Numerical Simulations of Granular Physics in the Solar System
Richardson, Derek C
MetadataShow full item record
Granular physics is a sub-discipline of physics that attempts to combine principles that have been developed for both solid-state physics and engineering (such as soil mechanics) with fluid dynamics in order to formulate a coherent theory for the description of granular materials, which are found in both terrestrial (e.g., earthquakes, landslides, and pharmaceuticals) and extra-terrestrial settings (e.g., asteroids surfaces, asteroid interiors, and planetary ring systems). In the case of our solar system, the growth of this sub-discipline has been key in helping to interpret the formation, structure, and evolution of both asteroids and planetary rings. It is difficult to develop a deterministic theory for granular materials due to the fact that granular systems are composed of a large number of elements that interact through a non-linear combination of various forces (mechanical, gravitational, and electrostatic, for example) leading to a high degree of stochasticity. Hence, we study these environments using an N-body code, pkdgrav, that is able to simulate the gravitational, collisional, and cohesive interactions of grains. Using pkdgrav, I have studied the size segregation on asteroid surfaces due to seismic shaking (the Brazil-nut effect), the interaction of the OSIRIS-REx asteroid sample-return mission sampling head, TAGSAM, with the surface of the asteroid Bennu, the collisional disruptions of rubble-pile asteroids, and the formation of structure in Saturn's rings. In all of these scenarios, I have found that the evolution of a granular system depends sensitively on the intrinsic properties of the individual grains (size, shape, sand surface roughness). For example, through our simulations, we have been able to determine relationships between regolith properties and the amount of surface penetration a spacecraft achieves upon landing. Furthermore, we have demonstrated that this relationship also depends on the strength of the local gravity. By comparing our numerical results to laboratory experiments and observations by spacecraft we can begin to understand which microscopic properties (i.e., grain properties) control the macroscopic properties of the system. For example, we can compare the mechanical response of a spacecraft to landing or Cassini observations of Saturn's ring to understand how the penetration depth of a spacecraft or the complex optical depth structure of a ring system depends on the size and surface properties of the grains in those systems.