TP7
This session focuses on the mechanical, geotechnical, dynamical (flow) and thermophysical properties of regolith on planetary bodies such as the Moon, Mars, asteroids, comets, and natural satellites. We welcome contributions based on laboratory experiments, numerical simulations, and theoretical analyses, in addition to studies using in-situ, remote sensing, or returned sample data and simulants.
The session aims to provide a forum for advancing our understanding of regolith physics across planetary environments and for enhancing connections between disciplines: planetary science, granular physics, and geotechnical engineering. Interdisciplinary approaches that bridge experiments, modelling, and observations are encouraged and we particularly encourage early career scientists to submit an abstract for an oral presentation.
Introduction:
Recently, Bierhaus et al. (2023) argued for the presence of a global, near-surface layer (~1-4 m thick) of fine-grained material (particles between approximately 100 μm and 1 cm) on asteroid Bennu, based partly on the presence of small (20 m diameter) craters with smooth interiors. This layer may have evolved due to percolation of fines into the subsurface by dynamic processes such as seismic shaking from impact cratering and/or mass movement. If present, a subsurface layer with a distinct particle size-frequency distribution (PSFD) may have different physical properties (e.g., cohesion, friction angle) that affect the evolution of Bennu’s surface—a stronger, more cohesive layer at depth could, for example, stabilize the near-surface and inhibit creep-style mass movement relative to an unlayered target.
We have therefore undertaken a study to test the hypothesis of a global, near-surface, fine-grained layer on Bennu, and to determine how such a layer would affect mass movement on the asteroid’s surface. Our investigation includes: (1) mapping smooth exposures, characterizing their geologic setting and relative stratigraphy, and determining whether they are consistent with such a layer; (2) carrying out laboratory experiments simulating mass movement in granular targets of varying properties and structures; and (3) complementing laboratory experiments with numerical simulations of surface creep under Bennu’s gravity using the Soft-Sphere Discrete Element Model (SSDEM) code PKDGRAV.
Confirming the presence of layering and its role on surface evolution would further our understanding of both rubble pile asteroid interior structure and rubble pile resurfacing—such understanding could help to address a question regarding the mismatch between rubble pile NEA surface ages (~few to tens Ma) and the proposed breakup of their parent bodies (~1 Ga) (Bierhaus et al., 2022; Bottke et al., 2015; Cho et al., 2021).
Results:
Mapping Smooth Regions: Our first task involves identifying all exposures of smooth material on Bennu. We identified smooth regions using two methods, (1) morphologic identification using the ~5 cm/pix Detailed Survey global mosaic (Bennett et al., 2021), and (2) automated identification of regions with low facet tilt variation (<4º)—a proxy for surface roughness—on 20 cm DTMs. This dual mapping technique (Figure 1) yielded abundant smooth exposures >1 m diameter, with complementary results across both methods. Both methods identified smooth exposures in similar regions; the automated DTM search found more small exposures, while the morphologic mapping better distinguished apparent exposures of fine-grained materials from other features such as smooth surfaces of boulders. We find that smooth exposures are concentrated within the Smooth Unit of the global geologic map (Jawin et al., 2022), with relatively fewer exposures in the Rugged Unit. Our next step is to determine the stratigraphic position of smooth exposures—are fine-grained regions consistent with exposure of a subsurface layer, or is smooth material present at the surface?
Figure 1. (A) Distribution of morphologically smooth exposures (blue circles) compared to geologic unit boundaries (yellow outline). Smooth regions are concentrated in the Smooth Geologic Unit. (B) Distributions of low-tilt regions found with OLA, closely matching the results of morphologic mapping.
Laboratory experiments: Our second task involves a series of experiments simulating mass movement in granular targets. For each experiment, we fill a Plexiglass box with different target materials and slowly raise one edge of the box using a hand crank. The box is tilted until the target material fails (Figure 2). We record each run using high-speed cameras and take still images and 3D scans throughout each run. We vary unlayered and layered targets (2 and 3 layers) using lava sand/gravel, play sand, and colored fishtank gravel.
Our results show evidence of size sorting associated with mass movement, with coarse material accumulating on the distal ends of deposits, and finer-grained material becoming exposed upslope. Failure events are observed to enhance the percolation of fines.
Figure 2. Time series of 2-layer experiment. This shows coarse lava gravel (>1/4”) on play sand.
Numerical assessments: Our third task involves a suite of SSDEM simulations similar to the setup of our laboratory experiments, but with a pre-defined range of particle sizes and SFD under Bennu gravity. Initial results show that PSFD significantly influences the bulk mechanical properties of the granular beds, in particular smaller particles can reduce the overall shear strength of the bed, although bulk cohesion remains negligible for cm-scale grains and becomes detectable only when particle sizes decrease to mm-scale. Additionally, our simulations show that slow landslides can effectively drive downward percolation of fine particles via kinetic sieving of fines. Even low-velocity landslide events can reorganize regolith under Bennu’s extremely weak gravity and low effective normal stress, leading to modification of the surface PSFD and stratification of the granular bed (Figure 3). In addition, segregation efficiency appears to increase with particle size contrast and increasing granular bed depth. Taken together, these results show that slow, creep-style mass movement is an efficient mechanism for producing vertical particle size heterogeneity, which could potentially lead to layering within Bennu’s near-surface—although does not exclude a well-mixed interior.
Figure 3 (Left) Modeling results showing changes in surface particle SFD and (right) size segregation due to landslides under Bennu gravity
Current status of findings:
Our findings show multiple lines of evidence that mass movement can size-segregate particles by moving larger particles to the surface and driving finer-grained materials into the subsurface. However, our results do not currently indicate that fine-grained materials will concentrate in a subsurface layer—rather, they appear to become mixed with coarser particles.
References: Barnouin, OS et al. Nat. Geosci. 12, 247-252 (2019). Bennett, CA et al. Icarus, 357, 113690 (2021). Bierhaus, EB et al. Nat. Geosci. 15, 440–446 (2022). Bierhaus, EB et al. Icarus 406, 115736 (2023). Bottke, WF et al. Icarus 247, 191–217 (2015). Cho, Y et al. JGR: Planets 126, e2020JE006572 (2021). Jawin, ER et al. Icarus 381, 114992 (2022). Scheeres, DJ et al. Science Advances 6, eabc3350 (2020).
How to cite: Jawin, E., Barnouin, O., and Zhang, Y.: Does Bennu contain a near-surface, fine-grained layer?, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-85, https://doi.org/10.5194/epsc2026-85, 2026.
How to cite: Gasteiner, L., Murdoch, N., and D'Angelo, O.: Centralizing Lunar Regolith Properties: An Open-Source Database for Geomechanical Data, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-529, https://doi.org/10.5194/epsc2026-529, 2026.
We introduce a ready-to-use regolith stratification model for airless Solar System bodies, built on laboratory and literature data describing granular-media compression. Under the influence of gravity, regolith becomes progressively denser with increasing depth (Schräpler et al. 2015, Bürger et al. 2024) . We quantify this densification by extracting key parameters from experimentally determined compression curves (Blum et al. 2026).
To establish the model, we analysed compression data for various granular SiO2 samples and determined the turnover pressure, i.e. the stress pm at which the material transitions from a loose to a compact state, and the logarithmic width of this transition Δ. We then correlated the packing fraction with grain size r to link particle radius to the turnover pressure. Our analysis covers grain radii from approximately r=0.5 µm to r=100 µm and reveals a power-law relationship pm~r-2, in agreement with a theoretical dust-aggregate compression model (Tatsuuma et al. 2023).
