The collisional and dynamical evolution of the main belt, NEA and TNO populations

Abstract

The size distribution of main-belt of asteroids is determined primarily by collisional processes. Large asteroids break up and form smaller asteroids in a collisional cascade, with the outcome controlled by the strength-vs.-size relationship for asteroids. We develop an analytical model that incorporates size-dependent strength and is able to reproduce the general features of the main-belt size distribution. In addition to collisional processes, the non-collisional removal of asteroids from the main belt (and their insertion into the near-Earth asteroid (NEA) population) is critical, and involves several effects: Strong resonances increase the orbital eccentricity of asteroids and cause them to enter the inner planet region; Chaotic diffusion by numerous weak resonances causes a slow leak of asteroids into the Mars- and Earth-crossing populations; And the Yarkovsky effect, a radiation force on asteroids, is the primary process that drives asteroids into these resonant escape routes. Yarkovsky drift is size-dependent and can potentially modify the main-belt size distribution. The NEA size distribution is primarily determined by its source, the main belt population, and by the size-dependent processes that deliver bodies from the main belt. All of these processes are simulated in a numerical collisional evolution model that incorporates removal by non-collisional processes. This model yields the strength-vs.-size relationship for main-belt asteroids and the non-collisional removal rates from the main belt required for consistency with the observed main-belt and NEA size distributions. Our results are consistent with other estimates of strength and removal rates, and fit a wide range of constraints, such as the number of observed asteroid families, the preserved basaltic crust of Vesta, the cosmic ray exposure ages of meteorites, and the observed cratering records on asteroids. Finally, our analytical and numerical models are applied to the collisional evolution of the trans-Neptunian objects (TNOs). We show that the TNO population likely started with a shallow initial size distribution, and that bodies ≳ 10 km in diameter are likely not in a collisional steady state. In addition, we show that the population of bodies in the TNO region below the size range of recent observational surveys is likely large enough to explain the observed numbers of Jupiter-family comets.