Acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67P/Churyumov–Gerasimenko

Jessica Agarwal (1) , Michael F. A'Hearn (2) , J.-B. Vincent (1) , C. Güttler (1) , S. Höfner (1) , H. Sierks (1) , C. Tubiana (1) , C. Barbieri (3) , P. L. Lamy (4) , R. Rodrigo (5, 6, 7) , D. Koschny (8) , H. Rickman (9, 10) , M. A. Barucci (11) , Jean-Loup Bertaux (12) , I. Bertini (13) , S. Boudreault (1) , Gabriele Cremonese (14) , Vania Da deppo (15, 14) , B. Davidsson (16) , S. Debei (17) , M. De cecco (18) , J. Deller (1) , S. Fornasier (11) , M. Fulle (19) , A. Gicquel (1) , O. Groussin (4) , Pedro J. Gutiérrez (20) , M. Hofmann (1) , S. F. Hviid (21) , W.-H. Ip (22) , L. Jorda (4) , H. U. Keller (23) , J. Knollenberg (21) , J.-R. Kramm (1) , Ekkehard Kührt (21) , M. Küppers (24) , L. M. Lara (20) , M. Lazzarin (3) , J. J. Lopez moreno (20) , F. Marzari (3) , G. Naletto (13, 25, 15) , N. Oklay (1) , X. Shi (1) , N. Thomas (26)

1 MPS - Max-Planck-Institut für Sonnensystemforschung
2 Department of Astronomy [College Park]
3 Dipartimento di Fisica e Astronomia "Galileo Galilei"
4 LAM - Laboratoire d'Astrophysique de Marseille
5 CAB - Centro de Astrobiologia [Madrid]
6 ESAC - European Space Astronomy Centre
7 ISSI - International Space Science Institute [Bern]
8 ESTEC - European Space Research and Technology Centre
9 CBK - Space Research Centre of Polish Academy of Sciences
10 Department of Physics and Astronomy [Uppsala]
11 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
12 PLANETO - LATMOS
13 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
14 OAPD - INAF - Osservatorio Astronomico di Padova
15 IFN - CNR Institute for Photonics and Nanotechnologies
16 JPL - Jet Propulsion Laboratory
17 Department of Industrial Engineering [Padova]
18 University of Trento [Trento]
19 OAT - INAF - Osservatorio Astronomico di Trieste
20 IAA - Instituto de Astrofísica de Andalucía
21 DLR Institut für Planetenforschung
22 Institute of Space Science [Taiwan]
23 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
24 Operations Department (ESAC)
25 DEI - Department of Information Engineering [Padova]
26 Physikalisches Institut [Bern]

We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained in January 2016 when the comet was at 2 AU from the Sun out-bound from perihelion. We measure the acceleration of individual aggregates through a two-hour image series. Approximately 50% of the aggregates are accelerated away from the nucleus, and 50% towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10% of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2km) of a comet, where gas drag is still significant.

Keywords: zodiacal dust; Comets; 67P/Churyumov-Gerasimenko

Domains: Sciences of the Universe [physics]; Astrophysics [astro-ph]; Earth and Planetary Astrophysics [astro-ph.EP]

Jessica Agarwal, Michael F. A'Hearn, J.-B. Vincent, C. Güttler, S. Höfner, et al.. Acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, 2016, 462 (Suppl. 1), pp.S78-S88. ⟨10.1093/mnras/stw2179⟩. ⟨insu-01367854⟩