The maximum slope of a granular surface—beyond which particles start to move through avalanches—is determined by the balance of gravitational and interparticle forces and is independent of gravity when only frictional forces act between the particles. Additional forces like cohesion can modify (increase) this slope angle, though, and break the degeneracy on gravity. In addition, it has been proposed that a subsurface Knudsen compressor induced by insolation can specifically decrease maximum granular slope angles on Mars. To test the latter hypothesis, we study particle motion on slopes under Martian conditions in parabolic flights. That is, we simulate low gravity (0.37 g), low ambient pressure (100, 300, 600 Pa), and weak sunlight (590, 1235 W m ^−2 ) while increasing the slope of the granular media. As simulant we use JSC Mars-1A of two different particle size ranges centered at 50 and 160 μ m and a mixture of both. Without illumination, we find avalanches starting on slopes reaching about 35°, not significantly depending on the size fraction. In illuminated parts, avalanching systematically starts at lower slope angles, i.e., as low as about 10°. Here details depend on particle size, light flux, and ambient pressure, as elucidated by a mathematical model that corroborates our experimental findings. At high pressure, we see a tendency that particle motion starts at the light/shadow boundary. In any case, we see for the first time that the low Martian light flux is sufficient to drive significant Knudsen compressors within the Martian soil and reduce slope angles.

Models applicable on Earth tend to fail in low gravity. Beyond a critical gravitational acceleration, cohesive forces become predominant, resulting in a shift in macroscopic behavior. For granular processing in space, notably to sustain human presence on the Moon, this can have disastrous consequences; yet, the influence of (low) gravity on granular flows is not accounted for in most existing models. We present results from hopper discharge experiments, conducted in low gravity using an active drop tower and parabolic flights. To access partial gravity, a centrifuge is placed inside the plane, allowing us to generate lunar gravity inflight. The flow rate, Q, and clogging probability, Pc, are measured for different granular materials. We find an increased clogging probability and deviations from the generally accepted gravity-scaling of the Beverloo equation, Q - √g. We generalize the granular behavior in low gravity for all materials studied, and propose an explanation based on dominant cohesive interparticle forces in low gravity.

In recent years, the tribocharging of colliding and bouncing submillimeter (submm) particles has been studied as a possible mechanism promoting the formation of large pebbles on centimeter (cm) to decimeter (dm) scales in protoplanetary disks. Here, we observe, for the first time, that it is not only monolithic, spherical particles, but also real dust aggregates, that become tribocharged and end up forming large clusters. For aggregates of ~0.4 mm consisting of ~1 micrometer (µm) sized dust, we determined net charge densities up to 10−7 C/m2 during our drop tower experiments. These charged aggregates form compact clusters up to 2 cm in size via collisions with other clusters and aggregates at collision velocities on the order of 1 cm/s. Size and speed are the only lower limits for growth, currently set by the limits of the experiment. However, these clusters already form under conditions that are well beyond the expected transition to bouncing for uncharged aggregates and clusters. Our findings further support the idea that collisional charging can leapfrog the traditional bouncing barrier and form larger clusters that then serve as large pebbles. These cm-sized clusters are more susceptible to further evolutionary steps via particle trapping, concentration, and planetesimal formation.

If two solid particles collide, charge is exchanged. However, this transfer is not restricted to the surface of the particle. Ions are also dispersed into the environment. They form a charge cloud around the particle. In this way, all particle-laden atmospheres from volcanic plumes on Earth over exoplanet atmospheres to protoplanetary disks might be subject to gas-phase ionization by means of particle collisions. In laboratory experiments, we quantify the amount of ions produced in a collision of glass beads with 2.8 mm diameter. We extract the ions by applying an external electrostatic field and measuring the generated current. The ions are detected at all the pressures studied, i.e., from 0.3 mbar to 100 mbar. However, the ionization rate peaks at about 1 mbar. Scaled to individual bouncing collisions, charge as high as ∼1 pC of each polarity was detected. This implies collisions of grains can be a significant source of ions in various atmospheres.

The aim of our work is to analyze size distributions of particles and their agglomerates in imagery from astrophysical microgravity experiments. The data acquired in these experiments are given by sequences consisting of several hundred images. It is desirable to establish an automated routine that helps to assess size distributions of important image structures and their dynamics in a statistical way. The main technique we adopt to this end is the morphological granulometry. After preprocessing steps that facilitate granulometric analysis, we show how to extract useful information on size of particle agglomerates as well as underlying dynamics. At hand of the discussion of two different microgravity key experiments we demonstrate that the granulometric analysis enables to assess important experimental aspects. We conjecture that our developments are a useful basis for the quantitative assessment of microgravity particle experiments.

