OSIRIS was built by a consortium led by the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS, University of Padova, Italy; the Laboratoire d’Astrophysique de Marseille, France; the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain; the Scientific Support Office of the European Space Agency (ESA), Noordwijk, The Netherlands; the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain; the Universidad Politéchnica de Madrid, Spain; the Department of Physics and Astronomy of Uppsala University, Sweden; and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the contributing research institutes of the OSIRIS consortium and of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. M.A. acknowledges NASA funding through Jet Propulsion Laboratory contract no. 1267923 and from the Akademie der Wissenschaften zu Göttingen. W.-H.I. acknowledges the Ministry of Science and Technology, Taiwan (grant no. NSC 102-2112-M-008), and Macau University of Science and Technology (grant no. FDCT 017/2014/A1). We thank the ESA teams at European Space Astronomy Centre, European Space Operations Centre, and European Space Research and Technology Centre for their work in support of the Rosetta mission. P. H. Hasselmann and D. Perna are acknowledged for helpful discussion. Rosetta/OSIRIS data are available through the ESA’s Planetary Science Archive (PSA) at www.cosmos.esa.int/web/psa/rosetta. Some images used in this paper that are not yet available at PSA can be downloaded from the MPS OSIRIS website at https://planetgate.mps.mpg.de/WebFileShare/Released_Images/Fornasier+Science.

The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond.We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface.We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties. © 2017 Elsevier B.V., All rights reserved.

Fornasier, S. et al.--Full list of authors: Fornasier, S.; Mottola, S.; Keller, H. U.; Barucci, M. A.; Davidsson, B.; Feller, C.; Deshapriya, J. D. P.; Sierks, H.; Barbieri, C.; Lamy, P. L.; Rodrigo, R.; Koschny, D.; Rickman, H.; A'Hearn, M.; Agarwal, J.; Bertaux, J. -L.; Bertini, I.; Besse, S.; Cremonese, G.; Da Deppo, V.; Debei, S.; De Cecco, M.; Deller, J.; El-Maarry, M. R.; Fulle, M.; Groussin, O.; Gutierrez, P. J.; Güttler, C.; Hofmann, M.; Hviid, S. F.; Ip, W. -H.; Jorda, L.; Knollenberg, J.; Kovacs, G.; Kramm, R.; Kührt, E.; Küppers, M.; Lara, M. L.; Lazzarin, M.; Moreno, J. J. Lopez; Marzari, F.; Massironi, M.; Naletto, G.; Oklay, N.; Pajola, M.; Pommerol, A.; Preusker, F.; Scholten, F.; Shi, X.; Thomas, N.; Toth, I.; Tubiana, C.; Vincent, J. -B.