The timescales and mechanisms controlling deformation of partially molten, anisotropic crust during continental indentation remain poorly understood. To address this, we present a case study of the southern Chinese Altai fold belts, through an integration of geological observations, geochronological constraints and analog modeling. The region features a series of antiforms formed in response to northward indentation of the East Junggar domain. Vertical felsic dikes transecting the antiform hinges record a progressive south-to-north younging in emplacement age (similar to 300 Ma in the south to similar to 270 Ma in the north), mirroring the spatiotemporal pattern observed in host rock 40Ar/39Ar cooling ages (from similar to 285 Ma in the south to similar to 220 Ma in the north). Scaled analog experiments employing paraffin wax reproduce this pattern, generating serial folds of decreasing ages and amplitudes away from the indenter, accompanied by development of tensile fractures analogous to the field-observed felsic dikes. The experiments reveal that pressure gradients associated with serial folds development drove lateral melt transfer toward distal fold regions and maintained elevated thermal gradients over extended timescales. This mechanism explains observed increase in cooling durations-from similar to 15 Ma in the southernmost antiforms to similar to 50 Ma in the northernmost. We propose an indentation model that reconciles spatiotemporal variations in deformation intensity, melt migration, and thermal evolution in the region. This model provides a robust framework for understanding role of melt and stratified crust in indentation-dominated orogens in general.

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.

Abstract. Pre-existing mechanical heterogeneities significantly influence the evolution of continental rifts and their resulting fault architecture. The Cenozoic Eger Graben (EG) in the northwestern Bohemian Massif (Central Europe) represents a fossil rift system in which the role of structural inheritance has been relatively unaddressed. This study examines how basement structures inherited primarily from Paleozoic geodynamic events may have controlled syn-rift fault development in a setting where the orientation of the rift axis and basement fabric varied in space relative to the extension direction. Two series of crustal-scale analogue models inspired by the EG setting were designed to investigate the effects of both discrete and pervasive weaknesses on evolving fault populations. The first series simulated reactivation of a segmented basement weakness (velocity discontinuity, VD) under uniform or changing extension directions, testing an existing two-phase extension model. The second employed a uniformly extending elastic model base and approximated the role of pervasive fabrics by imprinting grooved patterns into the basal silicone layer. Surface strain evolution was analysed using digital image correlation, while final fault patterns were compared with the generalised structure of the EG. The results highlight the dominant influence of the initial extension phase during polyphase rifting, with the segmented geometry of the VD exerting further control on fault development. Some two-phase scenarios reproduce the observed fault pattern more successfully than others, suggesting that temporal variations in paleostress orientation remain plausible. However, experiments involving spatially variable basement fabrics demonstrate that faults of contrasting orientation and geometry can develop simultaneously above suitable inherited structures. Several structural features of the Eger Graben may therefore reflect synchronous reactivation of different inherited structures rather than rotation of the extension direction. The influence of pervasive fabrics on modelled fault geometries compares well with natural examples and demonstrates the broader applicability of this experimental approach.

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.

Sedimentary volcanism is a widespread geological phenomenon on Earth. Similar processes are theorized to occur on extraterrestrial bodies like Mars, potentially representing surface expressions of subsurface liquid water reservoirs. As we are currently missing ground truth, recognizing signs of extraterrestrial mud volcanism is based on comparative analysis with terrestrial mudflows. Despite the ubiquitous presence of salts on planetary surfaces, it remains unclear how different dissolved salts in mud mixtures influence the behavior of erupted mud. Our study integrates laboratory experiments, thermodynamic calculations, and rheometry measurements to investigate how concentrations of NaCl, MgSO4, Na2SO4, and CaSO4 affect such behavior. The results show that 10 wt.% MgSO4 and only 2.5 wt.% NaCl maximizes mud propagation, producing ropy patterns and narrow flows, while higher salt concentrations result in sheet-like flows covering wider areas. When the mud is supersaturated in salt, the liquid state is prolonged but propagation is reduced. Hence, mud salinity has a significant influence on mud flow behavior in low-pressure environments, reflecting the balance between anti-freezing effects and viscosity-compositional effects. Our experimental results are limited to bentonite muds, yet the sensitivity to salt type/concentration is also expected for other muds, leading to a much broader range of morphology, longevity and spatial dispersion than previously assumed.

