryovolcanism might be expected.

The Dust Impact Sensor and Counter (DISC), part of the payload of Comet Interceptor mission, will determine the coma dust features of the mission target comet. DISC sensing plate will be exposed to cometary dust hypervelocity impacts (HVI), because of the high flyby speed (10– 60 km/s) foreseen for the space probe. For this measurement configuration, an efficient dust shield is mandatory, to protect the electronics of the detector and the housing S/C. Aiming at withstanding the predicted HVI without affecting DISC measurements and significantly increasing the payload mass, we designed a light aerogel-stuffed Whipple shield. In this work, we present an analysis of the dust-shield performances by computing the related Ballistic Limit Equations (BLEs) with the support of laboratory tests and numerical simulations. Our results reveal that the DISC dust shield exhibits resilience against particles up to 7 mm in size impacting DISC at 20 km/s—resulting in a momentum 100 times larger than impact predicted during the Comet Interceptor flyby. In addition, we find that the DISC dust shield withstands, for fixed impact velocity, critical particle diameters about four times larger than those stopped by a classic Whipple shield, with a Bumper and Rear wall configuration of equal areal density.

Landforms created by flowing water with sediment have left deposits on the surface of Mars, allowing study of the ancient environment. These features could provide constraints on surface water activity and past habitability. However, only a few lab studies have investigated the appearance and behavior of sediment-rich flows at relevant Mars surface conditions. We conducted experiments in a Mars environment chamber to understand the rheology and deposit morphology of mud under atmospheric pressures from 5 to 1000 mbar and surface temperatures between 248 and 297 K. We found that sediment flows in the Noachian era, when most aqueous activity occurred, could behave similarly to Earth analogs, but only under certain climate conditions. However, in the Hesperian and Amazonian periods, the dominant physical regime changed due to global atmospheric loss. Sediment flows during these eras would not have been similar to Earth analogs, and would have been dominated by freezing, evaporative cooling, and boiling depending on the microclimate (local pressure and temperature). Thus, regional climate and compositional context are important factors for interpreting satellite remote sensing images of these features on Mars. The results suggest we may be able to discover the paleo-atmospheric pressure record on Mars by analyzing sediment flow morphology.

Planetary surfaces are transformed by various processes at different atmospheric pressures. On Mars, fluids are expected to have flowed at reduced atmospheric pressure during the last 3 billion years. Nevertheless, the influence of low pressure on surface processes has been investigated in only a few studies. Our understanding of the influence of gas in fluids on Martian surface morphologies is poor due to the low number of experiments performed at low atmospheric pressure (similar to 7 mbar) and the limited potential analogs on Earth. Here, we present laboratory and numerical experiments to explore the influence of gas production on fluid-sand mobility at low atmospheric pressure. Energetic cohesive sand pellets uplifted by escaping gas were generated, forming narrow channels with levees with lengths of 80-90 cm, which were approximately 1800 times their widths. The maximum experimentally determined velocity of the pellets ranged from 60 to 150 cm/s. The channel morphologies were formed by a pseudo-levitation mechanism corresponding to transport with reduced friction. We propose that the mechanisms observed in our experiments at low atmospheric pressure may explain some past Martian surface changes.

We performed the first laboratory study on the formation of molards by sublimation processes. On Earth, permafrost molards are cones of loose debris in landslide deposits that can be used as a marker for mountain permafrost retreat (Morino et al., EPSL 2019). They originate from ice-cemented blocks of sediment that are transported downslope within the landslide and melt to form conical mounds over time. Molard candidates have also been found on Mars in the ejecta flows of the one billion-year-old Hale Crater. These show similar morphology and spatial distribution to molards found on Earth (Morino et al., Icarus 2022). In contrast to Earth, these molards likely formed by sublimation, because water is not stable in its liquid form (Harbele et al., JGR 2001). To investigate how molards that formed by sublimation could differ from those formed by melting on Earth we performed experiments at the Open University’s Mars Chamber facility. We created cylindrical (Ø13 cm) initial frozen blocks of sediment with either H₂O or CO₂ ice. To condense CO₂ gas within the sediment we modified the approach of Kaufmann and Hagermann (Icarus 2017). Because CO₂ has a faster sublimation rate than H₂O, this allowed us to investigate a wider range of sublimation conditions, and reveal processes which may be applicable to comets and/or icy satellites. We let the initially frozen blocks of sediment degrade on a board in the Mars Chamber while monitoring them with a time-lapse photogrammetry system at a 15 minute interval. This allowed us to quantify the volume transport during the degradation phase. We performed experiments for both ice types at terrestrial and martian pressure for coarse sand, gravel, and JSC-Mars-1a (a Mars regolith simulant). We successfully recreated conical morphologies resembling terrestrial permafrost molards for coarse sand and gravel with CO₂ and H₂O ice under Martian pressure. JSC-Mars-1 fully degrades into conical mounds with CO₂ ice, but only partially degrades for H₂O ice under Martian conditions and does not degrade under terrestrial conditions. The sublimation gas flux produced by the ice makes the largest difference in morphology between the experiments for the finest sediments. For the JSC-Mars-1a under martian pressure, the CO2 ice cemented block degrades into a mound that is spread over a wider area than the same block under terrestrial conditions. We infer that the higher the gas production the more likely the grains are to be ejected, rather than just fall. Sublimation is not the dominant degradation process for the H2O ice cemented JSC-Mars-1a block. All the blocks with coarse sand and ice degrade by sublimation processes. Yet because the grains are barely entrained by the gas flux (even at the highest forcing), the differences are more subtle. The gravel is not influenced by the sublimation gas flux. Our results reveal that sublimation can change the expected morphologies when the gas flux is able to entrain the sediment and has implications for interpreting sublimation pit morphologies on Mars and other planetary bodies where sublimation dominates (Mangold, Geomorphology 2011).