The regolith-stratification model was indirectly validated by providing a unique solution for grain size, transition width and deep-layer density for the lunar regolith when thermal radiation from LRO/Diviner and Chang'E-2/MRM over a wide wavelength range are taken into account and compared to a thermophysical model (Bürger et al. 2026).
Finally, we present the full stratification model along with all relevant free parameters determined from the data. Valid across a wide range of grain sizes and materials, this model provides a practical tool for estimating density profiles in the regolith of moons, asteroids, and comets.
References:
J. Blum, J. Cybulski, G. Meier, B. Gundlach, C. Güttler, J. Bürger 2026. The stratification of planetary regolith: investigating the relation between turnover pressure and grain size, Astronomy & Astrophysics 707, A361.
J. Bürger, J. Feng, M.A. Siegler, J. Blum 2026. A microphysical thermal model for the lunar regolith: determining the lunar regolith properties using a combination of LRO/Diviner and Chang’E-2/MRM data, Astronomy & Astrophysics, accepted.
J. Bürger, P. Hayne, B. Gundlach, M. Läuter, T. Kramer, J. Blum 2024. A microphysical thermal Model for the lunar regolith: investigating the latitudinal dependence of regolith properties, JGR Planets 129, e2023JE008152.
R. Schräpler, J. Blum, I. von Borstel, C. Güttler 2015. The stratification of regolith on celestial objects, Icarus 257, 33-46.
M. Tatsuuma, A. Kataoka, S. Okuzumi, H. Tanaka 2023. Formulating compressive strength of dust aggregates from low to high volume filling factors with numerical simulations, The Astrophysical Journal 953, id.6.
How to cite: Blum, J., Cybulski, J., Meier, G., Gundlach, B., Güttler, C., and Bürger, J.: A verified physical model for the stratification of planetary regolith, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-134, https://doi.org/10.5194/epsc2026-134, 2026.
Introduction
Lunar regolith is repeatedly subjected to mechanical shaking from impacts, moonquakes, tidal forcing, and thermal cracking [1]. Future in-situ activities such as lander operations, rover traffic, construction, and mining will extend the range of disturbances acting on the surface. These disturbances may alter regolith density, strength, and thermal properties without producing obvious visible modification, making the vibratory response of regolith important for both surface evolution and future exploration.
Lunar cold spots provide the primary motivation for this work. Cold spots are distal regions of reduced thermal inertia surrounding young impact craters, commonly interpreted as areas of lowered near-surface bulk density [2]. Although cold spots are associated with impacts, their formation mechanism remains unknown. Recent work suggests that sustained, low-acceleration shaking following lunar impacts may contribute to regolith de-compaction [3]. However, cold spots are not the only setting in which mechanically induced de-compaction may matter: any repeated forcing capable of perturbing the contact network may modify the bulk state of regolith.
The response of a granular assembly to shaking is not controlled by forcing alone. Initial packing fraction influences whether applied vibrations lead to dilation or compaction [4], and preparation history (how the material reached its current packing and contact state) can also influence its later response [5]. This is especially relevant for lunar regolith, which is typically compacted outside cold spot regions [6,7], though the exact process responsible for that state remains uncertain (impact processes, quaking, micrometeoroid impact, etc.). Here we ask whether sustained low-frequency shaking can dilate lunar-like regolith, and how that response changes with preparation history.
Methods
We use discrete element method simulations [8] to examine the history-dependent response of lunar-like regolith to sustained low-frequency shaking. We prepare two slender 50 cm columns of synthetic lunar regolith following the procedures and rocky-regolith-like material parameters used in [9]. The spherical particles experience Hertzian contact forces, damping, high sliding and rolling/twisting friction, and cohesion, giving them effective properties intended to represent rough, dissipative, cohesive regolith grains. Periodic boundary conditions in the horizontal directions represent a laterally continuous slice of regolith.
The two columns differ in preparation history. In the “poured” case, particles are inserted near the maximum column height and settle slowly under lunar gravity, producing a relatively compact poured state. In the “pre-tapped” case, particles are inserted rapidly, settle into a looser initial state, and are then regularized by tapping until the average packing fraction oscillates about a compacted steady state. This produces poured and pre-tapped states with comparable average packing fractions (60.8% vs 61.8%, respectively), but different preparation histories and contact-network structures.
We then apply shaking using a sinusoidally oscillating particle shake plate at the floor. The imposed shaking has frequency f = 20 Hz and varying non-dimensional peak acceleration , Γ=ap/gL, where ap is the peak shake-plate acceleration and gL is lunar gravity. This procedure follows the tapping framework of [4] and was previously implemented in [10]. We quantify the bulk response using column-height change, Δh , relative to the initial 50 cm state. Positive Δh indicates dilation; negative Δh indicates compaction.
Results
The preliminary results show that the pre-tapped and poured columns follow similar qualitative trends, but differ substantially in magnitude and detailed evolution (Fig. 1). For Γ=1, both cases undergo brief initial dilation followed by net compaction. For Γ≤0.1, both beds slowly dilate and reach steady state after several thousand taps as in [4]. However, the magnitude of dilation is much smaller in the poured case (Fig.1, left) than in the pre-tapped case (Fig. 1, right), differing by roughly an order of magnitude. The pre-tapped column also evolves more intermittently, including abrupt rearrangement or slip-like events, while the poured column evolves more smoothly.
Figure 1. Column-height change relative to the initial 50 cm state as a function of tap number for varying at f=20 Hz. Time is obtained by dividing tap number by frequency. The left panel shows the pre-tapped column and the right panel shows the poured column. Positive height change indicates dilation, while negative height change indicates compaction and is not shown on the logarithmic scale. Insets show late-time steady-state behavior, though we have not yet reached steady state for the poured case. The same legend applies to both panels.
These results show that the response to repeated shaking is not determined by forcing amplitude and frequency alone. Instead, the magnitude and evolution of bulk density change depend on initial packing state, contact-network structure, and preparation history. For cold spots, the low acceleration vibrations considered in this work can contribute to near-surface dilation with an efficiency that depends on how the regolith’s high compaction state was achieved. More broadly, repeated low-frequency forcing from non-impact induced surfaces may produce changes in surface density and mechanical state. Mechanically induced regolith modification should therefore be treated as a history-dependent granular process.
Acknowledgements
EF and FF acknowledge funding of the European Union’s Horizon Europe research and innovation programme under grant agreements No. 101264707 (Marie Skłodowska-Curie Actions Postdoctoral Fellowship, SEISMOR) and No. 101077758 (ERC, TRACES). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union, the European Research Council Executive Agency, or the European Research Executive Agency. Neither the European Union nor the granting authorities can be held responsible for them. NS and VL acknowledge support from NASA SSERVI GEODES grant 80NSSC19M0216.
References
[1] Lognonné et al. 2009, Journal of Geophysical Research: Planets, 114(E12), E12003.
[2] Bandfield et al. 2014, Icarus, 231, 221–231.
[3] Schmerr et al. 2025, 56th Lunar and Planetary Science Conference, LPI Contributions 3090, abstract 2160.
[4] Knight et al. 1995, Physical Review E, 51(5), 3957–3963.
[5] Mouraille et al. 2009, Engineering Fracture Mechanics, 76(6), 781–792.