The Reykjanes Peninsula (RP) in southwestern Iceland represents a zone of oblique rifting where the divergent boundary of the Mid-Atlantic ridge is offset to the eastern Iceland along a left-lateral transform fault - the South Iceland Seismic Zone (SISZ). RP and the SISZ represent regions of the most abundant earthquake activity on Iceland, development of fissure arrays and occasional lava eruptions. A series of earthquake swarms at RP in the 2021-2023 period indicates development of distributed fracture networks along ENE direction of the transform fault and two new fissure arrays trending NE divided by a gap in seismicity. In the last 3 years, the volcanic activity culminated two times in volcanic eruptions, bringing magmas from Moho depth at 15 km. Inspired by the recent tectonic activity at RP, we conducted a series of analogue experiments consisting of a silicone magma chamber embedded in a photoelastic gelatine crust. The aim of our study is to constrain the links between the depth level of the magma chamber, the crustal scale fracture arrays, faults, magma pathways, superficial fractures and the location of related potential volcanic activity in a transform setting. Inducing strike slip deformation of the system, we explored the influence of shape and orientation of the magmatic chamber on the evolution and pattern of progressively developed fractures along the central shear domain. During the experiment, we captured the stress fringe patterns in the fractured gelatine. The surface deformation was traced by a stereoscopic digital image correlation (DIC) system employing two high-speed LaVision cameras. Analog magma spreading was traced using fluorescent dye mixed to the silicone or into the gelatine interlayer. Modelling results show that decoupling of the crust above the magma reservoir in strike-slip setting produces a domain with higher vorticity bounded by a conjugate set of tensional fractures. The largest open fractures initiate at and propagate from the intersection of the principal strike-slip fault plane with the vertical contact of the magma chamber and the surrounding crust. Including other open fractures, the orientation of the fracture set is oblique (~ 60°) to the fault plane. With formation approximately coeval to those of the fractures, fine wrinkles at the crust surface are observed with orientation of ~ 120° with respect to the fault plane.

Salt diapirism leads to the complex internal deformation of multilayered salt sequences. Competent intrasalt layers undergo intense disruption, décollement, and detachment, coherently reflecting salt flow. Fragments of these layers (stringers) can be dragged to the surface where salt dissolves, accumulating in the dissolution residue (caprock), and potentially preserving the structural configuration given by salt flow. Thus, stringers may act as strain markers to shed light on the diapir evolution. In this contribution, we present the structural configuration of an array of decameter-thick, carbonate stringers, preserved in the softer, evaporitic caprock that forms the surfacing section of the Les Avellanes Diapir (South-Central Pyrenees). This diapir has a complex geometry obtained as Triassic salt arose to the surface assisted by Pyrenean contraction during the late Eocene-early Oligocene and then spread laterally southwards as an extrusive salt sheet. The diapir exposure has been mapped in detail, collecting structural data to build a N-S cross-section that portrays the structural architecture of the stringers. To observe the dynamic behavior of stringers within a vertical and lateral flow, the diapir emplacement has been reproduced using analogue models. The experiments were monitored by a stereoscopic system of two digital cameras, obtaining a set of images that have been processed with DIC techniques (LaVision GmbH) to provide a dynamic analysis of strain quantification. We used the comparison between the modeled theoretical scenario and the field prototype to relate the structural configuration of the stringers with the salt flow during multistage diapirism. From north to south, the stringers present four domains: (1) verticalized parallel to the stem wall; (2) subhorizontal stacked stringers; (3) alternating overturned, subvertical stringers, with north-dipping stringers; and (4) south-vergent imbricated stringers. In the experiments, verticalized stringers in the stem rotated over the feeder as the diffluent flow became lateral. Lateral flow refolded the stringers into isoclinal, recumbent sets verging toward the advancing lobe. These stringers also created temporal embankments concealing the flow and inducing deformation in the brittle caprock. Since the experimental and the field scenario present comparable arrays, the Les Avellanes Diapir evolution is discussed to support its geometry.

Continental lithosphere undergoing the process of rifting has typically previously experienced a complex deformation history resulting in a highly heterogeneous mechanical structure. This structural inheritance can affect the developing continental rift across all scales, from rift localization and segmentation to individual fault geometries. Assessing the impact of such inherited structures on extensional basin geometries can be difficult, especially in the case of fossil rifts where uncertainties may arise about the orientation of regional stresses during extension. One such example is the Eger Rift which developed during the Oligocene to early Miocene as the easternmost branch of the European Cenozoic Rift System (ECRIS). Earlier interpretation proposed a two-phase extensional history for the rift. We use a series of crustal-scale, brittle-viscous analogue models, based on the geometry of the central and eastern parts of the Eger Rift, to explore the development of a segmented rift in a multiphase setting with evolving extension direction. Our model crust rests on a basal velocity discontinuity (VD), a discrete boundary of a mobile base plate simulating a reactivated basement weakness localizing our model rift. The geometry of this weakness is a simplified representation of the geometry of older, mainly Upper Paleozoic basins, which are hypothesized to have greatly influenced the localization and geometry of principal fault systems and rift segments that they define. The VD thus consists of 3 segments oriented at various angles with respect to extension direction. The Model surface is imaged by stereoscopic cameras and analyzed by Particle Image Velocimetry (PIV) techniques to track surface deformation and topography evolution during the run. Our results confirm that in a setting with an abrupt change in extension direction, the first extensional phase plays a key role in defining the final observed fault pattern with new second-phase faults generally being few in number and of limited length. This effect is enhanced above a segmented VD. If a larger portion of the VD is optimally oriented with respect to the first-phase extension, the final fault pattern is dominated by first-phase structures with the growth of second-phase faults being nearly inhibited. In a contrasting scenario, where most of the VD is initially oblique to extension direction, second-phase faults are more abundant, leading to a bimodal final fault pattern. By comparing our results with newly mapped fault populations in the Eger Rift we conclude that the proposed two-phase history for the rift is plausible with a major role of the initial phase of approximately N-S extension. This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S.

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.

