Comets are fragments: Determining the degree of processing during the collisional grinding of the primordial Kuiper-belt.

There is a general consensus that cometesimals formed in an original reservoir, the primordial Kuiper-belt (PKB), between 20-40 au from the Sun (e.g., Nesvorný 2018). From there, they were scattered into the current trans-Neptunian region. The so-called scattered disk is the source reservoir for Jupiter family comets, JFCs (e.g., Duncan & Levison 1997). Crucially, the scattering phase of cometesimals must occur as the gas disk dissipates to prevent the damping of the scattered orbits. This is well described by the final phase of planetesimal-driven migration, particularly Neptune’s migration (e.g., Nesvorný et al. 2018). 

Current planetesimal formation models predict an initial Gaussian size distribution centred around 100 km (e.g., Polak & Klahr 2023). Therefore, the question arises if such an initial size frequency distribution (SFD) of the PKB can reproduce the observational constraints of the current Kuiper-belt. Indeed, we find in Bottke et al. 2023 that the collisional evolution of the PKB based on planetesimal formation models will evolve into a SFD that is consistent with (i) crater SFDs on icy satellites and KBOs and (ii) observed SFDs of populations derived from the PKB (e.g., Jupiter’s Trojans). The craters on icy satellites allow us to infer the SFD of the PKB population scattered onto planet-crossing orbits and those that went to the scattered disk (i.e., source of Centaurs/JFCs).

Because comets originate from that same PKB but are then scattered into the Kuiper-belt, they similarly go through the collisional evolution described above. This raises the question of whether the collisional grinding from 100 km cometesimals to the few km sized comets can preserve the primitive properties of comets (i.a., they contain highly volatile species such as CO and have very low densities). Here, we will present recent simulations showing the degree of compaction and heating due to the collisional evolution.