[6] Colwell et al. 2007, Reviews of Geophysics, 45, RG2006.
[7] Hayne et al. 2017, Journal of Geophysical Research: Planets, 122(12), 2371–2400.
[8] Kloss et al. 2012, Progress in Computational Fluid Dynamics, 12(2/3), 140–152.
[9] Frizzell and Hartzell 2024, Granular Matter, 26(4), 90.
[10] Frizzell and Hartzell 2023, Granular Matter, 25(4), 75.
How to cite: Frizzell, E., Schmerr, N., Hartzell, C., Lekić, V., and Ferrari, F.: History-dependent bulk density changes of lunar regolith under sustained shaking, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-360, https://doi.org/10.5194/epsc2026-360, 2026.
Recent exploration missions to asteroids have demonstrated that low-energy material mobility occur in asteroid environments. Low energy in this case may be defined as the energy level below that of the escape speed of the asteroid, allowing take off from the surface, orbital motion and re-impact. The episodes of ejection and reimpact of particles observed on Bennu by OSIRIS-REx mission [1], the evidence of material transport between binary companions in Didymos system [2] and potential sesquinary impacts following the DART impact on Dimorphos [3] suggest that such activity may be more common than previously thought, shaping asteroid surfaces incessantly. This would complicate the interpretations of asteroid surface evolution. It was indeed previously suggested that small craters of Bennu may have formed as a result of low-speed impact cratering through a study of low-energy impacts with respect to surface normal [4]. However, most re-impacting material would impact the surface at an angle due to the uncontrolled initial ejection conditions and irregular surfaces of asteroids.
This study therefore investigates the low-energy oblique impact cratering under low gravity. The impacts are simulated in a discrete element method code GRAINS, which can handle the contacts between non-spherical particles with a non-smooth contact model [5]. A gravity level (~9.81 x 10-4 m/s2) representative to asteroid Didymos is selected. The granular bed is prepared with polygonal particles sized 2 to 7 cm equivalent radii and densities (3.3 g/cm3) similar to Didymos. The impact of a 20-cm diameter spherical impactor at the same density as the particle is then simulated on a granular bed with impact speeds of 10 and 20 cm/s and at angles between 30 and 90 degrees from local horizontal. Normal impacts are simulated for comparison with the authors’ previous work [4,6]. Qualitatively, the results show shallow non-circular craters and asymmetric ejecta field except of normal impacts. The impactor remains near the crater wall in the direction of lateral motion, which may be used as an indication of a low-speed impact structure in asteroids. The craters are quantitatively analysed with crater-scaling relationships [7] to extract scaling coefficients by including the non-circularity of the craters via the impact angle as a parameter the relationships. The presented results will have direct implications to understand low-speed activity on asteroids. It will particularly be relevant to identify sesquinary impacts on Didymos and to reveal the surface mechanical properties as observed by the Hera mission. The implications of the results can also be used to predict the outcomes of ballistic CubeSat landings during Hera missions, which will occur at a similar energy level, offering a further opportunity to understand small body surfaces.
References
[1] D. 1047 Lauretta, C. Hergenrother, S. Chesley, J. Leonard, J. Pelgrift, C. Adam, M. Al Asad, P. Antreasian, R.-L. Ballouz, K. Becker, et al., 2019. Episodes of particle ejection from the surface of the active asteroid (101955) Bennu, Science 366 (6470), doi:10.1126/science.aay3544.
[2] Sunshine, J.M., Rizos, J.L., Barnouin, O.S., Daly, R.T., Ernst, C.M., Farnham, T.L., Agrusa, H.F., Wright, E., Wiggins, S.E., Bruck Syal, M. and Stickle, A.M., 2026. Evidence of Recent Material Transport within a Binary Asteroid System. The Planetary Science Journal, 7(3), p.56.
[3] Langner, K., Marzari, F., Rossi, A., Zanotti, G., 2024. Long-term dynamics around theDidymos–Dimorphos binary asteroid of boulders ejected after the DART impact. Astron. Astrophys. 684, A151. http://dx.doi.org/10.1051/0004-6361/202348675.
[4] Çelik, O., Ballouz, R.L., Scheeres, D.J. and Kawakatsu, Y., 2026. Material Dependency in the Scaling of Low-Speed Craters under Microgravity, Icarus, 450, 116981.
[5] Ferrari, F., Lavagna, M. and Blazquez, E., 2020. A parallel-GPU code for asteroid aggregation problems with angular particles. Monthly notices of the Royal Astronomical society, 492(1), pp.749-761.
[6] Çelik, O., Ballouz, R.L., Scheeres, D.J. and Kawakatsu, Y., 2022. A numerical simulation approach to the crater-scaling relationships in low-speed impacts under microgravity. Icarus, 377, 114882. doi: 10.1016/j.icarus.2022.114882
[7] Holsapple, K.A., 1993. The scaling of impact processes in planetary sciences. Annu.Rev. Earth Planet. Sci. 21, 333–373.
How to cite: Çelik, O., Fodde, I., and Ferrari, F.: Low-speed oblique impacts under low gravity with implications of asteroid surfaces and spacecraft landings, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1216, https://doi.org/10.5194/epsc2026-1216, 2026.
On Earth, hillslope processes are typically driven by gravity and lubricated by liquid water. The slope angle, availability of water, and material composition ultimately determine the type of mass-movement, the flow dynamics, and the morphology of the resulting depositional landforms. Therefore, terrestrial hillslope landforms have served as our guide in the interpretation of hillslope landforms and their formation processes on other planetary bodies (e.g. the Moon, Mars). However, pioneering work has shown that gravity has a significant effect on the dynamic angle of repose (Kleinhans et al., 2011), the transition of bedload to suspended load in fluvial sediment transport (Braat et al. 2024), and the settling speed of fine sediment in water (Kuhn et al., 2015). This raises the questions if and how gravity affects the non-linear flow dynamics of hillslope mass movements and the morphology of their depositional landforms.
In this study, we experimentally explored the effects of gravity on the dynamics of dry mass movements and those lubricated by a liquid. We performed rotating drum experiments under varying gravity (from ~0.1g to 2g, with g = 9.81 ms-2). The lower and hyper-gravity conditions were created by flying, respectively, parabolic trajectories and steep turns with a Cessna Citation II aircraft (PH-LAB), in which the rotating drum set-up was installed. In the rotating drum (d = 50 cm), we tested how dry and wet granular flows responded to different gravity by measuring flow depth, density, compaction and dilation, and internal grain dynamics. Reference experiments with varying drum-rotation speeds were performed under Earth gravity to determine the relative effects of centrifugal force versus gravity, and aircraft vibrations.
Our experiments show that gravity changes the dynamics of both dry and wet granular flows in our drum, and that these effects are more pronounced for wet granular flows. Under higher gravities (>1g), the granular flows become more compacted, which pushes the water out of the mixture and decreases the water content of the granular flow itself. As a result, the interparticle friction increases and the centre of mass shifts upslope in the drum. At lower gravities (<1g), the granular flows dilate, increasing the pore space in the sediment-water mixture, resulting in an increase in air in the inter-particle pore space. Furthermore, we see that under lower gravity conditions (<1g), the friction angle of the flowing material increases significantly. For dry granular flows the friction angle increases by 5 degrees when going from Earth to Martian gravity. For water-driven granular flows this increase in friction angle is even larger; i.e. 20 degrees.