The surfaces of many icy bodies in the Solar System have been resurfaced by cryovolcanism, during which liquid and vapour are released from the subsurface into cold, near-vacuum conditions. Water is one of the most commonly released liquids, but it is not stable at low pressure - boiling near the water surface causes rapid cooling and induces surface freezing. Despite previous theoretical works and laboratory experiments it remains unclear how the three coexisting phases interact. Here we expose large volumes of liquid water (17 and 5 litres) to low pressure to study how the phase transitions interact in the near-surface layer, and what controls the dynamics of the system. We observe that subsurface boiling and associated bubble formation significantly affects the rate and manner of freezing. Ascending vapour deforms the ice and causes it to crack, which releases subsurface pressure. Once the pressure is released, the underlying liquid water is again exposed to the reduced atmospheric pressure, triggering a new cycle of vigorous boiling, bubble formation, ice deformation, and subsequent cracking. Thereby, the period of boiling and freeze-over is prolonged. Additionally, we observe that fracturing and vapour accumulation beneath the ice layer create an uneven surface, characterized by bumps and depressions a few centimetres in height. This shows that ice solidification during effusive cryovolcanic eruptions is likely to be a highly complex process and could leave distinct, observable signatures on and within cryolava ponds and flows.

Recent observations by instruments aboard the ExoMars Trace Gas Orbiter (TGO) have revealed the seasonal presence of hydrogen chloride (HCl $\text{HCl}$) in the Martian atmosphere. This discovery may have important implications for Martian photochemistry as chlorine species are chemically active, and it may provide a link between the atmosphere and known surface reservoirs of chlorine. However, the global distribution of atmospheric HCl $\text{HCl}$ is unknown beyond the very sparse TGO observations, and the source and sink processes driving the observed variability of HCl $\text{HCl}$ are not currently understood. We used a Martian global climate model to investigate, for the first time, the spatial distribution of chlorine species in the Martian atmosphere, and the resulting distribution of surface perchlorates formed via adsorption of atmospheric chlorine species. We adapted an existing Martian photochemical scheme to include gas-phase chlorine chemistry with HCl as the source species, and the resulting atmospheric perchloric acid was allowed to deposit onto the Martian surface via a heterogeneous adsorption scheme. We found that odd-oxygen (O,O3 $\mathrm{O},{\mathrm{O}}_{3}$) and odd-hydrogen (H,OH,HO2 $\mathrm{H},\text{OH},{\text{HO}}_{2}$) species play a major role in controlling the distribution of atmospheric chorine species. Surface perchlorate deposition was found to occur preferentially at high latitudes; in the tropics, the perchlorate distribution was anti-correlated with surface thermal inertia and agreed qualitatively with observations of surface chlorine. Our model predicted a relative enhancement of HCl in polar regions, but it did not reproduce the observed strong seasonality of HCl, suggesting that heterogeneous chemistry may be required to explain the observed chlorine cycle.