IntroductionMud volcanism is a widely distributed geological phenomenon on Earth1. Likewise, it has been suggested that mud volcanoes might exist on some other solid-surface bodies in the Solar System, such as Mars2 and the dwarf planet, Ceres3. Since this phenomenon requires liquid water, extraterrestrial mud-volcano-like (MVL) structures represent key targets for studying the hydrology and potential habitability of subsurface environments on other planetary bodies. Thus, identifying the potential morphological signatures of these landforms beyond Earth is an important step in understanding the nature of aqueous environments within the Solar System. Previously performed low-pressure experiments have shown that in case of “cold” surfaces (-15 °C), low viscosity mud propagates similarly to pahoehoe lava types on Earth4, while in case of “warm” (+20 °C) and unconsolidated surfaces, mud “levitates'' and hence can be transported to longer distances5. The effect of composition, however, was not further tested nor discussed. We hypothesize that the potential muds on Mars or other planetary bodies may naturally contain a certain amount of salts which can affect their antifreezing and rheological properties. Therefore we address the question how the salt component in muds may affect their propagation over cold surface in reduced atmospheric pressures, namely those valid for recent Mars. MethodsTo test our hypothesis, we carried out 54 experiments by experimental procedure adapting the settings (Fig. 1) established for mud flow experiments performed in the Large Mars Vacuum Chamber at the Open University, UK by4,5. During the experiments, portions of mud were released onto a pre-cooled sand surface when the desired pressure (5.9±1 mbar) was reached. The mud was composed of D.I. water, bentonite and with various concentrations (0.5-10%) of salts, namely NaCl, MgSO4 (epsomite), Na2SO4 and CaSO4 were tested. Experiment progress was recorded by cameras situated at the top and sides of the sandbox and temperatures of sand and mud reservoir were measured by thermocouples. In complementary methods, we investigated the pressure-drop-induced evaporative cooling of the brine component on isolated samples in low pressure. Further, we calculated theoretical p-T paths by thermodynamic modeling and measured rheological properties of salty muds for reference Earth atmospheric pressure. Fig. 1: Setup of the (a) flow and (b) tube experiments. Results and discussionOur experiments confirmed expected contrasting behavior of salty mud in decreased pressure (Fig. 2). Individual flows are characterized by unique spatial dispersion and morphological patterns for mutually comparative salt content. Results also revealed thresholds when different muds produce similar patterns and spreading style for highly different concentrations. Performed brine evaporative-cooling experiments showed that the maximum antifreeze potential has NaCl and therefore solutions with this salt are capable of sustaining their liquid state in much smaller pressures than those with other tested salts. The rheological measurements, on the other hand, revealed a contrasting impact of salt addition to viscosity drop of mud samples until these are supersaturated by salt (typically >5-10% concentration in dependence on salt type) and viscosity is further increased. Due to the synergistic effect of decreased viscosity (Fig. 3a) and anti-freezing effect (Fig. 3b) flows are spatially longest for various salts and, not necessarily directly proportional, to their concentrations. For example, 2.5% NaCl mud has a higher anti-freezing effect and lower viscosity compared to 10% MgSO4 (epsomite), resulting in long and narrow flows (Fig. 3c). The increased viscosity of MgSO4 effectively slows the flow, supports the formation of a protective crust and the development of serial, long lobes that maintain liquid mud in their interiors (Fig. 2b,3c). Both salts then exhibit similar spatial dispersion but entirely different styles of propagation and surface geometric pattern. These results are contrasting to previously published experiments and reveal that the increased content of salts leads to different regimes of mud propagation, not necessarily similar to pahoehoe lavas.Fig. 2: Plan view of representative mud flows based on various salts and their concentrations. Visible are distinct patterns of individual flows and contrasting spatial dispersion. Fig. 3 Interpretation of the rheological and thermodynamical effects on mobility of the mud mixtures. (a) shows the progressive increase and decrease of the mud bulk shear velocity in dependence on salt content. (b) is the interpretation of how the individual salts in various concentrations (sorted from lowest to highest effect) impact the freezing delay during the pressure-temperature change. (C) three main contrasting evolutionary trends in dependence on salt type and concentration: 1. multiple short flows that form as parallel lobes, 2. single long-narrow flow - that forms as serial rope-shaped lobes for 10% MgSO4 and lobe-less gently roped pattern for 2.5 NaCl, and 3. single wide and longer "sheet-shaped" flow. The most effective propagation is observed with a combination of 10% MgSO4 and 2.5% NaCl, where the synergistic effects of minimum viscosity and lowered freezing point are most pronounced.All these findings therefore suggest that the salt type and concentrations dissolved within the muddy mixture are important factors in controlling the ultimate shapes, textures and dynamics of mud flows emplaced e.g. on Mars or on other bodies with a thin or non-existent atmosphere. Higher salinity levels extend the unfrozen state and promote the wider spatial dispersion of muds until they reach a certain point of saturation with salts. All this suggests that on Mars, contrary to Earth, salts can play an important role in shaping MVL structures. Variations in salt types and concentrations might help to, at least partly, explain the large variability in the shapes of these hypothesized martian MVL structures. References [1] Mazzini & Etiope (2017). Earth-Science Reviews, 168, 81-112; [2] Brož et al. (2023). Earth Surface Dynamics, 11(4), 633-661; [3] Ruesch et al. (2019). Nature Geoscience, 12(7), 505-509; [4] Brož et al. (2020). Nature Geoscience, 13(6), 403–407; [5] Brož et al. (2020). EPSL, 545.