Comparison of the results under varying gravity with those of the reference experiments with varying drum rotation speeds under 1g confirm that gravity has a unique effect on the flow dynamics of granular flows. In particular, the increase in friction angle. The increase in relative friction under lower gravity conditions can be explained by the relative increase of the influence of previously underestimated cohesion forces under lower gravity conditions (such as electrostatic forces and surface tension).
How to cite: Roelofs, L., van Dam, B., van Eijk, A., Klaassen, M., den Toom, G., Mulder, H., Kleinhans, M., ten Kate, I. L., van Westrenen, W., and de Haas, T.: Experimental debris flows and rock avalanches under different gravities – To the Moon and Mars in an airplane , Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-33, https://doi.org/10.5194/epsc2026-33, 2026.
One of the most used models in planetary science, geotechnical, and geophysical studies to describe the strength and behaviour of geological materials such as regolith is the Mohr-Coulomb equation. However, the Mohr-Coulomb equation is actually a linear approximation to a non-linear failure envelope and is not appropriate in very low-confining stress conditions. This has been widely documented in the geotechnical literature but, as far as we are aware, the consequences of the non-linearities in very low confining stress are not commonly considered in the context of small bodies. Indeed, on small bodies, where the gravitational acceleration and thus confining stress can be several orders of magnitude less than on Earth or the Moon, the surfaces should operate in a non-linear strength regime.
The primary goal of this work is to bring established models and literature from the geotechnical engineering community about the non-linear failure envelope to the attention of the planetary science and exploration communities. Then, using an established non-linear failure model, we demonstrate the consequences of non-linearity in the limit of extremely low confining pressure for both the surface and sub-surface mechanical properties of small planetary moons, asteroids and comets. We also discuss implications of the non-linearities for surface stability and regolith mobility, small body surface interactions, numerical modelling, laboratory experiments and planetary simulants.
This work is supported by the European Research Council GRAVITE project (grant N°1087060).
How to cite: Murdoch, N.: Strength and Stability of Regoliths in Low Confining Stress, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-311, https://doi.org/10.5194/epsc2026-311, 2026.
The Martian water cycle, particularly the exchange of water vapour between the atmosphere and shallow regolith, remains poorly constrained despite its importance for present-day climate, habitability, and future human exploration. On every sol, a Martian day, the near-surface regolith can act as a temporary sink and source of atmospheric water. Water vapour is adsorbed into the regoltih as temperatures drop after sunset, and released again as the surface warms after sunrise. This process has long been recognized as a key part of the near-surface water cycle, from early adsorption studies to later analyses using lander data, single-column models, and global climate models (e.g., Fanale and Cannon, 1971, 1974; Zent et al., 1993; Jakosky et al., 1997; Böttger et al., 2004; Savijärvi et al., 2016, 2019, 2021, 2024a,b; Steele et al., 2017; Rivera-Valentín et al., 2020). Laboratory studies have also measured water adsorption and desorption in Mars-relevant materials, including palagonite, basaltic analogues, clays, sulfates, and other mineral phases (Zent et al. 1993,1994, Pommerol et al. 2009, Beck et al. 2010, and Ramachandran et al. 2021).
This exchange also has astrobiological relevance. At Jezero crater, present-day environmental conditions may allow nighttime surface water activity to exceed 0.5 when temperatures fall below 190 K, while daytime conditions above 245 K correspond to water activity values below 0.02 (Zorzano et al., 2024). Despite decades of studies there are still gaps in laboratory measurements specifically under coupled Martian pressure and diurnal temperature cycles.
In this study, we present laboratory experiments designed to measure atmosphere–regolith water vapour exchange under simulated Martian near-surface conditions. This work builds on our previous single-column modelling study (Patel et al., 2025), which showed that regolith porosity can have a significant influence on the amount of water exchanged between the atmosphere and surface at Jezero Crater. We test this model prediction directly in the laboratory by exposing regolith simulants with differing porosity to Mars-like diurnal temperature cycles and pressures. The experiments are being carried out using the Dirty Under-vacuum Simulation Testbed for Icy Environments, DUSTIE, at JPL. These measurements will help test model predictions of diurnal exchange, support interpretation of rover humidity measurements, and identify which regolith properties are most important for controlling the water exchange on Mars.
References
Beck, P., Pommerol, A., Schmitt, B., & Brissaud, O. (2010). Kinetics of water adsorption on minerals and the breathing of the Martian regolith. Journal of Geophysical Research: Planets, 115, E10011.
Böttger, H. M., Lewis, S. R., Read, P. L., & Forget, F. (2005). The effects of the Martian regolith on GCM water cycle simulations. Icarus, 177(1), 174–189.
Fanale, F.P., & Cannon, W.A. (1971). Exchange of adsorbed water between the regolith and atmosphere of Mars. Nature, 230(5293), 502–504.
Fanale, F.P., & Cannon, W.A. (1974). Mars: Adsorption of water vapour by powdery mineral surfaces. Journal of Geophysical Research, 79(23), 3397–3402.
Martínez, G.M., & Renno, N.O. (2013). Water and brines on Mars: Current evidence and implications for MSL. Space Science Reviews, 175, 29–51.
Patel et al. (2025), Sensitivity Analysis of Regolith-Atmosphere Water Exchange on Mars, EPSC-DPS2025, 2025
Polkko, et al. (2025), Annual Evolution of Water Vapor at Jezero Crater Based on Observations and Modeling Journal of Geophysical Research: Planets, 130, e2025JE009124
Pommerol, A., Schmitt, B., Beck, P., & Brissaud, O. (2009). Water sorption on Martian regolith analogs: Thermodynamics and near-infrared reflectance spectroscopy. Icarus, 204, 114–136
Ramachandran, A., Zorzano, M.-P., & Martín-Torres, J. (2021). Experimental Investigation of the Atmosphere-Regolith Water Cycle on Present-Day Mars. Sensors, 21, 7421.
Rivera-Valentín, E.G., et al. (2020). The role of the regolith in the adsorption and desorption of water on Mars. Nature Astronomy, 4(8), 756–761.
Savijärvi, H., et al. (2016). The diurnal water cycle at Curiosity: Role of exchange with the regolith. Icarus, 265, 63-69
Savijärvi, H., et al. (2019). Water vapor mixing ratios and air temperatures for three martian years from Curiosity. Icarus, 326, 170-175
Savijärvi, H., et al. (2021). Water vapor adsorption on Mars. Icarus 357, 114270
Savijärvi, H., et al. (2024a). Moisture cycles in Jezero Crater, Mars. Icarus, 423, 116283.
Savijärvi, H., et al. (2024b). Wintertime column modeling in Jezero crater, Mars: Period of near-fog and a dust event. Icarus 421, 116-242
Steele, L. J., Balme, M. R., Lewis, S. R., & Spiga, A. (2017). Regolith-atmosphere exchange of water in Mars’ recent past. Icarus, 284, 233–248.
Zent, A.P., et al. (1993). A coupled subsurface-boundary layer model of water on Mars. Journal of Geophysical Research: Planets, 98(E2), 3319–3337.
Zent, A. P., & Quinn, R. C. (1995). Simultaneous adsorption of CO₂ and H₂O under Mars-like conditions and application to the evolution of the Martian climate. Journal of Geophysical Research, 100(E3), 5341–5349.