Introduction The surface of Enceladus could provide one of the best places in our Solar System to investigate the potential for life. Material from plumes seen emanating from Enceladus' South Polar Region[1], which are believed to be sourced directly from the subsurface ocean, is deposited onto the satellite's icy surface[2]. Rapid resurfacing means that material in this area has not been heavily processed by radiation, so could be an excellent indicator of oceanic composition[3, 4]. To date, the only surface species detected with certainty are H2O and CO2[5], with NH3, CH4 and low molecular weight organics tentatively detected[6, 7]. More in depth analysis will only come from future missions to the Saturnian system. The main aim of this work is to develop Enceladus ice analogues of a range of plausible compositions in conditions representative of the surface. The analogues will then be analysed using instrumentation that could be deployed on future missions. A cryogenic vacuum system has been designed in order to replicate the surface environment and grow ices. The analogues grown will be used to test protocols and analytical techniques relevant to future missions. They will then be subjected to simulated space weathering in order to cover all expected possibilities for different surface components and their fate. We will present the preliminary data of the ices grown and the results of testing of the system to ensure it is operating at optimum conditions. Methods In order to grow and analyse ices in conditions representative of Enceladus' surface, a vacuum chamber, previously used for analysis of pure H2O ice for the development of the Lunar Volatiles Mobile Instrumentation (LUVMI) rover[8], was repurposed. The chamber has a 35 L capacity and two vacuum pumps that can achieve internal pressures of 10-7 mbar. The chamber is also fitted with ports to allow it to be connected to analysis equipment such as a GC-MS. The analogues are grown within a sample holder made out of copper piping wrapped around a copper cooling plate. Liquid nitrogen is injected into the chamber and flows through the piping to cool it down to temperatures of -140 °C. Gas mixtures are injected into the chamber, so that they are condensed onto the cold sample holder and form ices. The system is currently undergoing preliminary calibration and testing in order to optimise operating procedures. H2O and H2O-salt mixtures will be inserted into the sample holder as liquids, then cooled and frozen, before the gas mixtures are inserted. A table showing the preliminary ice mixing ratios can be seen in Table 1. These preliminary compositions are based on upper limits estimated for surface components[5, 7, 9, 10]. The ices grown will be analysed initially using a GC-MS in order to fully characterise both them and the cryogenic system. Table 1. Expected mixing ratios for ices Mixing Ratio Surface Component Ice 1 Ice 2 Ice 3 Ice 4 H2O 99% 98% 98% 96% CO2 1% 1% 1% 1% NH3 - 2% 2% 1.5% CH4 - - 2% 1% NaCl - - - 0.5% Currently, the main system has the capabilities to be connected to a GC-MS, but in order to preserve the ices while in transport to other pieces of equipment for further experiments, an additional system is required. This system is in development, and is designed to replicate a perfect sample oven onboard a spacecraft. A series of valves connects to an external mini chamber, which is capable of being removed from the main system. Within the mini chamber, the sample holder is connected to an external copper bar, which is submerged in liquid nitrogen in order to cool the samples. This allows the ices to be kept in relatively pristine conditions over a longer period of time while being transported. The effects of thermal and vacuum leaks on the ices within the chamber will be determined, in order to mimic potential issues with sample handling onboard a lander. The preliminary results of ice analogue development, including physical and compositional characterisation, will be presented. In addition, the limitations of the system and how potential instrumentation issues during a mission could affect Enceladus ices will also be presented. Future work will include investigating the effects of space weathering on the ices and their subsequent analysis. The main focus will be on impacts that are representative of E-ring grains colliding with the surface and will be investigated using a Van de Graaff Dust Accelerator. References [1] Porco, C et al. (2006) Science 311 1393-1401 [2] Kempf, S et al. (2018) Enceladus and the icy moons of Saturn 195-210 [3] Kempf, S et al. (2008) Icarus 193 420-437 [4] Howett, C et al. (2018) Enceladus and the icy moons of Saturn 342-360 [5] Brown, R et al. (2006) Science 311 1425-1428 [6] Brown, R et al. (2004) Space Science Reviews 115 111-168 [7] Waite, J et al. (2006) Science 311 1419-1422 [8] Gancet, J et al. (2017) 13th Symposium on Advanced Space Technologies in Robotics and Automation 1-8 [9] Emery, J et al. (2005) Astronomy & Astrophysics 435 353-362 [10] Postberg, F et al. (2009) Nature 459 1098-1101

