Ciarniello, Mauro, Filacchione, Gianrico, Capaccioni, Fabrizio, Raponi, Andrea, de Sanctis, Maria Cristina, Tosi, Federico, Piccioni, Giuseppe, Migliorini, Alessandra, Cerroni, Priscilla, Capria, Maria Teresa, Erard, Stéphane, Bockelée-Morvan, Dominique, Leyrat, Cédric, Arnold, Gabriele, Barucci, Maria Antonella, Schmitt, Bernard, Quirico, Éric, Taylor, Frederic W., Kappel, David, Longobardo, Andrea, Henry, Florence, ITA, GBR, FRA, DEU, Istituto di Astrofisica e Planetologia Spaziali, INAF (IAPS), Observatoire de Paris, Université Paris sciences et lettres (PSL), Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), Pôle Planétologie du LESIA, Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), Institute for Planetary Research, Deutsches Zentrum fur Luft- und Raumfahrt (DLR) Berlin, Germany, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), and Oxford University
International audience; On 9-10 of April 2016, the Rosetta orbiter completed a close flyby around the nucleus of 67P/Churyumov-Gerasimenko, when the comet was at heliocentric distance of 2.76 AU, along the outbound leg of its orbit around the Sun. This allowed the VIRTIS-M imaging spectrometer to observe the surface of 67P at visible wavelengths (0.2-1 mum) in the 0.93°-89.7° phase angle range with a spatial resolution ranging from 7 to 46 m/pix, resulting in a total of 105 hyperspectral images. Previous observations of the comet at low phase angle were acquired by VIRTIS-M during the approach phase on July 2014 (3.7 AU), with pixel resolution varying from 450 to 3200 m/pix, preventing disk-resolved imaging. The April 2016 observations fill this gap, allowing us to constrain the spectrophotometric properties of the surface at higher spatial resolution in the opposition surge geometry, therefore investigating both the 'shadow hiding' and 'coherent backscattering' opposition effect. Extrapolation to 0° phase angle of the reflectance measured during the flyby at latitude between -10° and 30° indicates a surface normal albedo of 0.06 at 0.55 mum. This result is close to the previous average value derived by Ciarniello et al. (2015) from a full-disk analysis giving 0.062 /-0.002 (3.7 AU, inbound orbit). Subsequent measurements by Filacchione et al. (2016) and Ciarniello et al. (submitted), derived from pre-perihelion observations at lower heliocentric distances, revealed a progressive enrichment of water ice abundance on the nucleus, in northern hemisphere regions, with a consequent brightening of the surface. The measured normal albedo derived from the 9-10 April 2016 dataset seems to indicate that during the post-perihelion phase the northern hemisphere has returned to albedo values compatible to the ones measured during the inbound part of the orbit, when the comet was at 3.7 AU from the Sun. This could be an indication that the northern hemisphere has been either covered by dust emitted by the southern hemisphere during the very active perihelion phase, or that dehydration of the upper surface layers has taken place.