Zent, A. P., & Quinn, R. C. (1997). Measurement of H₂O adsorption under Mars-like conditions: Effects of adsorbent heterogeneity. Journal of Geophysical Research: Planets, 102(E4), 9085–9095.
Zorzano, M.-P., et al. (2024). Near-surface water vapour exchange at Jezero Crater: Implications for regolith hydration under current Martian conditions. Geophysical Research Letters, 51(3), e2023GL104567
How to cite: Patel, P., Tamppari, L., Kocian, C., and de la Torre Juarez, M.: Experimental Constraints on Near-Surface Water Vapour Exchange on Mars , Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-728, https://doi.org/10.5194/epsc2026-728, 2026.
Martian atmospheric and gravity conditions are unlike those found on Earth, presenting a unique aerodynamic regime for granular material. The CO2-dominatedatmosphere at 610 Pa and ~210 K is thin enough that the gas mean free path approaches the size of fine regolith particles, yet it is dense enough to sustain aerodynamic forces that causeparticle mobility.
Aerodynamic behavior of regolith has a great impact on the overall Martian environment. Global and local disruptive events such as dust storms and dust devils are commonplace, triggering constant regolith mixing, transport and size sorting. Furthermore, dust lifting, dune migration, aeolian transport and surface-atmosphere exchange are governed by the aerodynamics of regolith particles. Existing models often neglect the Cunningham slip effect, which becomes prominent at particle sizes below 100 microns. This work presents a theoretical analysis of Martian transport models using regolith-specific metrics and in-situ grain size data from Curiosity (Preston et al. 2024).
Specifically, Ganser drag with Cunningham slip correction using Rader constants are used (Ganser 1993; Rader 1990). As Curiosity measurements highlight, modelling non-spherical particle drag is essential to fully represent Martian conditions. The Ganser drag coefficient is given by,
Where Re is the Reynolds number, and K1 and K2 are functions of particle sphericity φ. Specifically,
Martian regolith sphericity has been computed to be 0.74 by calculating the geometric average of grains measured in-situ (Preston et al. 2024). For small particles, the Cunningham slip correction follows (Rader 1990),
With Kn, being the Knudsen number that compares the molecular mean free path with a representative physical scale, in this case the particle diameter, as we are correcting the effects of flow on small particles. Used constants, A, B and C are for CO2 taken from literature. The threshold friction velocity is modeled as the velocity limit necessary to be lifted using adapted sand transport equations that take into account particle cohesion (Kok et al. 2012). These equations are used in order to simulate the aerodynamic behavior of regolith particles under Martian conditions and in compressed Martian atmosphere applications.
Figure 1: Cunningham slip correction influence in terminal velocity for Martian regolith and environmental conditions. vgas represents the reference gas speed used in the simulations.
Figure 1 shows how the effect of the Cunningham slip correction is greatest for small particles. Specifically, at 5 microns the correction factor reaches CC≈4, reducing the effective drag by a factor of four relative to the continuum prediction.
Curiosity sampling locations show substantially different grain size distributions. This implies that aerodynamic mobility (i.e., which grains are lifted, transported or deposited) varies significantly across the Gale crater traverse. Simulation results presented in Figure 2 show how the lifted fine fraction of two distributions varies significantly (yields vary up to 1.7 times). The fine distribution is entrained by the wind field while the coarse fraction represents particles that settle under gravity. Influence of particle shape (φ=0.65-0.83 consistent with Curiosity observations) shifts the cut size, d50 , up to 10 microns. Across the full range φ=0.25-1.0, the shift exceeds 90 microns, meaning that particle mobility is sensitive to particle heterogeneity within a sample. Hence, grain morphology has a direct influence on surface-atmosphere exchange.
Figure 2: Particle size distribution of two Martian in-situ samples (Preston et al. 2024) and the simulated resulting coarse and fine distributions being lifted by vgas. Yf and Yc and represent the yield of the fine and coarse distribution.
These aerodynamic characterizations places the Martian atmospheric gases as active agents, collecting, transporting and classifying regolith by particle size. Therefore, this project investigated the possibility of actively utilizing the gas for pneumatic surface collection, pipeline transport and aerodynamic size classification. For each process, the theoretical models were adapted to assess their feasibility and identify the main physical constraints. Collection via suction nozzles is found to be feasible for typical subsystem Martian power budgets; yet, smaller particles require greater pickup velocities due to cohesion forces. Horizontal transport is shown to require gas velocities that remain in a turbulent regime due to the low atmospheric density, making pipe diameter the primary design variable. Aerodynamic size classification via vertical elutriation emerges as the most promising concept, offering a tunable cut size with low power budget. A trade-off analysis across mechanical and pneumatic regolith handling alternatives confirmed that pneumatic classification is the most competitive approach for producing size-sorted regolith feedstock under Martian surface conditions.
Together, these results suggest that a complete description of Martian regolith aerodynamics requires joint characterization of grain size, grain shape, and local atmospheric state. Crucially, the same atmosphere that causes regolith transport can be exploited as the working medium for pneumatic size classification at blower powers below 0.1 W, demonstrating that the Martian environment can enable resource for producing size-sorted regolith feedstock in support of in-situ resource utilization processes.
Bibliography
Andreotti, Bruno; Claudin, Philippe; Iversen, Jens Jacob; Merrison, Jonathan P.; Rasmussen, Keld R. (2021): A lower-than-expected saltation threshold at Martian pressure and below. In Proceedings of the National Academy of Sciences of the United States of America 118 (5). DOI: 10.1073/pnas.2012386118.
Ganser, Gary H. (1993): A rational approach to drag prediction of spherical and nonspherical particles. In Powder Technology 77 (2), pp. 143–152. DOI: 10.1016/0032-5910(93)80051-B.
Kok, Jasper F.; Parteli, Eric J. R.; Michaels, Timothy I.; Karam, Diana Bou (2012): The physics of wind-blown sand and dust. In Reports on progress in physics. Physical Society (Great Britain) 75 (10), p. 106901. DOI: 10.1088/0034-4885/75/10/106901.
Preston, Sarah L.; Siebach, Kirsten L.; Lapôtre, Mathieu G. A.; Banham, Steven G. (2024): Grain Size Measurements of the Eolian Stimson Formation, Gale Crater, Mars and Implications for Sand Provenance and Paleoatmospheric Conditions. In JGR Planets 129 (11), Article e2024JE008369. DOI: 10.1029/2024JE008369.
Rader, Daniel J. (1990): Momentum slip correction factor for small particles in nine common gases. In Journal of Aerosol Science 21 (2), pp. 161–168. DOI: 10.1016/0021-8502(90)90001-E.
Swann, C.; Sherman, D. J.; Ewing, R. C. (2020): Experimentally Derived Thresholds for Windblown Sand on Mars. In Geophysical Research Letters 47 (3), Article e2019GL084484. DOI: 10.1029/2019GL084484.
How to cite: Rejón López, M. and Zabel, P.: Theoretical analysis of pneumatic mobility and size classification of regolith particles under Martian environmental conditions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-814, https://doi.org/10.5194/epsc2026-814, 2026.
Regolith-covered surfaces exist in several planetological environments, including the Moon, Mars, NEAs, etc, some of which have been identified as first targets for future colonization outposts. Clearly, the development of advanced technologies in the framework of ISRU concept is a key asset to boost human space explorations on these celestial bodies. In particular, the use of regolith resources available in-situ will produce, through a significant decrease of transportation expenses, an intensification of manufacturing processes to fabricate constructions and protections, as well other useful installations and apparatuses possibly required, like for instance solar thermal energy harvesting systems and thermal energy storage media, which are the applications considered in the present study.
