The largest population of remnant planetesimals still found in the inner solar system is the main asteroid belt, located between 2.1–3.2 astronomical units (AU). According to meteorite studies, many of the largest bodies there formed ~4.56 Ga during planet formation processes. Since that time, however, they have been subject to collisional and dynamical evolution. A key goal of asteroid belt studies, therefore, is to turn back the clock, understand how the main belt has changed, and use the information to infer the original properties of primordial main belt planetesimals. By doing so, we not only provide powerful constraints on planetesimal and planet formation models, but we can also place meteorite constraints into their appropriate solar system context.
A problem in interpreting what we know about asteroids, however, is that collisonal and dynamical evolution are coupled to one other. For example, assuming a given dynamical excitation state for a small-body population, more collisional evolution takes place when a population is large than when it is small. Thus, if dynamical effects suddenly remove bodies from a population, disruption and cratering events must drop as well. Similarly, a population with low eccentricities and inclinations will undergo little collisional grinding, while one with large values will grind much faster.
For this reason, our discussion starts with what has been inferred about the collisional evolution of the asteroid belt. This leads into how it has been affected dynamically by the processes that led to the origin of our planets.
Constraints for Collisional Evolution
Given the enormous number of possibilities that can take place in collisional evolution models for given assumptions, and the importance of dynamical excitation, dynamical removal, and stochastic breakup events, it is critical that planetesimal formation and collisional/dynamical evolution models be tested against as many constraints as possible. This potentially allows us to rule out certain scenarios and place higher degrees of confidence in successful solutions. Here we discuss many of the constraints that need to be considered when modeling the collisional evolution of the main belt.
Wavy Main Belt Size Frequency Distribution
A key constraint comes from the main belt size frequency distribution (SFD). Reasonable estimates of this SFD can be found in several places, and reviews of this topic can be found in Jedicke et al. (2002), Mainzer et al. (2015), and Masiero et al. (2015).