Tidal distortion and disruption of rubble-pile bodies revisited -- Soft-sphere discrete element analyses

During close approaches to planets or stars, the morphological and dynamical properties of rubble-pile small bodies can be modified, and some may catastrophically break up. This phenomenon is of particular interest for understanding the evolution and population of small bodies, and for making predictions for future encounters. Previous numerical explorations typically used methods that do not adequately represent the nature of rubble piles. The encounter outcomes and influence factors are still poorly constrained. Based on recent advances in modeling rubble piles, we aim to provide a better understanding of the tidal encounter processes through soft-sphere discrete element modeling (SSDEM) and to establish a database of encounter outcomes and the dependencies on encounter conditions and rubble-pile properties. We performed thousands of numerical simulations using the SSDEM implemented in the N-body code pkdgrav to study the dynamical evolution of rubble piles during close encounters with the Earth. The effects of encounter conditions, material strength, arrangement, and resolution of constituent particles are explored. Three typical tidal encounter outcomes are classified, i.e., deformation, mass shedding, and disruption, ranging from mild modifications to severe damages of the progenitor. The encounter speed and distance required for causing disruption events are much smaller than those predicted by previous studies, indicating a smaller creation rate of tidally disrupted small body populations. Extremely elongated fragments with axis ratios ~1:6 can be formed by moderate encounters. Our analyses of the spin-shape evolution of the largest remnants reveal reshaping mechanisms of rubble piles in response to tidal forces, consistent with stable rubble-pile configurations derived by continuum theory. A case study for SL9 suggests a low bulk density (0.2-0.3 g/cc) for its progenitor.