Once a proper site choice is carried out, the lunar surface shows advantageous conditions for solar energy harvesting due to the high solar irradiance. However, due to the length of lunar days/nights, energy storage is a major issue to be faced. Thermal energy storage (TES) appears the most suitable energy storage approach for future extra-terrestrial human colonies or robotic stations. In a possible energy generation scheme, solar collectors on the Moon harvest sunlight and concentrate it into solar absorbers constituted by sintered regolith blocks exchanging heat with some heat transfer medium. Sensible-heat TES units could be made by processed regolith as well, and buried under the surface to benefit from thermally insulating characteristics of the surrounding powdered regolith. The use of the same starting material to be tailored, by means of optimized processing approaches, for various applications, is a formidable challenge to which the present work aims to bring its contribution.
Therefore, taking as case-study the JSC-1A lunar regolith simulant, we processed the powders into sintered ceramic bulks by the Spark Plasma Sintering (SPS) technique, taking the sintering temperature as variable process parameter to produce samples with diverse relative densities and surface porosities, and we characterized the obtained products in terms of composition, icrostructure and optical properties.
Powders were first sintered at 700-900°C, in vacuum and under mechanical pressure. Obtained pellets showed relative densities of about 86 and 98%, respectively. No additional phases were detected after SPS compared to original powder, even if some changes were observed in their relative amounts, in particular for the glassy part. Optical properties (spectral absorptance α(λ) or emittance ε(λ), integrated solar absorptance α and estimated integrated thermal emittance ε) were evaluated from experimental spectra both for ceramics and powders, in the temperature range from 100K, representative of the night temperature value on the lunar equatorial surface, to possible temperatures for the technological applications of interest in the present work (1300 K max, slightly below the melting temperature of the regolith simulant).
Powdered regolith simulant shows consistently lower solar absorptance α and thermal emittance ε at all temperatures, in comparison to sintered specimens (Fig. 1).
The ideal material for a thermal solar absorber should have a high solar absorptance and the lowest possible thermal emittance at operating temperatures. With a properly dimensioned sunlight collecting system, the working temperature of the absorber is quickly reached and steadily maintained during the illumination period, e.g., the lunar day. In this sense, the system is similar to solar receivers on Earth, with the advantage of the absence of thermal losses due to conductive and convective heat exchanges with the surrounding atmosphere.
Focusing on applications, the higher solar absorptance of sintered pellets makes them more suitable for solar receivers in cavity-like architectures. For operating temperatures <1000K, the sample sintered at 900°C appears the most promising, due to the highest solar absorptance and lowest thermal emittance. At 1000K both pellets show the same α/ε ratio, while, beyond such temperature, the sample sintered at 700°C shows lower emittance. As the latter specimen is also characterized by a lower solar absorptance, system-specific evaluations should be made case by case, according to the system architecture, to identify the most suitable material depending on the working temperature.
For buried sensible-heat TES applications, the material should operate between its maximum allowable temperature (melting temperature in the range 1100-1125°C) and ~250 K, i.e. the almost constant temperature of lunar regolith just below the surface [1], with thermal cycles corresponding to the day-night cycles. The most relevant optical parameter here is the thermal emittance only. Showing the lowest emittance for temperatures > 500K, the ceramic sintered at 700°C results therefore the best option with respect to the other processed pellet. Powdered regolith would actually show an even lower emittance, but its poor thermal conductivity would make difficult its actual usage as heat storage medium.
In conclusion, similarly to what reported in the literature for thermophysical properties, these results show that powder sintering changed the optical properties of the regolith simulant in a process-dependent way. Specifically, the process parameter investigated in the present study, the sintering temperature, is generally found to increase spectral absorptance/emittance with respect to pristine powders. Integrated solar absorptance and thermal emittance resulted therefore enhanced in all conditions [2].
These results open interesting perspectives for ISRU applications, allowing to exploit the different properties of both pristine powders and their sintered bricks, as well as of the proper interplay among them.
References
[1] M. F. Palos, et al (2020) https://doi.org/10.1016/j.actaastro.2020.02.005
[2] Licheri, et al (2022) https://doi.org/10.1016/j.actaastro.2022.09.016
Figure 1: Calculated integrated thermal emittance (ε) of investigated materials. Integrated
solar absorptance values are α=0.88 for the pellet sintered at 700°C, α=0.92 for the pellet
sintered at 900°C and α=0.77 for the powder.
How to cite: Sani, E., Dell'Oro, A., Licheri, R., Orrù, R., and Cao, G.: Spark Plasma Sintering and Optical Characterization of Lunar Regolith Simulant, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-401, https://doi.org/10.5194/epsc2026-401, 2026.
Introduction: On 13th April 2029, S-type asteroid 99942 Apophis will approach Earth to a close enough distance to cross the Earth outer electron belt for a short period of time. The ESA RAMSES mission (Rapid Apophis Mission for Space Safety) [1] will perform a one of a kind rendez-vous with the irregular object of typically 300 to 400m in diameter. RAMSES mission scenario includes the deployment of two CubeSats that will aim to characterise the asteroid surface, internal structure and close environment, embarking a dust analyser, radar and plasma instrumentation.
In this context, a unique opportunity arise to confront our current understanding of airless bodies regolith and dusty surfaces interactions with space plasmas. Effectively, the silicate based regolith surface will transition from a (comparatively) high velocity low temperature Solar Wind plasma (typically 400km/s bulk velocity, 1-10 particles / cm-3 and 10eV at 1AU) to a sequence of varying magnetospheric conditions when crossing the magnetosheath to reach tenuous hot geostationary (GEO) electrons and ions composed typically of several components including accelerated ions and electrons with several keVs to several 10s or keV in energy.
While the GEO environment is typically considered as a threat to spacecraft, as responsible for strong negative charging of spacecraft surfaces to several 10skeV negative with respect to the neutral plasma, it is not the case for a natural object such as Apophis. However, a number of phenomena are worth investigating :
- What charging level / surface potential(s) will Apophis develop at closest approach ?
- What consequences such potentials might have on Apophis regolith behaviour during observations, e.g. regarding potential exposed layer mobilization ?
- What near object (dusty) plasma environment might result from the interaction of Apophis surface with the local plasma ?
- What is the impact of the local plasma on the near surface dust population to be probed by RAMSES ?
- How will Apophis local plasma environment will be affected by the magnetospheric crossing (e.g. wake formation and related effects) ?
- What is the impact of the local plasma on the near surface dust population to be probed by RAMSES ?
- What level of differential charging between RAMSES and Apophis surface can be expected ?
- What impact could the differential charging between RAMSES and Apophis surface at closest approach have on target measurements such as plasma populations and the local asteroid dust population and the related scientific return ?
To start covering those questions, numerical simulations of Apophis surface charging and near surface plasma properties and dust transport in typical and extreme geostationary conditions will be presented and analysed.
References:
[1] https://www.esa.int/Space_Safety/Planetary_Defence/Introducing_Ramses_ESA_s_mission_to_asteroid_Apophis;
How to cite: Cipriani, F., Gutierrez, F., Déprez, G., and Johansson, F.: Apophis close encounter : surface charging predictions in geostationary environment, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1185, https://doi.org/10.5194/epsc2026-1185, 2026.
