Exposed bright features on the comet 67P/Churyumov–Gerasimenko: distribution and evolution

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, UPMC Univ. Paris 06, Sorbonne Universités, 5 place Jules Janssen, 92195 Meudon, France
e-mail: prasanna.deshapriya@obspm.fr
² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
³ INAF-OAPD, Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ NASA Ames Research Center, Mountain View, CA 94035, USA
⁵ Institut für Planetenforschung, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
⁶ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁷ Laboratoire d’Astrophysique de Marseille UMR 7326, CNRS, Aix-Marseille Université, 13388 Marseille Cedex 13, France
⁸ Centro de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
¹¹ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹² Polish Academy of Sciences, Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹³ LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d’Alembert, 78280 Guyancourt, France
¹⁴ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁵ CNR–IFN UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy
¹⁶ NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁷ Dipartimento di Ingegneria Meccanica, Università di Padova, Via Venezia 1, 35131 Padova, Italy
¹⁸ UNITN, Universitá di Trento, Via Mesiano 77, 38100 Trento, Italy
¹⁹ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
²⁰ Instituto de Astrofísica de Andalucía – CSIC, 18080 Granada, Spain
²¹ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
²² Institut für Geophysik und Extraterrestrisch Physik, TU Braunschweig, 38106 Braunschweig, Germany
²³ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
²⁴ Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo” (CISAS), Università di Padova, Via Venezia 15, 35131 Padova, Italy
²⁵ Dipartimento di Ingegneria dell’Informazione, Università di Padova, Via Gradenigo 6/B, 35131 Padova, Italy
²⁶ Physikalisches Institut, Sidlerstrasse 5, Universität Bern, 3012 Bern, Switzerland

Received: 17 October 2017
Accepted: 28 December 2017

Abstract

Context. Since its arrival at the comet 67P/Churyumov–Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta.

Aims. We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance.

Methods. We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel–Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest.

Results. We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H₂O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category.

Conclusions. Although the nucleus of 67P/Churyumov–Gerasimenko appears to be dark in general, there are localised H₂O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H₂O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet’s return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H₂O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.