Collisional heating of icy planetesimals – I. Catastrophic collisions

Abstract

ABSTRACTPlanetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code nimbus is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of CO2:CO mixtures and during crystallization of amorphous H2O is calculated, as well as mobilization and internal relocation of CO2. If an amorphous H2O host existed, and was protected by a CO2:CO heat sink, only diameter $D\lt 20\, \mathrm{km}$ (inner disc) and $D\lt 64\, \mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If CO2 was the only CO host, the critical diameters drop to D < 20–$32\, \mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of <9 Myr at 15 au and <50–70 Myr at 30 au. Alternatively, if a 450 Myr disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6–60 times below current estimates.