Regolith on asteroids - settling of granular and cohesive material

The surface of asteroids is typically covered by particles of varying size, ranging from large boulders (meters) to fine grained regolith. The mechanical properties of such surfaces, such as porosity, density, mechanical strength, or thermal conductivity are crucial for the dynamic evolution of the asteroids. The porosity of the regolith is probably the most important parameter, as the porosity of a granular bed also determines it's thermal and mechanical properties. In the low-gravity environment of an asteroid, cohesive forces between the particles are important or even dominate over the weight of the grains. To understand the physical properties of asteroid regolith, laboratory experiments with regolith simulants are a common approach. However, doing analog experiments under laboratory conditions does not necessarily lead to correct results, as the low gravity environment of asteroids must be considered.

 

Here, we present sedimentation experiments under reduced gravity to investigate how gravity affects the porosity of a regolith bed. On an asteroid, impacts or other disturbances will stir up regolith material, which then settles again. We simulate this process in a dedicated experiment setup at the drop tower Bremen. The whole setup is placed in a microgravity environment (residual acceleration < 10-5 m/s²). A linear stage is used to establish a controlled acceleration to the test cell, so the low gravity environment of an asteroid's surface is simulated. The level of artificial gravity ranged from 150 mm/s2 to 1000 mm/s2. As analog material two different basalt samples were used, with different grain size distributions. In addition, experiments were performed with glass spheres as sample, for a better comparison to theoretical studies.

 

The experiments show that the volume filling of a sedimenting regolith bed strongly depends on the local level of gravity. For fine basalt sand with particle sizes in the range of 100 µm, we see that the porosity of the resulting regolith bed increases by a factor of around 3, when the gravity level during the sedimentation is reduced from 1000 mm/s² to 250mm/s². This is of great importance, as the porosity of an asteroid's regolith surface strongly influences the thermal properties, such as the thermal conductivity of the surface or the thermal inertia. In addition, also the mechanical strength of a regolith scales with its porosity, which is important for future space missions.