Early dust evolution in protoplanetary disks is dominated by sticking collisions. However, this initial phase of particle growth faces constraints - notably from destructive encounters. To find the maximum particle size achievable, we studied collisional processes during a prolonged microgravity experiment aboard a suborbital flight. Here, we specifically report an impact erosion limit. We observed individual basalt beads, each measuring 0.5 mm in diameter, colliding with and either eroding or adhering to a cluster several centimeters in size. This cluster, formed from tribocharged particles, simulates an electrostatic growth phase that surpasses the classical bouncing barrier. We find a threshold velocity of about 0.5 m/s, distinguishing between additive and erosive impacts of individual beads. Numerical simulations of grain impacts into clusters, testing both low and high charge constituents corroborate the experimental findings of surface erosion within the observed velocity range. This specific velocity threshold suggests the potential formation of pebbles several centimeters in size within protoplanetary disks. Such dimensions place these pebbles well into a regime where hydrodynamic interaction might facilitate the formation of planetesimals.

Within the RoadMap project, we investigated the microphysical aspects of particle collisions during saltation on the Martian surface in laboratory experiments. In earlier works, we followed the size distribution of ejected particles, their aerodynamic properties, and aggregation status upon ejection. We now focus on the electrification and charge distribution of ejected particles. We analyzed rebound and ejection trajectories of grains in a vacuum setup with a strong electric field of 100 kV m(-1) and deduced particle charges from their acceleration. The ejected particles have sizes of about 10-100 mu m. They carry charges up to 10(5) e or charge densities up to >10(7) e mm(-2).

Particles regularly tribocharge in collisions. Here, we study how long charges can persist on such particles in the environment of a protoplanetary disc. We set up three complementary experiments to quantify discharge time-scales. We first directly measure the time dependency of charge on triboelectrically charged objects. For this aspect, we performed two long-time experiments under different environmental conditions. We find that the charge persists on the tribocharged bodies on time-scales between minutes and years. Discharge might be mediated by external ions or internally, by conduction. To constrain the latter, we also determined the specific electric resistance of dust samples as simulants for dust aggregates in protoplanetary discs. In this third experiment, we see an increase in resistivity at decreasing ambient pressure up to the limit of the instrument. These findings are consistent with the assumption that water on all relevant surfaces including dust grains within the pore space of aggregates is the main driver of discharge. Under disc conditions, the charge might persist for weeks to years. This leaves net-charged isolated grains in dense parts of the mid-plane of protoplanetary discs charged in between collisions.

In planet formation, the evolution of particles from dust to km-sized planetesimals goes through different stages of growth processes [1]. Starting with micron dust, hit-and-stick collisions create rather fluffy aggregates. With further collisions, these aggregates become more compact until the kinetic energy cannot dissipate into deformation anymore. At about mm-size, the dust aggregates reach the so-called bouncing barrier, where further growth is halted [2]. However, for subsequent growth mechanisms (e.g. hydrodynamic trapping through streaming instability) to set in, cm- to dm-sized pebbles are needed [3]. There are several approaches and explanations how one could overcome the bouncing barrier. One of them is collisional charging. Through tribocharging, there will be a transfer of electric charges between particles with each impact, leaving surface charges at the point of contact [4,5]. In previous experiments, these surface charges have already proven to be the cause of stable cluster formation for solid, monolithic grains [6]. Now, for the very first time, we also observed clustering of charged dust aggregates.We conducted microgravity experiments at the Drop Tower in Bremen with mm and sub-mm aggregates made from µm dust. The sample is placed in an experiment cell under vacuum conditions. During the 9 seconds of microgravity, the cell can be shaken to distribute the sample and induce inter-particle collisions. The video data show that the particles move rather randomly through the volume of the cell. For the sub-mm aggregates, clusters already form during the shaking phase. For the mm particles, the clustering only begins as soon as the shaking stops and the granular gas has somewhat cooled down. Both samples form cm-sized clusters, the smaller particles more efficiently than the larger ones. The clusters are stable when colliding with the wall but can be eroded by particle impacts at certain velocities. When an electric field is applied to the chamber walls, single particles and small clusters are accelerated to the walls, indicating that they are electrically charged.Our findings show that electric charges are capable of bridging the bouncing barrier. Sub-mm and mm particles that are made up of µm grains will stick together after impacts below a certain velocity, forming cm-clusters that are mostly stable against further impacts. This paves the way for pebbles to grow to a size range where further growth processes set in, eventually leading to km-sized planetesimals that can accrete more mass gravitationally.References:[1] G. Wurm, J. Teiser, 2021, Nature Reviews Physics, Vol. 3, No. 6, Springer Science and Business Media LLC, p. 405-421[2] A. Zsom, C. W. Ormel, C. Güttler, J. Blum, C. P. Dullemond, 2010, Astronomy & Astrophysics, 513, A57[3] A. Johansen, H. Klahr, T. Henning, 2006, The Astrophysical Journal, Vol. 636, No. 2, American Astronomical Society, p. 1121-1134[4] D. J. Lacks, T. Shinbrot, 2019, Nature Reviews Chemistry, Vol. 3, No. 8, Springer Science and Business Media LLC, p. 465-476[5] T. Steinpilz, F. Jungmann, K. Joeris, J. Teiser, G. Wurm, Gerhard, 2020, New Journal of Physics, Vol. 22, No. 9, IOP Publishing, p. 093025[6] J. Teiser, M. Kruss, F. Jungmann, G. Wurm, 2021, The Astrophysical Journal, 908, L22

