Presentation #304.03 in the session Comets and ISOs: Dynamics, Origins and Theory.
An ongoing debate about comets is whether they were formed directly by planetesimal formation processes or as a byproduct of collisions between larger Kuiper belt objects. To investigate this issue, we modeled the collisional evolution of the primordial Kuiper belt (PKB) and its destabilized population, defined as those objects ejected onto planet-crossing orbits by Neptune’s outward migration and the giant planet instability (i.e., Nice model). The destabilized population will hit the giant planets as well as their rings and satellites over 4.5 Gyr, giving us a wide range of potential constraints. Starting with a PKB containing 30 Earth masses, much of it in the form of D ~ 100 km diameter bodies (Klahr and Schreiber 2020), we tested thousands of possible KBO disruption laws. In our best fit case, we found the easiest bodies to disrupt were D ~ 20 m. Smaller objects are in the strength regime, and collisional grinding readily turns them into a size distribution with a cumulative power law slope of q = -2.6 (see Dohnanyi 1969). These bodies go on to decimate objects between a few tens of meters and 1 km, giving them a shallow slope of q = -1. We find the destabilized population’s size distribution takes on a wavy shape reminiscent of the main asteroid belt, as suggested in Morbidelli et al. (2021). When this size distribution is coupled to a dynamical evolution model of the destabilized population, we find it can reproduce (i) the crater size distributions found on most giant planet satellites, provided their surfaces are not dominated by secondaries or sesquinaries (e.g., Kirchoff and Schenk 2010), (ii) the impact flux of superbolides hitting Jupiter (Hueso et al. 2018), (iii) the impact flux of sub-meter objects striking Saturn’s rings (Tiscareno et al. 2013), and (iv) the debiased size distributions of km-sized Jupiter-family and long period comets (Bauer et al. 2017; Boe et al. 2019). All of this indicates that most observed comets are derived from a collisionally-evolved population. Like the asteroids, we can show that most D < 10-20 km comets are fragments of larger bodies. Their compositions are likely to be mixtures of materials from different depths within D > 100 km bodies, though the near-surface is probably favored. Intriguingly, our model size distribution shows a mismatch with sub-km comets; there are far fewer of them observed than predicted. Given that our model size distribution can reproduce crater constraints on Europa, Phoebe, and other worlds, we suspect this difference is from physical evolution, with sub-km comets disrupting before they become observable by ground-based surveys.