The role of fragment shapes in the simulations of asteroids as gravitational aggregates

Recent remote measurements and in-situ observations confirm the idea that asteroids up to few hundreds of meters in size might be aggregates of loosely consolidated material, or ‘rubble piles’. The dynamics of these objects can be studied using N-body simulations of gravitational aggregation. We investigate the role of particle shape in N-body simulations of gravitational aggregation. We explore contact interaction mechanisms and study the effects of parameters such as surface friction, particle size distribution and number of particles in the aggregate. As a case study, we discuss the case of rubble pile reshaping under its own self-gravity, with no spin and no external force imposed. We implement the N-body gravitational aggregation problem with contact and collisions between particles of irregular, non-spherical shape. Contact interactions are modeled using a soft-contact method, considering the visco-elastic behavior of particles' surface. We perform numerical simulations to compare the behavior of spherical bodies with that of irregular randomly-generated angular bodies. The simulations are performed starting from an initial aggregate in a non-equilibrium state. The dynamics are propagated forward allowing particles to settle through reshaping until they reach an equilibrium state. Preliminary tests are studied to investigate the quantitative and qualitative behavior of the granular media. The shape of particles is found to play a relevant role in the settling process of the rubble pile aggregate, affecting both transient dynamics and global properties of the aggregate at equilibrium. In the long term, particle shape dominates over simulation parameters such as surface friction, particle size distribution and number of particles in the aggregate. Spherical particles are not suitable to model accurately the physics of contact interactions between particles of N-body aggregation simulations. Irregular particles are required for a more realistic and accurate representation of the contact interaction mechanisms.