Sinking or bouncing in low gravity environments?

Understanding the physics of granular materials in a low gravity and vacuum environment is essential to predict the regolith behavior on the Moon and asteroids. Cohesive forces - interparticle attractive forces, are an important parameter to consider in the regolith dynamics. A given cohesive force may be considered negligeable in Earth’s gravity but become dominant when the gravitational acceleration is reduced (e.g. [1]). The surface properties, such as cohesion, influence the response of planetary surfaces (e.g., will an object sink or bounce?) and the performance of space missions (e.g., will a lander sink or bounce, how will a sampling arm perform?). The sinkage of an object, such as a CubeSat after landing or a boulder resting on fine regolith, enables estimates to be made of the bearing capacity of the surface – maximum load the surface can withstand before rupturing.

To quantify the importance of cohesive forces on the sinkage depending on the gravitational acceleration, sinkage experiments into granular materials were performed at different levels of cohesion and gravitational acceleration. The experiment campaign named SILOE (Surface Investigation in Low gravity Environment) was conducted in December 2024 in the GraviTower Bremen Pro at the ZARM drop tower facility (Bremen, Germany). The sinkage experiments were performed in terrestrial gravity (9.8 m/s²) and in two reduced gravitational accelerations: Mars gravity (3.8 ± 0.2 m/s²), and Moon gravity (1.8 ± 0.2 m/s²). These two levels of low gravity are achieved using the GraviTower in partial gravity.

The SILOE sinkage experiments (fig. 1) consist in the release of a spherical projectile (D = 25 mm, stainless steel) into a tray filled with a granular material. The projectile is released from a constant drop height of 6 mm above the surface. The associated collision velocities range from 0.1 to 0.5 m/s, depending on the gravitational acceleration. The experiments are conducted under low to medium vacuum conditions (50 – 400 Pa), to remove the interstitial air pockets and water molecules inside the material.

Figure 1: Design of the SILOE experiment.

The granular materials (fig. 2) studied are glass beads (120 µm) and quartz sand (500 µm). To increase the cohesive forces of the glass beads, the material was covered by a polymer coating enabling the addition of liquid capillary forces [2]. The cohesive force is controlled by the coating thickness applied to the beads. Glass beads with three levels of cohesion were produced following the preparation technique presented in Gans et al. (2020) [2]: at 50, 200, and 400 Pa (cohesive shear stress).

Figure 2: Granular materials used in the SILOE experiments.

 

The sinkage, defined as the final penetration depth of the projectile, is measured by tracking the vertical displacement of the projectile using a camera (sampling rate: 240 Hz) and a laser profilometer (sampling rate: 340 Hz), placed on the side and top of the chamber, respectively.

Cohesion increases the surface strength, which leads to smaller sinkage (fig. 3). In low gravity, the importance of cohesive forces increases, and smaller sinkage are observed for an identical material when the gravity is reduced. In the case of the very cohesive glass beads, rebounds can be observed (fig. 4).

Figure 3: Images captured with the camera after the projectile is dropped and finished sinking into the glass beads of different levels of cohesion (from cohesionless to very cohesive).

 

Figure 4: Projectile vertical displacement with respect to the surface (surface height at 0 mm) in vacuum and Earth gravity condition into the different cohesive glass beads.

 

The results of the SILOE drop tower campaign, and additional sinkage experiments performed at varied ambient pressure, will be presented during the conference. In addition to bringing a better understanding of cohesive forces with respect to gravity, the SILOE results will help to improve interpretations of the data from the upcoming ESA Hera mission (in particular with the CubeSat landings on asteroids Didymos and Dimorphos [3], and boulder tracks [4]), and the JAXA MMX mission (in particular the IDEFIX rover [5] on Phobos).

 

References

[1] Scheeres, et al. (2010). Scaling forces to asteroid surface: the role of cohesion. Icarus, 210, 2.

[2] Gans, Pouliquen, and Nicolas (2020). Cohesion-controlled granular material. Physical review E, 101:032904.

[3] Michel, et al. (2022). The ESA Hera mission: Detailed Characterization of the DART Impact Outcome and the Binary Asteroid (65803) Didymos. The Planetary Science Journal, 3:160.

[4] Bigot, Lombardo, et al. (2024). The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks. Nature Communications, 15:6204.

[5] Michel, et al. (2022), The MMX rover: performing in situ surface investigations on Phobos. Earth, Planets and Space, 74:2.

 

Acknowledgements

This project is carried out with the support of the Education Office of the European Space Agency (ESA) under the educational Experiments Programme. This project received funding through the grant EUR TESS N°ANR-18-EURE-0018 in the framework of the Programme des Investissements d’Avenir, and the Bourse Espace 2024 scholarship by the Fondation Ailes de France. A.D. acknowledges PhD funding from University of Toulouse III, and C.R. acknowledges PhD funding from CNES and ISAE SUPAERO. This project benefitted from funding from CNES in the context of the Hera mission and the MMX rover/wheelcams, the French ANR (ANR-23-ERCC-0003-01), and the European Research Council (ERC) GRAVITE project (Grant Agreement N°1087060).