AbstractWe present results of experiments performed inside a low-pressure chamber designed to investigate whether the volume of mud changes when exposed to a Martian atmospheric pressure. Depending on the mud viscosity, we observe a volumetric increase of up to 30% at the pressure of ∼6 mbar. The reason is that the low pressure causes instability of the water within the mud, leading to profuse bubble formation that increases the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lava or mud volcano eruptions caused by a rapid ascend along the conduit and associated pressure drop.IntroductionSubtle mounds have been discovered in the source areas of Martian kilometer-sized flows and on top of summit areas of some domes (Fig. 1; [1-2]). These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface [1-2]. Two theories have been proposed: (a) ascent of more viscous mud during the late stage of eruption and (b) expansion of mud within the conduit due to the instability of water under Martian conditions.Fig. 1. Examples of Martian mounds and knobs.Mud volcanoes on Earth release mud with a wide range of viscosities [3]; likewise similar variations are also expected in the mud extruded on other planetary surfaces. So far, experimental studies have been conducted only for low viscosity mud under low pressure conditions [4-5], and the behavior of more viscous muds under such conditions remains unclear. To overcome this knowledge gap, we completed analog experiments, to investigate the behavior of various viscous muds under martian pressure conditions.MethodsWe performed a set of experiments using the Mars Simulation Chamber at the Open University (UK), in which we inserted a two-part test bed that contained a reservoir which was able to accommodate 600 ml of mud and a ∼10 cm thick layer of frozen icy-sandy infill as the surrounding (Fig. 2). The temperature of this infill was around -20°C before experiment started. No active cooling of the icy-sandy infill was used, hence the infill slowly warmed up. All experiments were completed before the temperature of the icy-sandy infill reached the melting point of the water ice. The temperature of the mud when poured into the container was either 0.5-3°C (further refer as “cold”) or 13-22°C (“warm”).Figure 2. Schematic illustration showing the experimental setup inside the Mars Simulation Chamber.Three different viscosities were used (Fig. 3) and they are further refer as “low”, “medium” and “high.” The mud mixtures were prepared by mixing deionized water with 1% w/w of dissolved magnesium sulfate salts (MgSO4) and clay content varying depending on the required viscosity. Magnesium sulfate salt was added to the water to achieve Earth's average river salinity, which enables the suspension of submillimeter clay particles within low viscosity mixtures. During the experiment, the pressure was gradually reduced from 1 bar to 5-7 mbar to achieve the martian surficial pressure. Two different speeds of pressure drops were utilized, namely the pressure was reduced within a timeframe of minutes (rapid) or more than an hour (slow).Figure 3. Viscosity curves of “low”, “medium” and “high” viscosity aqueous bentonite.In order to quantify volumetric changes of the mud samples, we used semi-manual and automatized image analyses using the PIV (Particle Image Velocimetry; [6]) and photogrammetry methods.Results and discussionResults revealed a significant volume increase during the experiments with slow depressurization, high mud viscosity and low initial mud temperature (Fig. 4). The volumetric change occurs due to the formation of water vapor bubbles, which are temporarily trapped within the mud. This phenomenon occurs since the bubble buoyancy is insufficient to overcome the drag force within the viscous material. Hence, these bubbles remain trapped in the mud allowing their gradual growth up to centimeter-scale sizes. During their volume increase, they push the mud out from the container resulting in horizontal and vertical propagation of the mud over cm-scales. In those experiments where the mud bulge freezes due to the evaporative cooling, the internal structure is kept in (or beneath) the icy crust. Our experimental approach reveals that mud with identical characteristics features different morphologies depending if its extrusion occurs on Earth or Mars pressure conditions.Figure 4. Results of volumetric change measurements.As the surface gravity on Mars is nearly three times smaller than that on Earth, we performed numerical calculations to reveal to which depth the mud undergoes boiling. We reveal that the depth of boiling would be nearly three times larger on Mars, reaching meters for sufficiently warm muds. As a consequence, the boiling and associated volumetric changes observed during our small-scale experiments may apply to meter-scales for Mars natural conditions . This suggests that the observed mounds and knobs associated with putative martian sedimentary volcanoes might indeed be related to mud volumetric changes in response to surface exposure. We also show that mud flows on Mars, and elsewhere in the Solar System, could behave differently to those found on Earth since mud dynamics are affected by the formation of bubbles in response to the different atmospheric pressures.Figure 5. Simplified concept of gas migration and mud inflation due to the pressure drop.Results of these experiments also suggest that other types of liquid, that are unstable in the low pressure environment, might behave similarly if their viscosity is high enough to prevent the bubble escape. The results presented herein also have implications for cryovolcanic phenomena on icy moons (e.g., Enceladus or Europa) or dwarf planets like Ceres.References: [1] Komatsu et al., 2016. Icarus, 268; [2] Brož et al., 2019, JGR: Planets, 124(3); [3] Mazzini and Etiope, 2017, Earth-Science Reviews, 168. [4] Brož et al., 2020a, Nature Geoscience, 13(6); [5] Brož et al., 2020b, EPSL, 545; [6] Thielicke and Stamhuis, 2014, Journal of Open Research Software, 2(1).

