Bi-lobed Shape of Comet 67P from a Collapsed Binary

TL;DR

This paper investigates the origin of comet 67P's bi-lobed shape, proposing it as a collapsed binary formed through gravitational encounters and impacts, but finds such processes are unlikely to account for the high prevalence of bi-lobed comets.

Contribution

It introduces a model for the formation of bi-lobed comets via binary collapse and evaluates the likelihood of different physical mechanisms involved.

Findings

Approximately 30% binary collapse probability due to impacts.

Most km-class binaries are collisionally dissolved.

Only about 10% of binaries become contact binaries during disk dispersal.

Abstract

The Rosetta spacecraft observations revealed that the nucleus of comet 67P/Churyumov-Gerasimenko consists of two similarly sized lobes connected by a narrow neck. Here we evaluate the possibility that 67P is a collapsed binary. We assume that the progenitor of 67P was a binary and consider various physical mechanisms that could have brought the binary components together, including small-scale impacts and gravitational encounters with planets. We find that 67P could be a primordial body (i.e., not a collisional fragment) if the outer planetesimal disk lasted <10 Myr before it was dispersed by migrating Neptune. The probability of binary collapse by impact is 30% for tightly bound binaries. Most km-class binaries become collisionally dissolved. Roughly 10% of the surviving binaries later evolve to become contact binaries during the disk dispersal, when bodies suffer gravitational encounters to Neptune. Overall, the processes described in this work do not seem to be efficient enough to explain the large fraction (~67%) of bi-lobed cometary nuclei inferred from spacecraft imaging.