Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko

• Published in: Nature (London) Vol. 534; no. 7607; pp. 352 - 355
• Main Authors: Hirabayashi, Masatoshi, Scheeres, Daniel J., Chesley, Steven R., Marchi, Simone, McMahon, Jay W., Steckloff, Jordan, Mottola, Stefano, Naidu, Shantanu P., Bowling, Timothy
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.06.2016; Nature Publishing Group
• Subjects: 639/33/445/215; 639/33/445/848; Astronomical research; Astrophysics; Comets; Humanities and Social Sciences; letter; Morphology; multidisciplinary; Science; Solar system; Spacecraft; Sublimation
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature17670

A modelling study of the bilobate nucleus of comet 67P/Churyumov–Gerasimenko reveals that it has spun much faster in the past, but that its chaotically changing spin rate has so far prevented it from splitting; eventually the two lobes will separate, but they will be unable to escape each other and will ultimately merge again—a situation that seems to be common among cometary nuclei. Comet 67P nucleus keeps it together Rosetta spacecraft observations of comet 67P/Churyumov–Gerasimenko (67P) revealed a bilobate nucleus, with a structure that suggests that the two components were brought together at low speed after their separate formation. Here Daniel Scheeres and colleagues present a modelling study of the structure and dynamics of 67P's nucleus. They show that sublimation torques may have caused the nucleus of 67P to spin up and form the large cracks observed on its neck. These cracks are likely to propagate, eventually splitting the nucleus in two, but the separated lobes will be unable to escape each other and will ultimately merge again. The solid, central part of a comet—its nucleus—is subject to destructive processes 1 , 2 , which cause nuclei to split at a rate of about 0.01 per year per comet 3 . These destructive events are due to a range of possible thermophysical effects 4 ; however, the geophysical expressions of these effects are unknown. Separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes 5 , including the nucleus of comet 67P/Churyumov–Gerasimenko (67P), visited by the Rosetta spacecraft. Analysis of the Rosetta observations suggests that 67P’s components were brought together at low speed after their separate formation 6 . Here, we study the structure and dynamics of 67P’s nucleus. We find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. However, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. Once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. The components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. It has been shown 7 , 8 that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. The reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux.