Multivariate statistical analysis of OSIRIS/Rosetta spectrophotometric data of comet 67P/Churyumov-Gerasimenko

¹ LESIA – Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: davide.perna@obspm.fr
² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
³ University of Padova, Department of Physics and Astronomy, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴ Laboratoire d’Astrophysique de Marseille, UMR 7326 CNRS & Université Aix-Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ Centro de Astrobiologia (INTA-CSIC), 28850 Torrejon de Ardoz, Madrid, Spain
⁶ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
⁷ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁹ PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁰ University of Maryland, Department of Astronomy, College Park, MD 20742-2421, USA
¹¹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹² INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ CNR-IFN UOS Padova LUXOR, via Trasea, 7, 35131 Padova , Italy
¹⁴ Jet Propulsion Laboratory, M/S 183-301, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁵ Department of Mechanical Engineering University of Padova, via Venezia 1, 35131 Padova, Italy
¹⁶ University of Trento, via Mesiano 77, 38100 Trento, Italy
¹⁷ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
¹⁸ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34014 Trieste, Italy
¹⁹ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
²⁰ Instituto de Astrofisica de Andalucia (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain
²¹ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Asteroiden und Kometen, Rutherfordstrafle 2, 12489 Berlin, Germany
²² National Central University, Graduate Institute of Astronomy, 300 Chung-Da Rd, 32054 Chung-Li, Taiwan
²³ Space Science Institute, Macau University of Science and Technology, Macau, PR China
²⁴ Institut für Geophysik und Extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²⁵ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
²⁶ University of Padova, Department of Information Engineering, via Gradenigo 6/B, 35131 Padova, Italy
²⁷ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy

Received: 4 November 2016
Accepted: 15 December 2016

Abstract

Context. The ESA Rosetta mission explored comet 67P/Churyumov-Gerasimenko in 2014−2016, following its target before and after the perihelion passage on 13 August 2015. The NAC camera of the OSIRIS imaging system allowed to map the nucleus surface acquiring images with different filters in the visible wavelength range.

Aims. Here we study the spectrophotometric behaviour of the nucleus by a multivariate statistical analysis, aiming to distinguish homogeneous groups and to constrain the bulk composition.

Methods. We applied the G-mode clustering algorithm to 16 OSIRIS data cubes acquired on 5−6 August 2014 (mostly covering the northern hemisphere) and 2 May 2015 (mostly covering the southern hemisphere), selected to have complete coverage of the comet’s surface with similar observing conditions.

Results. We found four similar homogeneous groups for each of the analysed cubes. The first group corresponds to the average spectrophotometric behaviour of the nucleus. The second (spectrally redder) and the third (spectrally bluer) groups are found in regions that were already found to deviate from the average terrain of the comet by previous studies. A fourth group (characterised by enhancements of the flux at 700−750 nm and 989 nm, possibly due to H₂O⁺ and/or NH₂ emissions) seems connected with the cometary activity rather than with the bulk composition.

Conclusions. While our aim in this work was to study the spectrophotometric behaviour of the nucleus of 67P/Churyumov-Gerasimenko as a whole, we found that a follow-up application of the G-mode to smaller regions of the surface could be useful in particular to identify and study the temporal evolution of ice patches, as well as to constrain the composition and physical processes behind the emission of dust jets.