It is long known that particles of the same material but with different sizes charge with different polarities in mutual collisions. In most cases, the smaller grains become negative. Here, we study tribocharging of (sub-)mm dust aggregates in the course of microgravity experiments by determining the charges of particles through their motion within an electric field. Similar experiments were already conducted with monolithic grains. Here, the constituent dust grains in an aggregate add complexity to the process of tribocharging in various ways. This ranges from the dust size scale, setting local surface curvatures, over shifting grains during collisions, altering the outer surfaces and potentially generating sub-surface tribocharging, to material-dependent tribocharging with a non-homogeneous dust composition. Nevertheless, in concert with the usual size dependence, the small aggregates predominantly charge negatively, the large population charges predominantly positively.

In early phases of planet formation, bouncing and fragmentation barriers still represent major obstacles. Beginning at micrometer, dust can readily grow to sub-millimeter size in collisions due to cohesion before bouncing prevails. Later, streaming instabilities trigger further growth which might finally results into planetesimal formation by gravitational collapse. However, for streaming instabilities sub-millimeter grains might be too small, therefore there is gap of at least 1 order of magnitude in size which needs to be bridged.Here, we present our ongoing work how to bridge this gap by charge moderated aggregation [1]. When two (dielectric) grains collide they charge. This tribocharging or collisional charging is omnipresent in nature. We designed drop tower experiments in which we generated charges on glass and basalt grains by collisions in a shaker. In microgravity, the particle trajectories and collisions were observed, and charges were measured by applying an electric field.In early work, we analyzed millimeter-sized glass grain collisions with a copper plate. The coefficient of restitution increased with the charge on a single grain due to mirror charge forces. That means highly charged grains tend to stick more easily to surfaces than uncharged grains. The velocity where sticking is possible was increased by a factor of 100 up to several dm/s [2]. More recently, we used half millimeter basalt spheres and observed sticking events at several cm/s among grains themselves [3]. This is also way higher than predicted by adhesion. In a number of cases, we could observe the sequential formation of aggregates of up to ten single grains. During approach the grains are accelerated due to net charge Coulomb forces but likely also due to higher order charges on the surfaces in agreement to earlier measurements of strong permanent dipole moments [4]. Attraction increases collision cross-sections and the growth is sped up. Growth only stopped by the end of microgravity [3]. To observe the formation of still larger aggregates we developed a new setup, in which a dense cloud of 150 µm diameter basalt grains was continuously agitated slightly under microgravity and in vacuum. Here, the growth of a giant aggregate of centimeter size was observed collecting nearly all material in one cluster [5].To conclude, in experiments under various conditions, we see strong evidence that electrostatic charges on grains are able to conquer the bouncing barrier. We observed the bottom-up growth tracking individual particles, stable clusters emerging from dense regions and the formation of giant clusters during agitation. These are all bricks in the wall giving evidence that collisional charging might play a crucial role in planet formation.References:[1] Steinpilz, T.; Joeris, K.; Jungmann, F.; Wolf, D.; Brendel, L.; Teiser, J.; Shinbrot, T.; Wurm, G. Nature Physics 2020a, 16, 225-229.[2] Jungmann, F.; Steinpilz, T.; Teiser, J.; Wurm, G. Journal of Physics Communications 2018, 2 095009, 095009.[3] Jungmann, F.;Wurm, G. Astronomy and Astrophysics 2021, DOI: https://doi.org/10.1051/0004-6361/202039430.[4] Steinpilz, T.; Jungmann, F.; Joeris, K.; Teiser, J.; Wurm, G. New Journal of Physics 2020b, 22, 093025.[5] Teiser, J.; Kruss, M.; Jungmann, F.; Wurm, G. The Astrophysical Journal Letters 2021, 908, L22.

