Asteroid regolith strength: Role of fine-fractions

Abstract

Most smaller asteroids (<1 km diameter) are granular material loosely bound together primarily by self-gravity known as rubble piles. In an effort to better understand the evolution of rubble-pile asteroids, we performed bulk measurements using granular simulant to study the effects of the presence of fine grains on the strength of coarse grains. Our laboratory samples consisted of fine–coarse mixtures of varying percentages of fine grains by volume of the sample. We measured the material’s angle of repose, Young’s Modulus, angle of internal friction, cohesion, and tensile strength by subjecting the samples to compressive and shear stresses. The coarse grains comprising the fine–coarse mixtures ranged from 1 mm to 20 mm (2 cm) and the fines were sieved to sub-millimeter sizes (<1 mm). The measured angles of repose varied between 32°–45° which increased with increasing fine percentage. In compression, samples generally increased in strength with increasing fine percentage for both confined and unconfined environments. In all cases, the peak strengths were not for purely fine grains but for a mixture of fine and coarse grains. Shear stress measurements yielded angles of internal friction ranging between 25° and 45° with a trend opposite that of the angle of repose, 300–550 Pa for bulk cohesion, and 0.5–1.1 kPa for tensile strength. Using other published works that include data from telescopic and in-situ observations as well as numerical simulations, we discussed the implications of our findings regarding rubble-pile formation, composition, evolution, and disruption. We find that the presence of fine grains in subsurface layers of regolith on an asteroid (confined environment) aids the avoidance of disruption due to impact. However these same fines increase an asteroid’s chance to disrupt or deform from high rotation speeds due to reduced grain interlocking. In surface layers (unconfined environments), we find that the presence of fine grains between coarse ones generates stronger cohesion and aids in the prevention of mass loss and surface shedding.