Spectrophotometric variegation of the layering in comet 67P/Churyumov-Gerasimenko as seen by OSIRIS

¹ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, Via Venezia 15, 35131 Padova, Italy
e-mail: sabrina.ferrari@unipd.it
² Department of Physics and Astronomy “G. Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
³ Department of Geosciences, via G. Gradenigo 6, University of Padova, 35131 Padova, Italy
⁴ Department of Physics and Astronomy “G. Galilei”, University of Padova, via Marzolo 8, 35131 Padova, Italy
⁵ CNR-IFN UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy
⁶ INAF, Astronomical observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Max Planck Institute for Solar System Research, Justusvon-Liebig-Weg 3, 37077, Göttingen, Germany
⁸ Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁹ LESIA, Observatoire de Paris, Université PSL, CNRS, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France
¹⁰ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 23, 12489 Berlin, Germany
¹¹ Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary
¹² Laboratoire d’Astrophysique de Marseille, UMR 7326, CNRS & Aix Marseille Université, 13388 Marseille Cedex 13, France
¹³ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹⁴ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁵ Science Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹⁶ LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d’Alembert, 78280 Guyancourt, France
¹⁷ Physics Department, Auburn University, 206 Allison Laboratory, Auburn, AL 35849, USA
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 Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
²² Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²³ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
²⁴ Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomia s/n, 18008 Granada, Spain

Received: 14 December 2018
Accepted: 15 March 2019

Abstract

Context. Between August 2014 and September 2016, the ESA space mission Rosetta escorted comet 67P/Churyumov-Gerasimenko (67P) during its perihelion passage. The onboard OSIRIS Narrow Angle Camera (NAC) acquired high-resolution multispectral images of the cometary surface. These datasets allowed a characterization of the spectrophotometric variegation of the layering of the large lobe, correlated with the layer structural elevation.

Aims. We perform a spectrophotometric characterization of the outcropping stratification of the small lobe of 67P as a function of its structural elevation, and consequently, a spectrophotometric comparison of the layered outcrops of the two lobes.

Methods. We selected two sequences of post-perihelion OSIRIS NAC images (~2.4 au outbound tothe Sun), from which we built up two multispectral images, framing an extended geological section of the onion-like stratification of the small lobe. Then we classified the consolidated areas of the outcropping and the relative coarse deposits that were identified in the multispectral data with a two-class maximum likelihood method. For this, we defined the classes as a function of the structural elevation of the surface.

Results. As a result, we identified a brightness variegation of the surface reflectance that is correlated with the structural elevation. The outer class, which is located at higher elevations, appears darker than the inner class. This fits previously obtained results for the large lobe. The reflectance values of the nucleus of 67P tend to decrease with increasing structural elevation.

Conclusions. The observed spectrophotometric variegation can be due to a different texture as well as to a different content of volatiles and refractories. We suggest that the outer outcrops appear darker because they have been exposed longer, and the inner outcrops appear brighter because the surface has been more effectively rejuvenated. We interpret this variegation as the result of an evolutionary process.