We conducted experiments with ensembles of colliding sub-millimeter basalt particles under prolonged microgravity conditions on a suborbital flight. In these experiments, the sample motion was excited at different levels. Initial shaking of the particles simulates a classical phase of bouncing in early planet formation. During these collisions, the particles charge, as was measured by applying an electric field. Further moderate shaking then leads to the formation of compact clusters of the charged beads, up to several centimeters in size. These clusters grow by individual impacts up to a velocity threshold of about a meter per second. Beyond that velocity, clusters are eroded. We therefore find a shift in barriers from a bouncing particle at millimeters per second to an eroding cluster at meters per second, going along with a shift in particle size from sub-millimeter up to several centimeters. This allows growth well into a size regime that might make clusters prone to hydrodynamic instabilities and subsequent planetesimal formation.

We recently flew a new setup on parabolic flights for the first time to study particle motion on Martian slopes under Martian gravity. Here, we describe the initial experiments. We used dust/sand beds at varying ambient pressure of a few hundred pascals. The inclination of the particle bed was varied from 0 degrees to 45 degrees and parts of the surface were illuminated under varying conditions. We could observe downhill motion of material related to the insolation at the lowest light flux used of 591 +/- 11 W m-2 for JSC Martian simulant. Motion occurred at significantly lower inclinations under illumination than without illumination, i.e., down to about 10 degrees compared to about 20 degrees-30 degrees, respectively. We attribute this reduction in slope to thermal creep gas flow in the subsoil. This induces a Knudsen compressor, which supports grains against gravity and leads to smaller angles of repose. This is applicable to recurring slope lineae and slopes on Mars in general.

Wind erosion is a serious threat to the evolution of planetesimals in the early stages of planet formation. Since they move around the star at a different orbital velocity than the surrounding gas, they feel a permanent headwind of at least 50 m/s [1]. In the case of eccentric orbits, this difference in velocity can increase significantly. Planetesimals are not solid rocks, but rather a collection of loosely bound dust aggregates [2] that are held together by the low self-gravity. Although the pressure in protoplanetary disks is very low, wind erosion can dismantle planetesimals. As a result, their existence is ruled out in some areas close to the star [3,4].In a series of wind tunnel experiments in the laboratory and on microgravity platforms such as the drop tower Bremen and parabolic flights, we have recreated the conditions on planetesimals more and more realistically. Under pressures down to 1 Pa and gravity down to 10-5g, we observed and analysed wind-induced erosion of granular particles. Starting with simple glass spheres, we finally recreated the surface of a pebble pile planetesimal with millimetre sized SiO2 dust aggregates. These aggregates were produced by the collision of micrometre sized SiO2 particles in analogy to the dust growth up to the bouncing barrier in protoplanetary disks.As a result of our experiments, we found a single formula that predicts the erosion threshold of granular particles on the surface of a planetesimal. Its overall size, the volume and density of the pebbles forming it and its position in the protoplanetary disk can be set as parameters. Together with a suitable disk model, this allows us to define stable and unstable orbits for the evolution of planetesimals into full grown planets. References[1] Weidenschilling S. J., 1977, MNRAS, 180, 57[2] Wahlberg Jansson K., Johansen A., Bukhari Syed M., Blum J., 2017, ApJ, 835, 109[3] Cedenblad L., Schaffer N., Johansen A., Mehlig B., Mitra D., 2021, ApJ, 921, 123[4] Schönau, L., Teiser, J., Demirci, T., Joeris, K., Onyeagusi, F.C., Fritscher, M., Wurm, G., 2023, A&A, 672 A169

Planetesimals or smaller bodies in protoplanetary disks are often considered to form as pebble piles in current planet formation models. They are supposed to be large but loose, weakly bound clusters of more robust dust aggregates. This makes them easy prey for destructive processes. In microgravity experiments, we apply strong electric fields on clusters of slightly conductive dust aggregates. We find that this generates enough tensile stress on the fragile clusters to sequentially rip off the aggregates from the cluster. These experiments imply that electric fields in protoplanetary disks can dissolve pebble pile planetesimals. This process might induce a bias for the local planetesimal reservoir in regions with strong fields. Planetesimals prevail with certain kinds of compositions where they are either good isolators or compacted bodies. The less lucky ones generate pebble clouds which might be observable as signposts of electrostatic activity in protoplanetary disks.

