Comets as collisional fragments of a primordial planetesimal disk

TL;DR

This study explores whether comets are primordial objects or collisional fragments by simulating their early Solar System evolution, concluding that most km-sized comets are likely fragments from larger parent bodies due to collisional history.

Contribution

It provides the first detailed simulations of comet nucleus collisional evolution within the Nice Model framework, highlighting the likelihood of comets being fragments.

Findings

Comets have a low chance of surviving destructive collisions in the early disk.

Survivors of collisions would show many impact marks.

Most km-sized comets are likely fragments of larger bodies.

Abstract

The Rosetta mission and its exquisite measurements have revived the debate on whether comets are pristine planetesimals or collisionally evolved objects. We investigate the collisional evolution experienced by the precursors of current comet nuclei during the early stages of the Solar System, in the context of the so-called "Nice Model". We consider two environments for the collisional evolution: (1) the trans-planetary planetesimal disk, from the time of gas removal until the disk was dispersed by the migration of the ice giants, and (2) the dispersing disk during the time that the scattered disk was formed. Simulations have been performed, using different methods in the two cases, to find the number of destructive collisions typically experienced by a comet nucleus of 2km radius. In the widely accepted scenario, where the dispersal of the planetesimal disk occurred at the time of the Late Heavy Bombardment about 4Gy ago, comet-sized planetesimals have a very small chance to survive against destructive collisions in the disk. On the extreme assumption that the disk was dispersed directly upon gas removal, there is a chance for a significant fraction of the planetesimals to remain intact. However, these survivors would still bear the marks of many non-destructive impacts. Thus, the Nice Model of Solar System evolution predicts that typical km-sized comet nuclei are predominantly fragments resulting from collisions experienced by larger parent bodies. An important goal for further research is to investigate, whether the observed properties of comet nuclei are compatible with such a collisional origin.