Sedimentary volcanism is a widespread geological phenomenon on Earth. Similar processes are theorized to occur on extraterrestrial bodies like Mars, potentially representing surface expressions of subsurface liquid water reservoirs. Recognizing extraterrestrial mud volcanism relies on identifying emplaced mud flow features based on their morphological characteristics. Despite the ubiquitous presence of salts on planetary surfaces, it remains unclear how different types of dissolved salts in mud mixtures may influence the behavior of erupted mud. This study integrates laboratory experiments, thermodynamic calculations, and rheometry measurements to investigate how concentrations of NaCl, MgSO4, Na2SO4, and CaSO4 affect such behavior. The results show that 10 wt.% MgSO4 and only 2.5 wt.% NaCl maximizes mud propagation, producing ropy patterns and narrow flows, while higher salt concentrations result in sheet-like flows covering wider areas. When the mud is supersaturated, the liquid state is prolonged but propagation is reduced. These findings reveal that mud salinity variations in low-pressure environments have a powerful effect on mud flow behavior. Improved understanding of such parameters is crucial for investigating the mudflows on extraterrestrial bodies, and the study of salt-enriched muds may be therefore crucial for unraveling Mars' geological evolution and its potential for harboring signs of ancient life.

AbstractThis study focuses on investigating the ability of water to carry out the mm-sized grains when exposed to low pressure environments in the attempt to help understand the depositional and erosional capacity of Martian streams (if they are present on today's Mars). IntroductionMany edifices observed on Mars suggest that water once flowed over the martian surface and hence was present in the past (Figure 1) or might be still temporarily present even today [1-2]. However, as today’s Mars is a planet whose surface atmospheric pressure is ∼160 times weaker than that on Earth (~6 mbar vs. ~1000 mbar), water in a liquid phase cannot be present there for a prolonged period of time (e.g., [3]). This is because under such pressure, liquid water should boil, evaporate and eventually freeze due to the evaporative cooling (e.g., [4-5]). Figure 1: Central portion of Osuga Valles in false colors as seen by European probe Mars Express, which formed by movement of a large quantity of water in martian history. ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.As the boiling of water would lead to the formation of large quantities of bubbles, we expected that this would affect its ability to carry out the sediments and hence to change the erosional as well as depositional capacity of water streams on Mars. We speculate that two scenarios are possible: a) the boiling water will have a greater ability to entrain sediment because the emerging bubbles will formed around sediment grains and will be able to lift them, or b) that the bubbles in the water will reduce the ability of the sediment to entrain due to a change in the density contrast between the water and sediment due to the rapid boiling of water.Experimental setupTo find out which hypothesis is correct, we performed a set of 13 experiments using the Mars Simulation Chamber at the Open University (UK), in which we studied the ability of unstable liquid water to carry sediments under normal (1024 mbar) and reduced atmospheric pressures of 25 and 4.5 mbar respectively. To achieve that, we developed a dedicated experimental setup consisting of a steel container holding 500 ml of water with salinity of 0.5% (D.I. water mixed with NaCl salt), from which the liquid could be released to 1° inclined steel plate (Fig. 2). A gridded area of 60x100 cm was marked on the plate, into which 20 grams of 2-4 mm-sized calcite grains were emplaced before the start of the experiment, 10 cm from the start of the grid. The grains were spread out in a rectangular shape in a monolayer rectangle of size 20 x 3 cm. The chamber was then closed and when the required atmospheric pressure was reached (1024 mbar, 25 mbar and 4.5 mbar), the latch at the bottom of the container was released and the water drained to the steel plate. The experimental runs were recorded by cameras occupying the experiment from the top, side and front. This allowed further post-processing, including grain movement and distribution analysis by DIC and statistical methods.Figure 2: Image showing an experimental setup inside a low pressure chamber.Results and discussionExperiments conducted under normal pressure (1024 mbar) showed that most grains were able to get within 49-55 cm from the start of the grid (Fig. 3., Fig. 4a - blue points). In contrast, experiments conducted under reduced pressure of 25 mbar and 4.5 mbar showed that most grains only reached 38-50 cm (Fig. 3, Fig. 4a), respectively 37-49 cm (Fig. 3, Fig. 4a). In addition, with decreasing pressure, the grains were distributed along longer distances (typically 46 cm → 60 cm, black points) while the maximum reached distance by single grains was rather similar (Fig. 4a, green points).Fig. 3: Results of experimental runs performed for pressures of 1024 mbar (4 experiments), 25 mbar (4 experiments) and 4.5 mbar (5 experiments) where each point corresponds to the finite position of calcite grain. The initial position of grains is marked with a shadow rectangle.To quantify differences in sediment transport under different pressures, we performed a statistical analysis, specifically analysis of variance and cluster analysis. Analysis of variance, performed on the distributed calcite grains (Fig. 4b) for both orthogonal directions, shows increasing longitudinal (~5.4×104 → ~7.8×104) and decreasing lateral (12×104 → ~11×104) variances, while their ratio is near similar at about 0.5 in average. Fig. 4: Results of measured distances and analysis of variance. Experiments show that water has a greater ability to entrain grains under normal pressure when water is stable. Once the pressure drops and the water begins to boil and a large quantity of water vapor is produced within the flow, the ability of unstable water to entrain grains then decreases and therefore grains do not travel to such distances as in the case of normal pressure. Grains are also more scattered under decreased pressure. Hence, our preliminary results suggest that despite the current lack of knowledge of how atmospheric pressure has evolved in the Amazonian period, but once the atmospheric pressure dropped below the point at which water is stable, there would be changes in the water's ability to carry sediment.As a next step, we plan to investigate by numerical modeling how the different gravity on Mars affects this process. This parameter can still significantly affect how much unstable water can carry away sediment on the Red planet.References[1] McEwen et al. (2011), Science 333, 740-743, [2] McEwen et al. (2014), Nature Geoscience 7, 53-58, [3] Hecht (2002), Icarus, 156, 373–386 [4] Bargery et al. (2010), Icarus, 210(1), 488–506, [5] Brož et al. (2020). Nature Geoscience, 13(6), 403–407.

