Detection of exposed H₂O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

as observed by Rosetta OSIRIS and VIRTIS instruments

¹ LESIA, Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France
e-mail: antonella.barucci@obspm.fr
² INAF-IAPS, 00133 Rome, Italy
³ Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ Physikalisches Institut, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland
⁶ Dipartimento di Geoscienze, University of Padova, 35122 Padova, Italy
⁷ Observatoire des Sciences de l’Univers, 25000 Besançon, France
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 Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
¹³ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹⁴ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁵ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁶ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁷ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
¹⁸ LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d’Alembert, 78280 Guyancourt, France
¹⁹ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁰ Department of Information Engineering - University of Padova, via Gradenigo 6, 35131 Padova, Italy
²¹ JPL, 4800 Oak Grove Drive, Pasadena, CA91109, USA
²² Dipartimento di Fisica, Universita del Salento, Italy
²³ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²⁴ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
²⁵ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
²⁶ Instituto de Astrofísica de Andalucía – CSIC, 18080 Granada, Spain
²⁷ NASA Ames Research Center, CA 94035, USA
²⁸ UJF-Grenoble 1/CNRS-INSU, 38400 Saint Martin d’Hères, France

Received: 21 April 2016
Accepted: 11 August 2016

Abstract

Context. Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus.

Aims. The aim of this work is to search for the presence of H₂O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H₂O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS.

Methods. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination conditions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotometrically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes.

Results. Out of the 13 selected bright spots, eight of them present positive H₂O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H₂O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H₂O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.