Comets as collisional fragments of a primordial planetesimal disk

Abstract

Context. The Rosetta mission and its exquisite measurements have revived the debate on whether comets are pristine planetesimals or collisionally evolved objects. Aims. 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. Methods. We considered two environments for the collisional evolution: (1) the transplanetary 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. We performed simulations using different methods in the two cases to determine the number of destructive collisions typically experienced by a comet nucleus of 2 km radius. Results. In the widely accepted scenario, where the dispersal of the planetesimal disk occurred at the time of the Late Heavy Bombardment about 4 Gy ago, comet-sized planetesimals have a very low probability of surviving destructive collisions in the disk. On the extreme assumption that the disk was dispersed directly upon gas removal, a significant fraction of the planetesimals might have remained intact. However, these survivors would still bear the marks of many nondestructive impacts.Conclusions. 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 future research is to investigate whether the observed properties of comet nuclei are compatible with such a collisional origin.

Comet formation in collapsing pebble clouds

“…Since the resulting objects are bound to the same cloud, binary formation should be possible (Nesvorný et al 2010). An alternative possibility is the fragmentation of cometesimals in mutual collisions of big cometesimals after the collapse (Morbidelli & Rickman 2015). However, cometesimal collisions which are energetic enough to disrupt the colliding bodies can also be expected to significantly change the material properties, such as the density.…”

How primordial is the structure of comet 67P?

“…As shown in Morbidelli & Rickman (2015), it is clear that in this standard model, an object of the size of comet 67P would have experienced a high number of catastrophic collisions and thus could not have survived. However, it was also shown that in the (hypothetical) case that the dispersal of the disk occurred early, right after gas removal, the collisional evolution of km-size bodies ending in the Scattered Disk would have been less severe, and a fraction of these objects may have escaped all catastrophic collisions.…”

“…Whether the cometary nuclei structures as observed today are pristine and preserve a record of their original accumulation, or are a result of later collisional or other processes is much debated (e.g. Weissman et al 2004;Mumma et al 1993;Sierks et al 2015;Rickman et al 2015;Morbidelli & Rickman 2015;Jutzi & Asphaug 2015;Davidsson et al 2016). The shape, density, composition, and other properties of comet 67P/Churyumov-Gerasimenko (67P) have been determined in detail by the European Space Agency's Rosetta rendezvous mission (e.g.…”

“…The collisional evolution of an object of the size of comet 67P was studied by Morbidelli & Rickman (2015) using dynamical models of the evolution of the early solar system. In the "standard model", as defined by the so-called Nice model ), cometary nuclei, or their precursors, originated from an initial trans-planetary disk of icy planetesimals with a lifetime of a few hundred Myr.…”

“…In other words, it is crucial to know the critical specific impact energy at which the shape and structure of such an object are altered significantly. Previous studies of the collisional evolution of comet 67P (Morbidelli & Rickman 2015) used scaling laws for catastrophic disruption energies that are based on idealized spherical, solid icy bodies (Benz & Asphaug 1999), which may not represent well the properties of a highly porous cometary nuclei. It is well known that the impacts in highly porous material, given its dissipative properties, can lead to very different results compared to impacts involving solid materials (e.g.…”