The behavior and the rheology of mud during the emplacement of terrestrial sedimentary volcanism has been extensively investigated (e.g., [1,2]). In contrast, this is not the case for Mars and other planetary bodies within the Solar System for which sedimentary volcanism has been proposed [e.g., 3]. The propagation behavior of low viscosity mud in a low-pressure chamber, that partly simulated the environment of Mars, was firstly experimentally studied by [4,5]. Their work revealed that bentonite-based mud could flow in a completely different manner in such conditions. On Mars, mud flowing over cold surfaces would rapidly freeze due to evaporative cooling [6] forming an icy-crust leading to the behavior of some of the mud flows in a similar manner to pahoehoe lava on Earth [4]. However, we lack the knowledge how variations of salt types and their content would affect the flow style and finite pattern of such mudflows as a presence of various salts can be natural on Mars as well (e.g., [7,8]). Therefore increased content of salts can strongly affect the P-T-t dependent cooling and at the same time the rheology of mud which can lead to significantly different propagation potential and finite geometry. In a set of experiments, performed in the Mars Simulation Chamber (Open University, UK), we tested several selected salts relevant for the Mars environment (namely NaCl, MgSO4, Na2SO4 and CaSO4) and various salinities of these salts (0.5-15 wt%). These experiments were performed in metallic trays infilled with dry and precooled sand to -25 °C (to simulate the martian surface) and which were inclined to 5°. A container filled with 500 ml mud was positioned above the tray. Then we decreased the pressure to 4.5-6 mbar and released mud. Experiments were documented by a system of video cameras situated around the model box. At the same time, referential cooling experiments of binary solutions (water-salt) were performed. Results revealed contrasting scenarios of mud propagation which result in a wide range of shapes. We also found several transitional regimes in behavior between current concentrations and various salts. It was confirmed that the high content of salt in a mud or mud composed by different salts can undergo slightly to significantly different cooling according to thermodynamic equilibria which shifts both freezing and boiling point. Thus, the resultant style of flow process and finite morphology of such mudflows can be highly variable. For example, high content of MgSO4 (typically 5-10 wt%) leads to development of long and narrow streams and with increasing content also develops a “ropy pattern” structure, whereas the same behavior occurs for 2.5 wt% of the NaCl. References: [1] O’Brien and Julien (1988), Journal of Hydraulic Engineering 114 [2] Laigle and Coussot (1997), J. Hydraul. Eng., 123 [3] Ruesch et al. (2019) Nature Geoscience 12 [4] Brož et al. (2020), Nature Geoscience [5] Brož et al. (2020), EPSL 545 [6] Bargery et al. (2010), Icarus 210(1), Chevrier et al. (2020), The planetary science journal, 1(3) [8] Nuding, et al. (2014), Icarus, 243.

Stringers represent fragments of competent, brittle layers, formerly interstratified within a layered evaporite sequence (LES) that are entrained and deformed by viscous flow. To resolve the impact of the stringers on the deformation style of an extrusive salt sheet, an array of decameter-thick, carbonate stringers, preserved in the caprock matrix of the Les Avellanes Diapir (South-Central Pyrenees) was analysed. The diapir exposure was mapped in detail to produce a cross-section that shows the stringer assemblage. To understand the dynamic behaviour of stringers, this natural prototype has been reproduced using scaled analogue models. In the model, stringers were carried into the salt sheet, rotating and deforming while migrating toward the sheet front. The array of stringers can be divided into the following three structural domains: 1) a feeder domain mainly hosting vertical stringers parallel to the stem wall that rotate to a subhorizontal orientation as they approach the allochthonous sheet, 2) a sheet domain containing stack-like sets of subvertical and overturned, to subhorizontal stringers, as well as isoclinally and recumbently folded stringers, and 3) a sheet front domain characterised by stringers dipping toward the salt sheet front. Compartmentalization of the horizontal flow by the transported stringers controlled the deformation in the caprock matrix above. Finally, the similarity between the field observations and the model is evaluated and discussed to shed light on the Les Avellanes Diapir kinematic evolution.

Subtle mounds have been discovered in the source areas of martian kilometer-sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: a) ascent of more viscous mud during the late stage of eruption and b) expansion of mud within the conduit due to the instability of water under martian conditions. Here we present experiments performed inside a low-pressure chamber, designed to investigate whether the volume of mud changes when exposed to a reduced atmospheric pressure. Depending on the mud viscosity, we observe volumetric increase of up to 30% at the martian average pressure of ~6 mbar. This is because the low pressure causes instability of the water within the mud, leading to the formation of bubbles that increase the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lavas or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative martian sedimentary volcanoes might indeed be explained by volumetric changes of the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth, because mud dynamics are affected by the formation of bubbles in response to the low atmospheric pressure.