3D Structure and Stability of Particle Clusters close to the Bouncing Barrier – New Experiments Collisions of particle clusters in the protoplanetary disk are a key process in the matter of planet formation and therefore formation of solar systems. [1] It is generally accepted that the early phases of planet formation are governed by such collisions. Collisions can result in different outcomes, such as growth, fragmentation, or restructuring of the cluster. Disk models cannot resolve the detailed physical processes involved, so laboratory experiment help to understand this phase of planet formation. Growth by collision is stalled once a critical aggregate size (Stokes number ≈ 1) is reached, as particles bounce off each other instead of sticking or transferring mass. This critical size range is referred to as the bouncing barrier.Recent experiments showed that collisional charging is a key process to allow growth beyond the growth barrier. [2] Collisions among particles lead to charge separation resulting in a broad charge distribution within the ensemble, changing the collision dynamics and the resulting structure of growing agglomerates.Which collision outcome occurs depends on numerous properties of the participating agglomerates. The structure of these clusters is an important aspect in determining how stable the cluster is. In experiments with only one camera perspective much of the structural information gets lost. Due to the projection of a three-dimensional object on the two-dimensional plane depth information is not easily accessible. Using stereo vision, one can calculate the position of particles a cluster in three-dimensional space and further get a more precise description of the clusters before and after the collision to get an in depth understanding of cluster stability. We present novel experiments on the stability of growing clusters and their three-dimensional structure.To analyze the stability of clusters the experiments are executed in low gravity at the Gravitower Bremen, providing around 2 s of microgravity. Additionally, the experiments are performed under vacuum to exclude the influence of gas drag. The main part of the experiment is a test cell (free volume of 5 cm x 5 cm x 5 cm) with a smaller particle compartment at the bottom. The test cell is agitated in vertical direction, inducing particle-particle collisions while the experiment is still on ground, leading to the formation of clusters of charged particles (either basalt or glass particles).In microgravity, clusters and single particles are ejected into the free volume by shaking the chamber. Inside the chamber the cluster interacts with the granular gas, which results in abrasion of the cluster surface due to ongoing collisions with small particles. When the cluster hits the wall, the cluster can shrink even further or may be fragmented in smaller clusters and single particles. Therefore, these experiments enhance our understanding of the cluster stability while interacting with a granular gas and when colliding with more solid objects. The experiments are observed with a high temporal resolution and stereo vision. The stereo vision is obtained with a pair of mirrors generating two different images onto different sections of the sensor. Together with an exact calibration of the optical system, this enables a detailed 3D reconstruction of the agglomerates. Therefore, these experiments help to correlate the 3D structure of clusters with the collisional dynamics. [1] Wurm, Gerhard, and Jens Teiser. "Understanding planet formation using microgravity experiments." Nature Reviews Physics 3.6 (2021): 405-421.[2] Steinpilz, Tobias, et al. "Electrical charging overcomes the bouncing barrier in planet formation." Nature Physics 16.2 (2020): 225-229

ABSTRACT Particles regularly tribocharge in collisions. Here, we study how long charges can persist on such particles in the environment of a protoplanetary disc. We set up three complementary experiments to quantify discharge time-scales. We first directly measure the time dependency of charge on triboelectrically charged objects. For this aspect, we performed two long-time experiments under different environmental conditions. We find that the charge persists on the tribocharged bodies on time-scales between minutes and years. Discharge might be mediated by external ions or internally, by conduction. To constrain the latter, we also determined the specific electric resistance of dust samples as simulants for dust aggregates in protoplanetary discs. In this third experiment, we see an increase in resistivity at decreasing ambient pressure up to the limit of the instrument. These findings are consistent with the assumption that water on all relevant surfaces including dust grains within the pore space of aggregates is the main driver of discharge. Under disc conditions, the charge might persist for weeks to years. This leaves net-charged isolated grains in dense parts of the mid-plane of protoplanetary discs charged in between collisions.