Understanding the behaviour of planetary regolith under low-gravity conditions is essential for the design of future exploration systems and surface operations. Over the past two decades, the European Space Agency (ESA) has conducted various microgravity experiments on granular matter, combining vibration-driven dynamics and compaction studies across sounding rockets, parabolic flights, and long-duration investigations onboard the International Space Station (ISS).
Sounding rocket and parabolic experiments have enabled the study of dilute granular gases under near-weightless conditions, revealing strongly non-equilibrium behaviours such as the breakdown of energy equipartition and modifications of collisional dynamics in anisotropic particle systems. Complementary parabolic flight campaigns have provided insights into dense granular regimes relevant to regolith, demonstrating three-dimensional compaction, non-linear collision scaling, and vibration-driven convection phenomena, while also quantifying the influence of residual accelerations.
More recently, ISS experiments have enabled systematic investigations of granular systems subjected to controlled vibration forcing over extended durations. These studies have highlighted key mechanisms such as segregation, convection-like transport driven by energy gradients, and the transition between dilute and dense regimes. Such observations provide direct analogues to processes expected in planetary regolith exposed to mechanical perturbations, including rover interactions, drilling, or seismic shaking.
These multi-platform results will be presented, bridging the gap between fundamental granular physics and the behaviour of regolith in planetary environments, supporting the development of predictive models and technologies for surface mobility, sampling, and in-situ resource utilisation on the Moon, Mars, and small bodies.
How to cite: Sebastien, V.-B.: Granular Matter in Space: Microgravity Experiments and Implications for Regolith Behaviour, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1207, https://doi.org/10.5194/epsc2026-1207, 2026.
Introduction
On 13 April 2029, asteroid (99942) Apophis will fly by Earth with a distance of approximately 38,000 km. Several authors suggest that the gravity field of Earth may lead to surface reshaping, e.g., by avalanches [1,2,3]. These events will be observed through planned space missions, providing insights into the granular dynamics of surfaces of small rubble-piles asteroids. A key parameter of the physics of granular material is the so-called static angle of repose, which describes the angle of a slope that must be exceeded to start an avalanche [4]. Understanding this parameter and its variation with grain size and gravity, especially the extrapolation to low gravity conditions as present on Apophis, is an important field of research.
To provide ground truth on this parameter under Earth and reduced gravity conditions, the static angle of repose was experimentally investigated in a rotating tumbler setup using highly monodisperse glass beads. Several grain sizes ranging from 37 µm up to 193 µm were investigated to specifically examine the influence of cohesion on the angle of repose. Vacuum experiments were conducted in the laboratory and under reduced gravity conditions in the GraviTower Bremen (ZARM). Future campaigns will expand the dataset to include hyper-gravity, as well as additional reduced gravity data.
Laboratory Experiments
To investigate the influence of particle size on the angle of repose, highly monodisperse, spherical soda-lime-glass beads were selected as sample material. With particle grain sizes noted above, vacuum experiments were conducted under Earth’s gravity, particularly focusing on the increasing dominance of cohesion (Bond number, [5,6]) below roughly 100 µm. Cohesive behaviour has been observed through a significant increase in the angle of repose with decreasing grain size, as shown in Figure 1. Furthermore, the current results largely follow the model of the grain-size-dependent angle of repose developed by Elekes & Parteli (2021) [6].
Figure 1: Static angle of repose over sample material grain size. Highly monodisperse spherical soda-lime-glass beads have been used as sample material. Error bars denote standard deviation from experiment repetition.
GraviTower Experiments
To thoroughly investigate the effect of gravity on the angle of repose as a function of particle size, a first series of experiments was conducted in February 2026 at the GraviTower Bremen Pro at ZARM in Bremen. Thanks to the GraviTower’s very high repetition rate and our simultaneous execution of two experiments per flight, a large dataset was obtained, counting 354 flights (=708 data points) in total. Experiments were conducted under various reduced gravity conditions (Mars-g, 3.0 m/s², 2.5 m/s², Moon-g) using the sample materials from the laboratory experiments described above.
In some experiments, the time of an avalanche start - and thus the measured static angle of repose - was significantly influenced by vibration induced by the drop tower capsule, as well as by mechanically induced vibrations within the experimental setup. Temporarily negative gravitational values additionally led to a brief “lift off” of the sample material in some cases. This effect was reduced through mechanical adjustments made on-site and the high experiment repetition rate also enables to apply a quality filter on the data. Analysed experiment data show a clear and significant increase in the static angle of repose for all grain sizes examined under reduced gravity compared to Earth’s gravity. Contrary to expectations, a relative decrease is observed from Martian to smaller gravity. This behaviour has not yet been fully understood, and may have technical origin (disturbances as described above) or founded in granular physics: A reduction of the volume filling factor for reduced gravity conditions, as suggested by Elekes & Parteli (2021) [6], may reduce contact networks and thus increase system instability, potentially making the granular heap more susceptible to disturbances. However, it is important to note that the downward trend of the static angle of repose for accelerations smaller than Martian gravity is comparably low to the generally observed trend of a strong increase for all grain sizes when compared to Earth’s gravity.
Perspective
Future campaigns at ZARM planned for this year will build on the currently obtained dataset with the goal to minimise disturbances and expand it to include hyper-gravity, as well as further broadening the data for reduced gravity conditions. We aim to connect and interpret our experimental results in the context of granular physics to enhance knowledge about the effect of cohesion (Bond number, [5,6]) and its influencing parameters on the angle of repose. Together with live observations during the flyby of Apophis in 2029, the understanding of the behaviour of granular materials, specifically small rubble-piles asteroids, under various gravity conditions will be substantially increased.
Acknowledgements
We acknowledge financial support by the DLR Agency (50WM2544) and the support of the ZARM team during the experiment campaigns.
References
[1] G. Noiset et al., 2022. EPSC 2022-1159. https://doi.org/10.5194/epsc2022-1159
[2] Ballouz et al., 2024. The Planetary Science Journal, 5 (11), 251. https://doi.org/10.3847/PSJ/ad84f2
[3] Kim et al., 2023. MNRAS, 520 (3), 3405-3415. https://doi.org/10.1093/mnras/stad351
[4] M.G. Kleinhans et al., 2011. J. Geophys. Res. 116, E11. https://doi.org/10.1029/2011JE003865
[5] Scheeres et al., 2010. Icarus, 210:968. https://doi.org/10.1016/j.icarus.2010.07.009
[6] Elekes & Parteli, 2021. PNAS, 118 (38), e2107965118. https://doi.org/10.1073/pnas.2107965118
How to cite: Kost, P.-M., Güttler, C., Bannemann, L., and Gundlach, B.: Avalanches on asteroid (99942) Apophis: static angle of repose of granular material under reduced gravity, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-146, https://doi.org/10.5194/epsc2026-146, 2026.
Frictional granular collapse is a fundamental process underlying surface evolution on Solar System bodies, including crater-wall collapse and slope failure. Laboratory experiments on granular column collapse indicate that the column aspect ratio is the dominant control parameter governing collapse behaviour. The normalized run-out length and normalized final deposit height vary approximately linearly with aspect ratio (a) for a ≤ 3, whereas for a ≥ 3 both quantities follow power-law relationships, with the scaling exponent influenced by geometry and the proportionality constant governed by the material’s internal friction angle. However, whether these scaling relations remain valid across the wide range of gravity regimes encountered in the Solar System remains unclear. Recent reduced-gravity studies [1] suggest that gravity significantly influences collapse dynamics while exerting comparatively weak control on normalized deposit geometry.