Salt sequences usually contain interbedded, non-saline, sedimentary layers (carbonates, sulphates, and siliciclastics) which behave as brittle, competent layers entrained within the weak, viscous salt. These layers become fragmented, and further stretched and folded as the host rock salt is mobilized. In diapirs reaching the surface, fragments of these brittle layers (stringers) can be transported upwards along the diapir stem from their source layer and then laterally and sometimes gravitationally downwards in the allochthonous salt sheets, becoming embedded in the diapir caprock as the salt is dissolved by unsaturated fluids. Therefore, the stringers arrangement may serve as a proxy to understand salt flow. To test this hypothesis, we have examined the internal structure of the Les Avellanes diapir rocks (South-Central Pyrenean fold-and-thrust belt) which represents a syn-orogenic laterally advancing salt sheet, early Oligocene in age. To understand the internal structure, the diapir exposure has been mapped in detail and projected in a cross-section along the expected flow direction. Then, to evaluate this structure in terms of flow kinematics and dynamics, we have reproduced the Les Avellanes lateral salt sheet with analogue models equipped with a stereographic system of strain quantification (LaVision GmbH).The Les Avellanes Diapir exposes a gypsum rich caprock with numerous Triassic carbonate and subvolcanic stringers, which were carried along within the diapir stem and salt sheet. The carbonate stringers show contrasting bottom and top facies (laminated vs. tabular) constraining their stratigraphic polarity. The stringers are mainly subvertical in the diapir stem and around the probable crestal/feeder area, flat lying stringers are disrupted by several extensional faults. Towards the allochthonous salt body, they are obliquely or vertically imbricated with some of the stringers overturned. This suggests that stringers were carried through the feeder conduit to the surface, then became stretched horizontally in the feeder area and imbricated and stacked within the laterally advancing salt sheet.This hypothesis has been evaluated using analogue models. The modelling setup consisting of a box with a moving wall to simulate shortening, and two silicone layers (polydimethylsiloxane) separated by two thin, colored granular layers (simulating carbonate layers disrupted into stringers). The host rock overburden, represented by colored sand, is continuously sieved around a rectangular vertical conduit of the diapir. During shortening, caprock made of cohesive material (glass beads) is added on top and syn-kinematic sand layers are added adjacent to the laterally advancing silicone extrusion. In the cross-sections of the models, stringers are verticalized in the diapir conduit, parallel to the walls, and distorted into isoclinal folds with downflow vergence in the advancing allochthonous extrusion. The surface strain pattern revealed extension around the crestal area, and belts of contraction downslope in the advancing body developing in sequence backwards from the front as caprock rafts and stringers continuously became imbricated, blocking and decelerating the flow. As the internal structure of the deformed stringers is compatible with field observations, similar strain patterns visible in the model may be attributed to the development of this salt sheet.

&lt;p&gt;Growth rate of salt diapirs usually oscillates depending on several factors. The growth can be arrested by depletion of the source layer or diapir burial, conversely, diapir reactivation occurs through erosion of the overburden and/or introduction of tectonic forces. Examples of reactivated diapirs can be observed in the Zagros Mountains in Iran. There, tectonic shortening responsible for development of the Zagros Fold and Thrust Belt simultaneously squeezes the diapirs, which extrude salt onto the surface. The top section of the diapir is usually affected by meteoric water, which dissolves the salt and leaves behind insoluble material embedded within the source layer, forming the so called caprock. This caprock can be assumed to be already present before the reactivation of the diapirs during shortening, hence it may play a role in the development of the salt extrusions. Geometry, composition and mechanical properties of the caprock can vary widely depending on factors such as original composition of diapiric material, dissolution and growth rates, etc. Additionally, exact mechanical properties of any caprock are difficult to determine and are currently largely unknown.&amp;#160;&lt;/p&gt; &lt;p&gt;Hence, we present a series of 2D numerical simulations utilising finite element method to investigate how different geometries and rheologies of the caprock affect the shape of the subaerial extrusions. The analysis was performed with three variable parameters (caprock viscosity, cohesion, and thickness) for three scenarios of diapirism (1 - purely shortening-based, with depleted source layer; 2 - purely buoyancy-based, with preserved source layer and no tectonic forces, and; 3) a combined scenario). We analysed the general deformation patterns as well as quantifiers such as velocities, displacements, strains, strain rates and ratios between vertical and horizontal components of the quantifiers. We investigated variability of averaged values of the quantifiers in time as well as detailed spatial distribution for the finite state of simulation.&amp;#160;&lt;/p&gt; &lt;p&gt;The simulations revealed the strong contrast between less (low cohesion and viscosity) and more competent rheologies in term of deformation patterns. The former tends to result in caprock material being thinly spread over the surface of the salt extrusion, whereas in the latter case the caprock fractures into &amp;#8220;rafts&amp;#8221; floating on top of the extrusion. The exact geometry of the rafts (size, spacing, distribution) is highly dependent on the geometry and type of diapirism. We also compare the resultant patterns to the quantifiers, especially velocities and their ratios, establishing clear ties between the patterns and deformation dynamics.&amp;#160;&lt;/p&gt;

