Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
e-mail: sonia.fornasier@obspm.fr
² Center for Technical Physics, Institute of Physics, Vietnam Academy of Science and Technology, Hà Nội, Vietnam
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077, Göttingen, Germany
⁴ Department of Physics and Astronomy Galileo Galilei, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁵ Center of Studies and Activities for Space (CISAS) G. Colombo, University of Padova, Via Venezia 15, 35131 Padova, Italy
⁶ CNR-IFN UOS Padova LUXOR, Via Trasea, 7, 35131 Padova, Italy
⁷ Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & Université de Versailles Saint-Quentin-en-Yvelines, 11 boulevard d’Alembert, 78280 Guyancourt, France
⁸ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
⁹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹¹ Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹² Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹³ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁴ Department of Mechanical Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy
¹⁵ Faculty of Engineering, University of Trento, Via Mesiano 77, 38100 Trento, Italy
¹⁶ Dipartimento di Geoscienze, University of Padova, Via G. Gradenigo 6, 35131 Padova, Italy
¹⁷ INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 34014 Trieste, Italy
¹⁸ Instituto de Astrofísica de Andalucia (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹⁹ National Central University, Graduate Institute of Astronomy, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
²⁰ Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²¹ Deutsches Zentrum für Luft und Raumfahrt (DLR), Institut für Planetenforschung, Asteroiden und Kometen, Rutherfordstrasse 2, 12489 Berlin, Germany
²² Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universitat Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²³ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
²⁴ MTA CSFK Konkoly Observatory, Budapest, Hungary

Received: 9 July 2018
Accepted: 27 August 2018

Abstract

Aims. The Rosetta space probe accompanied comet 67P/Churyumov-Gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. This has enabled us to study its activity almost continuously from 4 au inbound to 3.6 au outbound, including the perihelion passage at 1.24 au. This work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. We use observations acquired with the OSIRIS/NAC camera during July–October 2015, that is, close to perihelion.

Methods. We analyzed more than 2000 images from NAC color sequences acquired with 7–11 filters covering the 250–1000 nm wavelength range. The OSIRIS images were processed with the OSIRIS standard pipeline up to level 3, that is, converted in radiance factor, then corrected for the illumination conditions. For each color sequence, color cubes were produced by stacking registered and illumination-corrected images.

Results. More than 200 jets of different intensities were identified directly on the nucleus. Some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the visible-to-near-infrared region. We attribute this spectral behavior to icy grains mixed with the ejected dust. Some of the jets have an extremely short lifetime. They appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. We also report a resolved dust plume observed in May 2016 at a resolution of 55 cm pixel⁻¹, which allowed us to estimate an optical depth of ~0.65 and an ejected mass of ~2200 kg, assuming a grain bulk density of ~800 kg m⁻³. We present the results on the location, duration, and colors of active sources on the nucleus of 67P from the medium-resolution (i.e., 6–10 m pixel⁻¹) images acquired close to perihelion passage. The observed jets are mainly located close to boundaries between different morphological regions. Some of these active areas were observed and investigated at higher resolution (up to a few decimeter per pixel) during the last months of operations of the Rosetta mission.

Conclusions. These observations allow us to investigate the link between morphology, composition, and activity of cometary nuclei. Jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. This study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered byillumination conditions. Faint jets or outbursts are not associated with a particular terrain type or morphology.