Rosetta/OSIRIS observations of the 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, 92195 Meudon Cedex, France
e-mail: clement.feller@obspm.fr
² Center of Studies and Activities for Space (CISAS) G. Colombo, University of Padova, Via Venezia 15, 35131 Padova, Italy
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077 Goettingen, Germany
⁴ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁵ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, Via Venezia 15, 35131 Padova, Italy
⁶ CNR-IFN UOS Padova LUXOR, Via Trasea, 7, 35131 Padova, Italy
⁷ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
⁸ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
⁹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Science Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹¹ Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹² Physics Department, 206 Allison Laboratory, Auburn University, Auburn, AL 36849, USA
¹³ INAF, Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁴ Department of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy
¹⁵ Faculty of Engineering, University of Trento, Via Mesiano 77, 38121 Trento, Italy
¹⁶ INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁷ Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹⁸ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
¹⁹ Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²⁰ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Asteroiden und Kometen, Rutherfordstraße 2, 12489 Berlin, Germany
²¹ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²² Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA

Received: 9 July 2018
Accepted: 17 November 2018

Abstract

Context. From August 2014 to September 2016, the Rosetta spacecraft followed comet 67P/Churyumov–Gerasimenko along its orbit. After the comet passed perihelion, Rosetta performed a flyby manoeuvre over the Imhotep–Khepry transition in April 2016. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 observations with mainly three broadband filters (centered at 480, 649, and 743 nm) at a resolution of up to 0.53 m/px and for phase angles between 0.095° and 62°.

Aims. We have investigated the morphological and spectrophotometrical properties of this area using the OSIRIS/NAC high-resolution observations.

Methods. We assembled the observations into coregistered color cubes. Using a 3D shape model, we produced the illumination conditions and georeference for each observation. We mapped the observations of the transition to investigate its geomorphology. Observations were photometrically corrected using the Lommel–Seeliger disk law. Spectrophotometric analyses were performed on the coregistered color cubes. These data were used to estimate the local phase reddening.

Results. The Imhotep–Khepry transition hosts numerous and varied types of terrains and features. We observe an association between a feature’s nature, its reflectance, and its spectral slopes. Fine material deposits exhibit an average reflectance and spectral slope, while terrains with diamictons, consolidated material, degraded outcrops, or features such as somber boulders present a lower-than-average reflectance and higher-than-average spectral slope. Bright surfaces present here a spectral behavior consistent with terrains enriched in water-ice. We find a phase-reddening slope of 0.064 ± 0.001%/100 nm/° at 2.7 au outbound, similar to the one obtained at 2.3 au inbound during the February 2015 flyby.

Conclusions. Identified as the source region of multiple jets and a host of water-ice material, the Imhotep–Khepry transition appeared in April 2016, close to the frost line, to further harbor several potential locations with exposed water-ice material among its numerous different morphological terrain units.