Growing salt diapirs emerging at the sedimentary surface can produce outflowing salt extrusions, as observed, for instance, in many locations in the Zagros fold-and-thrust belt (Southern Iran). Flow patterns of such salt extrusions are controlled by gravity spreading and gliding. Furthermore, internal structures and shapes of salt extrusions are affected by factors like the local topography, the width of the diapir and the tectonic stress field. Many field examples of outcropping diapirs reveal, however, that the highly soluble evaporites (mainly halite) are already dissolved at the surface and that extrusions are covered by a ‘caprock’ layer, which is built of a multi-compositional residuum of less soluble minerals and rocks. Thickness, composition and mechanical properties of the caprock (density, shear strength, etc.) strongly vary between individual diapirs depending on the original composition of the salt layer, overlying host rock sediments, erosion rate, etc. Hence, the influence of such a caprock on the dynamics of the salt extrusion might also be highly variable and has not yet been investigated. It is unclear, if the caprock deforms by ductile shearing similar as viscous rock salt or if it acts as competent, brittle cover layer deforming by fracturing and brecciation during flow of the underlying salt. We present a series of 3D analogue experiments and 2D numerical models in which we systematically investigated deformation patterns of the caprock layer during diapiric extrusion of a viscous material. In the analog experiments, we tested different types of granular materials as caprock equivalent to simulate different rheologies. Specifically, a fine-grained powder was used to mimic a competent, high-cohesive rock and a coarse-grained, low-density granulate for a less competent rock. In the numerical models, we tested a wide range of caprock parameters, such as thickness, viscosity, and shear strength. Our study is specifically focused on salt extrusions of the Iranian Zagros fold and thrust belt. Thus, the extrusion patterns in both, analog and numerical models, were tested on 1. passively growing diapirs and 2. diapirs reactivated by lateral shortening. The results of this modelling study provide insights into the conditions (e.g. minimum thickness or strength) under which a caprock layer has a significant influence on the style of the salt extrusion or only acts as a passive veneer floating on top of the flowing salt. The model results show that a competent or thick caprock forms a polygonal fracture pattern at the beginning of the extrusion, while the separated blocks slide downslope during later stages of the extrusion. An incompetent or thin caprock rather deforms by flow, shear thinning and folding coupled to the flow of the underlying salt. These characteristics can help us to interpret deformation structures observed on natural salt extrusions, in terms of thickness and deformation behavior of the caprock.

The behavior and the rheology of mud during the emplacement of terrestrial sedimentary volcanism has been previously investigated (e.g., [1,2]). In contrast, this is not the case for Mars nor for other planetary bodies within the Solar System for which sedimentary volcanism has been proposed [e.g., 3]. The propagation behavior of low viscosity mud in a low-pressure chamber that partly simulated the environment of Mars was firstly experimentally studied by [4,5]. Their work revealed that low viscosity mud could flow over cold (<273 K) and warm (>273 K) surfaces at martian atmospheric pressure, however, the mechanism of such propagation would be very different from that observed on Earth. On Mars, mud flowing over cold surfaces would rapidly freeze due to evaporative cooling [6] forming an icy-crust leading to the behavior of some of the mud flows in a similar manner to pahoehoe lava on Earth [4]. In contrast, the mud propagating over the warm surface boils and levitates above the surface. However, as the viscosity of ascending mud can vary, depending on water content, it remains unclear how this affects the mud behavior. To investigate the behavior of muds more viscous than that studied by [4,5] in low pressure conditions, we used the Mars Simulation Chamber at the Open University (UK). In a set of experiments, we tested how the volume of mud (water-bentonite mixture) changed depending on different depressurization rates, mud initial viscosity and initial temperature. These experiments were performed in plastic boxes infilled with frozen (wet) sand (to simulate the martian surface). In the center of these boxes we placed a container filled with a mud volume, then we decreased the pressure to 7 mbar. Experiments were documented by system of video cameras situated around the model box. Quantification of the volumetric changes used semi-manual and automatized image analyses using the PIV (Particle Image Velocimetry) and photogrammetry methods. Results revealed a significant volume increase during the experiments with slow depressurization, higher mud viscosity and low initial mud temperature. The volumetric change occurs due to the formation of water vapor bubbles, which are temporarily trapped within the mud. This phenomenon occurs since the bubble buoyancy is insufficient to overcome the drag force within the viscous material. Hence, these bubbles remain trapped in the mud allowing their gradual growth up to centimeter-scale sizes. During their volume increase, they push the mud out from the container resulting in horizontal and vertical propagation of the mud over cm-scales. In those experiments where the mud bulge freezes due to the evaporative cooling, the internal structure is kept in (or beneath) the icy crust. Our experimental approach hence shows that when mud with identical characteristics is extruded on Earth and Mars, different morphologies would result. References: [1] O’Brien and Julien (1988), Journal of Hydraulic Engineering 114 [2] Laigle and Coussot (1997), J. Hydraul. Eng., 123 [3] Ruesch et al. (2019) Nature Geoscience 12 [4] Brož et al. (2020), Nature Geoscience [5] Brož et al. (2020), EPSL 545 [6] Bargery et al. (2010), Icarus 210(1).