Here we investigate granular column collapse under gravity levels representative of planetary, lunar, and asteroid environments. We quantify the dependence of run-out distance, collapse duration, deposit morphology, and flow mobility on gravitational acceleration and derive corresponding scaling relations.
To this end, we combine Coupled Eulerian–Lagrangian (CEL) numerical simulations with shallow-flow theoretical modelling. The CEL formulation employs a Mohr–Coulomb plasticity model with negligible cohesion, while the shallow-flow framework provides a vertically averaged continuum description with basal friction. The resulting gravity-dependent scaling laws provide a framework linking laboratory-scale granular mechanics to planetary-scale mass-wasting processes and may aid interpretation of landslide mobility and regolith transport across Solar System bodies.
References
[1] Yucheng Li, Raul Fuentes (2025) Granular column collapse: Analysing the effects of gravity levels. Computers and Geotechnics, Vol. 183, 107207.
How to cite: Dubey, R., Gaurav, K., and Sharma, I.: Granular column collapse on extra-terrestrial bodies, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-197, https://doi.org/10.5194/epsc2026-197, 2026.
Seismology is a frequently used technique to understand the interior of planetary bodies. It has been widely used on Earth, as well as other celestial bodies such as the Moon (with Apollo missions [1]), on Mars (with the InSight mission [2]), and will be used in upcoming missions such as RAMSES to the asteroid Apophis [3] and even Dragonfly to Titan [4]. Planetary bodies are covered by a layer of regolith, a fine grain layer with a non-linear elastic behaviour that deforms seismic signals. Since planetary seismometers are placed on the surface or in the near sub-surface, they are directly coupled with this regolith layer. It is thus crucial to understand the behaviour of seismic waves in such material to fully interpret seismometer data. The propagation of seismic waves is influenced by the regolith properties, but also by the different levels of gravity. However, while the velocity of seismic waves is known to be pressure dependent the dependency factor often deviates from elastic theory predictions [e.g. 5,6]. Here we perform experiments to study the pressure dependence of the seismic velocities of planetary simulants.
In this context, we use a modified bender element experiment (Figure 1). A sample is placed inside a latex membrane of 10 cm in diameter and 20 cm height, with one bender element at each side. A vacuum pump is connected to the bender elements to apply a partial vacuum inside the sample. A pressure controller and a barometer are also used to have a precise estimation of the pressure inside our sample, and prevent leaks. With respect to classical bender element experiments, the sample is placed horizontally to minimize the effect of gravity and reach lower confining pressure levels (currently 10 to 85 kPa) [7]. Seismic waves (4 kHz sine pulse) are then transmitted through the sample. Experimental results for several different lunar, martian and asteroid regolith simulants will be presented during the conference and their pressure sensitivity factors will be compared.
Figure 1: Horizontal bender element experiment set-up.
This work is funded by the European Research Council (ERC) GRAVITE project (Grant Agreement N° 1087060).
References
[1] Bates, James R. ALSEP termination report. Vol. 1036. National Aeronautics and Space Administration, Scientific and Technical Information Office, 1979.
[2] Lognonné, Philippe, et al. "SEIS: Insight’s seismic experiment for internal structure of Mars." Space Science Reviews1 (2019): 12.
[3] Murdoch, Naomi, et al. Seismic Instrument for Asteroids (SIA): the RAMSES seismometer. No. EPSC-DPS2025-417. Copernicus Meetings, 2025.
[4] Lorenz, Ralph D., et al. "Dragonfly: A rotorcraft lander concept for scientific exploration at Titan." Johns Hopkins APL Technical Digest3 (2018): 14.
[5] Johnson, David Linton, et al. "Nonlinear elasticity of granular media." Physica B: Condensed Matter1-3 (2000): 134-138.
[6] Makse, H. A., et al. "The apparent failure of effective medium theory in granular materials." Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy1-2 (2001): 107-111.
[7] Betancourt, JP Castillo, et al. "Wave velocities and Poisson ratio in a loose sandy Martian regolith simulant under low stresses: 1. Laboratory investigation." Journal of Geophysical Research: Planets11 (2023): e2023JE007988.
How to cite: Amsili, A., Peignon, A., Margerin, L., and Murdoch, N.: Seismic velocity characterisation of planetary simulants, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-193, https://doi.org/10.5194/epsc2026-193, 2026.
The regolith of rocky bodies, such as planets or asteroids, generally settles under gravity conditions different from those of the earth. The behavior of granular material is not easily scalable for different gravities. To predict these highly complex systems where cohesive inter particle forces can be comparable to gravitational forces, we need simulations and experiments. We did experiments on settling of three different granular samples in varying reduced gravities and examined their packing densities. For this we used a high precision linear motor as a stage to artificially induce milligravity inside the zero g environment provided by the ZARM drop tower and observe the settling of opur samples. The three samples were fine basalt with particle diameters of 1–200μm, coarse basalt with 1–3mm diameters and glass beads with 825–1000 μm. The artificial gravities were 150, 250, 500, 750 and 1000 mm/s2 and therefore ranged from large asteroid gravity to almost moon gravity. We unsurprisingly saw the granular samples with higher volume in lower gravities and therefore lower packing densities, however we also saw the fine basalt be the most sensitive to changes in gravity, up to +19.6% in volume for 250 mm/s2 , followed by the coarse basalt particles, up to 12.2% in volume for 150 mm/s2 and the glass beads packing density being the least sensitive to changes in gravity, up to 4.25% in volume for 250mm/s2. With these experiments we show change in volume is not solely dependent of particle size but also roughness and uniformity, we provide real life experimental data to validate theoretical works and highlight the role of cohesive forces in low gravity environments.
How to cite: Keulen, M., Giese, T., Joeris, K., and Kollmer, J. E.: Experiments on Settling of Granular and Cohesive Material in Low Gravity, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-802, https://doi.org/10.5194/epsc2026-802, 2026.
Granular flow models tend to fail in low gravity. When gravity decreases below a critical acceleration, cohesive forces become predominant, resulting in a shift in macroscopic behavior. Yet, these models are still used to prepare planetary exploration missions, for lack of better ones. For granular processing in space, essential for sustaining human presence on the Moon, this can have disastrous consequences.
We investigate the influence of partial gravity on granular flows, focusing on hopper discharge (granular discharge flow through an orifice). Using an hourglass with different aperture sizes and multiple materials, including regolith simulants, we perform experiments across a wide range of effective gravitational accelerations using drop tower and parabolic flight experiments. A centrifuge installed inside the aircraft allows us to generate any level of partial gravity in-flight.
We find that lunar gravity consistently increases the clogging probability and slows down the flow rate, which deviates from the scaling predicted by the Beverloo equation. We also find that tribocharging occurs, and its effect on flow can be isolated in reduced gravity. We propose a correction to the Beverloo equation and a clogging state diagram based on the granular Bond number: the cohesion-to-gravity ratio. Scaling the data with this parameter collapses scattered observations into a universal framework for predicting granular flow and clogging in low gravity.
How to cite: D'Angelo, O., Gaida, O., and Kollmer, J. E.: Granular charge and discharge in low gravity , Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-552, https://doi.org/10.5194/epsc2026-552, 2026.
