Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

¹ Max-Planck-Institut für Sonnensystemforschung (MPS), Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
² Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
³ INAF Osservatorio Astronomico di Trieste, via Tiepolo 11, 34014 Trieste, Italy
⁴ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
⁵ Jet Propulsion Laboratory, M/S 183-301, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁶ Ames Research Center, Moffett Field, CA 94035, USA
⁷ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁸ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ LESIA-Observatoire de Paris, CNRS, Université Pierre et Marie Curie, Université Paris Diderot, 5 place J. Janssen, 92195 Meudon, France
¹⁰ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astro-physique de Marseille) UMR 7326, 13388 Marseille, France
¹¹ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹² Laboratoire d’Astrophysique de Marseille, UMR 7326 CNRS & Aix-Marseille Université, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹³ Centro de Astrobiología (CSIC-INTA), 28850 Torrejón de Ardoz, Madrid, Spain
¹⁴ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁵ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 Noordwijk, The Netherlands
¹⁶ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹⁷ PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁸ LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d’Alembert, 78280 Guyancourt, France
¹⁹ Centro di Ateneo di Studi ed Attivitá Spaziali “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy
²⁰ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
²¹ Department of Industrial Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²² University of Trento, via Sommarive 9, 38123 Trento, Italy
²³ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
²⁴ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80301, USA
²⁵ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomia, 18008 Granada, Spain
²⁶ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, 32054 Chung-Li, Taiwan
²⁷ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Cañada ( Madrid), Spain
²⁸ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
²⁹ Center for Space and Habitability, University of Bern, 3012 Bern, Switzerland

Received: 15 October 2016
Accepted: 4 April 2017

Abstract

Context. Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P/Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology.

Aims. We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission.

Methods. Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes.

Results. Surface changes discussed here were concentrated at mid-latitudes, between about 20◦N and 40◦N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0.5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night.

Conclusions. The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or/and redeposited over other areas of the nucleus.