Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft^⋆

¹ LESIA, Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Pricipal Cedex, France
e-mail: sonia.fornasier@obspm.fr
² Univ Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
³ Observatório Nacional, General José Cristino 77, São Cristovão, Rio de Janeiro, Brazil
⁴ Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
⁵ Instituto de Astrofísica de Andalucía – CSIC, 18080 Granada, Spain
⁶ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁷ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
⁸ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁹ Laboratoire d’Astrophysique de Marseille UMR 7326, CNRS & Aix Marseille Université, 13388 Marseille Cedex 13, France
¹⁰ Centro de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain
¹¹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹² Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹³ PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁴ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁵ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁶ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹⁷ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁸ CNR–IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹⁹ Department of Mechanical Engineering – University of Padova, via Venezia 1, 35131 Padova, Italy
²⁰ UNITN, Universitá di Trento, via Mesiano, 77, 38100 Trento, Italy
²¹ INAF−Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²² Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
²³ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
²⁴ Institut für Datentechnik und Kommunikationsnetze, 38106 Braunschweig, Germany
²⁵ Department of Information Engineering − University of Padova, via Gradenigo 6, 35131 Padova, Italy
²⁶ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy
²⁷ Physikalisches Institut, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland
²⁸ Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

Received: 15 February 2015
Accepted: 12 May 2015

Abstract

Context. The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted are onboard Rosetta.

Aims. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The observations presented here were performed during July and the beginning of August 2014, during the approach phase, when OSIRIS was mapping the surface of the comet with several filters at different phase angles (1.3°–54°). The resolution reached up to 2.1 m/px.

Methods. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted into I/F radiance factors and corrected for the illumination conditions at each pixel using the Lommel-Seeliger disk law. Color cubes of the surface were produced by stacking registered and illumination-corrected images. Furthermore, photometric analysis was performed both on disk-averaged photometry in several filters and on disk-resolved images acquired with the NAC orange filter, centered at 649 nm, using Hapke modeling.

Results. The disk-averaged phase function of the nucleus of 67P shows a strong opposition surge with a G parameter value of −0.13 ± 0.01 in the HG system formalism and an absolute magnitude H_v(1,1,0) = 15.74 ± 0.02 mag. The integrated spectrophotometry in 20 filters covering the 250−1000 nm wavelength range shows a red spectral behavior, without clear absorption bands except for a potential absorption centered at ~290 nm that is possibly due to SO₂ ice. The nucleus shows strong phase reddening, with disk-averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3°−54° phase angle range. The geometric albedo of the comet is 6.5 ± 0.2% at 649 nm, with local variations of up to ~16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700−750 nm that is associated with coma emissions.