Introduction: The Martian surface hosts a variety of active surface processes [1-3] whose regular monitoring is key to providing us insights into past and present-day surface, geologic and climatic conditions [4]. Most change detection studies on Mars utilize time-series image acquisitions from the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE; 25-50 cm/px) [5] and the Context Camera (CTX; 5-6 m/px) [6]. However, the relatively narrow HiRISE colour swath (~20% of the image swath) results in a lower probability of observing surface changes with multiple wavelengths. The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO) [7] permits 4-band VNIR colour coverage at 4.6 m/px and an image swath >6 km. Furthermore, TGO/CaSSIS is able to observe Mars at multiple times of day, permitting detection/monitoring of diurnal processes. While TGO has only been in operation for a short period of time (MY34-35), the development of simulated CaSSIS images from MRO datasets [8] permits the monitoring of long-term surface changes with CaSSIS from as early as MY28 to present. This work assesses the change detection capabilities of CaSSIS by using a combination of simulated and actual CaSSIS images of one of the most active Martian gully sites to date – Gasa Crater [9-12] . Methods: We initially restricted the simulated CaSSIS images for this study to pre-2012 acquisitions, as our ability to fully photometrically correct CRISM targeted observations for along-track variations in emission/phase is confounded due to the loss of the full gimbal range of CRISM in late 2012 [8]. Three coordinated CRISM/CTX pairs were selected for production into simulated CaSSIS cubes based on a combination of favourable geometries, coverage, estimated atmospheric dust opacities and notable changes. Simulated CaSSIS images were generated using the procedures in [8] where spectrally and spatially resampled CaSSIS-compatible CRISM and CTX products are combined into a rigorous fully-simulated CaSSIS image using a Gram-Schmidt spectral pan-sharpening algorithm, which retains I/F information and minimises colour/spectral distortions [13]. To reduce atmospheric contributions, a dark-object-subtraction technique [14] was applied to both simulated and actual images. All images, including the first actual CaSSIS image acquired on Ls 350, MY34 were overlain and compared to one another to identify visible colour and/or morphologic changes. Notable changes were then compared with previously documented activity. Results and Discussion: We observe 28 possible changes between the simulated and actual CaSSIS image cubes spanning MY28 to MY34 (Fig. 1). Of these, 20 are previously undocumented changes, including 8 putative new changes and 12 fading flows (black-arrows in Fig. 1). All new/previously unrecognized changes are currently under active investigation with associated HiRISE coverage to verify if observed physical changes are not a manifestation of variable illumination conditions. Prominent changes previously observed between MY28 to 30 by [9,10] are all on the northern and north-eastern crater walls (red-arrows in Figs. 1a-c). While one prominent physical change (orange-arrow in Fig. 1c) was previously identified with a simulated CaSSIS image by [8], we note that 5 meter-scale physical changes noted by [9,10] are unresolved by CaSSIS products. Putative new colour changes are observed between MY29 and MY34 (Figs. 1, 2). Six of these were not readily visible with HiRISE due to lack of colour coverage. One prominent example includes a bright-bluish deposit in the eastern part of the crater (Figs. 1b-c). These deposits have an NIR-signature that suggests they are possible ferrous-bearing materials sourced from the gully alcoves [8,12,15]. The most-notable recent putative change based on our first CaSSIS acquisition of Gasa (Ls 350, MY34) shows a bright-bluish deposit on the eastern crater wall that spills partially onto the crater floor (Figs. 1d, 2). Although previous HiRISE acquisitions between MY31 and MY34 seem to show possible morphological changes, lack of HiRISE colour coverage over this deposit makes it difficult to verify the activity. A new optimally-positioned HiRISE acquisition later this year will enable us to verify this surface change (if it has not since faded). Conclusions: This study demonstrates how both simulated and actual CaSSIS cubes are useful for detecting both previously documented and potentially new gully activity at Gasa Crater. While CaSSIS may not capture all small meter-scale physical changes that HiRISE does, it allows for a much-improved colour-change capability over HiRISE. However, continuing to monitor with both instruments is pivotal, as CaSSIS detections of prominent colour changes can be used to reposition HiRISE to better target colour-coverage to validate and characterise meter-scale surface changes. Despite anticipated photometric complications that post-2012 CRISM targeted observations offer, future work will also include an assessment of simulated CaSSIS products generated with post-2012 CRISM and CTX coordinated data to assess identification of both new and previously documented HiRISE changes [16] after MY30. References: [1] McEwen et al. (2010), Icarus 205(1) [2] Bridges et al. (2013), Aeol. Res. 9 [3] Munaretto et al. (2020), PSS 187 [4] Greeley and Iversen (1985), Cambridge Univ. Press [5] McEwen et al. (2007), JGR 112 [6] Malin et al. (2007), JGR 112 [7] Thomas et al. (2017), SSR 212 [8] Tornabene et al. (2018), SSR 214(18) [9] Dundas et al. (2010), GRL 37 [10] Dundas et al. (2012), Icarus 220 [11] Dundas et al. (2015), Icarus 251 [12] Harrison et al. (2019), Geol. Soc. 467(1) [13] Laben et al. (2000), US Patent No: 6011875 [14] Chavez (1988), RSE 24(3) [15] Okubo et al. (2011), Icarus 211(1) [16] Dundas et al. (2019), Geol. Soc. 467(1). Acknowledgements: We thank the spacecraft and instrument engineering teams for the successful completion of the instrument. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA's PRODEX programme. The instrument hardware development was also supported by the Italian Space Agency (ASI), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the University of Arizona (Lunar and Planetary Lab.) and NASA are also gratefully acknowledged. The lead author acknowledges support from Tornabene’s funding from the CSA’s Planetary and Astronomy Missions Co-Investigator programme (19PACOI07) and the Canadian NSERC Discovery Grant programme (06418-2020 RGPIN).