Context. The initial particle growth in protoplanetary disks is limited by a bouncing barrier at submillimeter wavelengths. Bouncing leads to tribocharging and the electrostatic attraction of tribocharged aggregates may eventually draw them into large clusters. A charge- mediated growth phase allows for the formation of larger entities, namely, clusters of aggregates that are more prone to further particle concentrations, such as the streaming instability. Aims. We aim to quantify the strength of the electrostatic forces. Methods. In laboratory experiments, we used an acoustic trap to levitate small aggregates of tribocharged submm grains. These aggregates spin up within the trap until they lose grains. Thus, we used the centrifugal force as a measure of the local force. Results. Grains are regularly bound strongly to their neighbors. In comparison, the force at ejection can be stronger than the attractive scattering forces of the trap and can therefore be several orders of magnitude larger than expected. We note that these forces are long- ranging, compared to van der Waals forces. Thus, charged aggregates are much more stable than uncharged ones. Conclusions. Particle aggregates in disks might grow to centimeter clusters or larger as tribocharging increases the effective binding forces. This allows for hydrodynamic concentration and planetesimal formation to eventually take place throughout a wide part of the disk.

Protoplanetary disks are thought to harbor deadzones, regions in their interior, where the ionization fraction is too low to trigger MRI turbulence. This has quite severe consequences for accretion and planet formation. So far, the dominant source considered for ionization is high energy radiation from external cosmic rays over stellar radiation to radioactive decay. Especially, external radiation might indeed be attenuated too much before reaching parts of the midplane in disks. Radiation might not be the only source of ionization though.In recent years, we carried out a number of laboratory experiments on collisional charging (tribocharging) of grains related to planet formation. Being mm-size, in a range where fragmentation and bouncing dominate, these particles are large and, although charged, might not be relevant in the context of MRI themselves directly. An underlying assumption though is that triboelectric charge generated in collisions among grains stays on the grains. In this case, grain charging and aggregation as part of early particle growth on one side and MRI building on gas ionization on the other side would be distinct processes. However, what we recently found in a number of different experiments, is that tribocharging of grains also charges the ambient gas.In one set of microgravity experiments, we observed that the charge on two particles before and after a collision is not conserved. On the order of 20\% of the pre-charge on a grain is discharged into the environment. To mention a second experiment, the number of ions detected in a gas flow after passing a granular medium increases strongly if the particles undergo collisions, i.e. are charged. There are experiments with further evidence but these two clearly show that charge is leaking into the ambient atmosphere during collisions.In detail, the charging depends on the collision frequencies and particle sizes, which are model dependent. We also caution that these are only first experiments in this direction, but, in any case, estimates suggest that ionization rates within the gas might regularly be way larger than e.g. provided by radiation at the surface of a disk. If this holds, the charge balance in the midplane of protoplanetary disks might be quite different than previously assumed. The obvious and severe consequence might simply be that there are just no deadzones.

In an experiment at Martian gravity (on parabolic flights), Martian pressure, and Martian insolation we simulated the motion of particles on Martian slopes with an inclined particle bed, representing Martian soil. The insolation generates temperature gradients within the soil which lead to thermal creep gas flow. This generates a sub-soil overpressure millimetres below the surface. The increased pressure supports particles against gravity. Here, we show for the first time that this support allows particles to move downward at much shallower slopes than on non-illuminated parts of the slopes. We also show for the first time, that on inclined slopes, the low Martian light flux is already sufficient to induce this motion (Bila et al. 2024). While low ambient pressure is usually not in favour of particle motion as it requires high wind speeds to pick up grains, this is not the case in the presence of thermal creep gas flow. In this case, the low ambient pressure on Mars is rather a booster for particle motion. The pressure at the surface of Mars is exactly in the pressure range, where thermal creep works best (Bila et al. 2023).At a given pressure, thermal creep depends on grain size. For soils of predominantly large sand-sized particles, no support will be present on Mars but for soils with significant fractions of smaller grains and dust, the flow features on slopes might be influenced, i.e. shaping Recurring Slope Lineae. Also, an easier downward motion of grains might increase the amount of dust that becomes entrained into the atmosphere.T. Bila, G. Wurm, K. Stuers, K. Joeris, and J. Teiser, Dry Downhill Particle Motion on Mars, Planetary Science Journal, (accepted), 2024. T. Bila, J. Kollmer, J. Teiser, and G. Wurm, Thermal Creep on Mars: Visualizing a Soil Layer under Tension, Planetary Science Journal, 4:16 1-8, 2023.