Phase-curve analysis of comet 67P/Churyumov-Gerasimenko at small phase angles

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: masoumzadeh@mps.mpg.de
² Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
³ Department of Earth and Planetary Sciences, Birkbeck College, University of London, London, UK
⁴ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 23, 12489 Berlin, Germany
⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁶ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁷ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, Via Venezia 15, 35131 Padova, Italy
⁸ CNR-IFN UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy
⁹ LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d’Alembert, 78280 Guyancourt, France
¹⁰ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹¹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹² Science Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹³ Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
¹⁵ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁶ Physics Department, 206 Allison Laboratory, Auburn University, Auburn, AL 36849, USA
¹⁷ INAF – Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁸ Department of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy
¹⁹ Faculty of Engineering, University of Trento, Via Mesiano 77, 38121 Trento, Italy
²⁰ INAF – Astronomical Observatory of Trieste, Via Tiepolo 11, 34143 Trieste, Italy
²¹ Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain
²² Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
²³ Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²⁴ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²⁵ Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary

Received: 12 December 2018
Accepted: 25 April 2019

Abstract

Aims. The Rosetta-OSIRIS images acquired at small phase angles in three wavelengths during the fly-by of the spacecraft on 9–10 April 2016 provided a unique opportunity to study the opposition effect on the surface of comet 67P/Churyumov-Gerasimenko (67P). Our goal is to study phase curves of the nucleus at small phase angles for a variety of surface structures to show the differences in their opposition effect and to determine which surface properties cause the differences.

Methods. We used OSIRIS NAC images that cover the Ash-Khepry-Imhotep region to extract the phase curve, that is, the reflectance of the surface as a function of phase angle. We selected six regions of interest (ROIs) and derived the phase curves for each ROI. We fit a linear-exponential function to the phase curves. The resulting model parameters were then interpreted by spectrophotometric, geomorphological, and phase-ratio analyses, and by investigating the influence of structural and textural properties of the surface.

Results. We find evidence for the opposition effect (deviation of the phase curve from linear behavior) in phase curves for all areas. We found an anticorrelation between the phase ratio and reflectance in a small phase angle range. This provides evidence for the shadow-hiding effect. We conclude that the decrease in the slope of the phase ratio versus reflectance indicates a decrease in the proportion of shadowed regions and reduces the contribution of the shadow-hiding effect. Large uncertainties in the determination of the opposition effect parameters with respect to wavelength do not allow us to conclusively claim coherent backscattering in the opposition effect phenomenon. Based on the two analyses, we conclude that the opposition effect of comet 67P in the Ash-Khepry-Imhotep region is mainly affected by shadow-hiding.