Geomorphology and spectrophotometry of Philae’s landing site on comet 67P/Churyumov-Gerasimenko

¹ Dipartimento di Fisica e Astronomia “G. Galilei”University of Padova, vic. Osservatorio 3, 35122 Padova, Italy
e-mail: fiorangela.laforgia@unipd.it
² Dipartimento di Geoscienze, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
⁵ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy
⁶ INAF Osservatorio Astronomico di Padova, vic. dell’Osservatorio 5, 35122 Padova, Italy
⁷ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
⁸ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
⁹ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
¹⁰ Laboratoire d’Astrophysique de Marseille, UMR 7236, CNRS & Aix-Marseille Université, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹¹ Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz, 28850 Madrid, Spain
¹² International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
¹³ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹⁴ PAS Space Research Centre, Bartycka 18A, 00716 Warszawa, Poland
¹⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁶ Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, 38106 Braunschweig, Germany
¹⁷ University of Maryland, Department of Astronomy, College Park, MD 20742-2421, USA
¹⁸ LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
¹⁹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
²⁰ Department of Industrial Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²¹ University of Trento, via Sommarive 9, 38123 Trento, Italy
²² Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cadex 13, France
²³ INAF Osservatorio Astronomico di Trieste, via Tiepolo 11, 34014 Trieste, Italy
²⁴ Instituto de Astrofísica de Andalucía-CSIC, 18008 Granada, Spain
²⁵ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd Chung-Li 32054, Taiwan
²⁶ Operations Department European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
²⁷ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany

Received: 27 February 2015
Accepted: 13 July 2015

Abstract

Context. On 12 November 2014 the European mission Rosetta succeeded in delivering a lander, named Philae, on the surface of one of the smallest, low-gravity and most primitive bodies of the solar system, the comet 67P/Churyumov-Gerasimenko (67P).

Aims. The aim of this paper is to provide a comprehensive geomorphological and spectrophotometric analysis of Philae’s landing site (Agilkia) to give an essential framework for the interpretation of its in situ measurements.

Methods. OSIRIS images, coupled with gravitational slopes derived from the 3D shape model based on stereo-photogrammetry were used to interpret the geomorphology of the site. We adopted the Hapke model, using previously derived parameters, to photometrically correct the images in orange filter (649.2 nm). The best approximation to the Hapke model, given by the Akimov parameter-less function, was used to correct the reflectance for the effects of viewing and illumination conditions in the other filters. Spectral analyses on coregistered color cubes were used to retrieve spectrophotometric properties.

Results. The landing site shows an average normal albedo of 6.7% in the orange filter with variations of ~15% and a global featureless spectrum with an average red spectral slope of 15.2%/100 nm between 480.7 nm (blue filter) and 882.1 nm (near-IR filter). The spatial analysis shows a well-established correlation between the geomorphological units and the photometric characteristics of the surface. In particular, smooth deposits have the highest reflectance a bluer spectrum than the outcropping material across the area.

Conclusions. The featureless spectrum and the redness of the material are compatible with the results by other instruments that have suggested an organic composition. The observed small spectral variegation could be due to grain size effects. However, the combination of photometric and spectral variegation suggests that a compositional differentiation is more likely. This might be tentatively interpreted as the effect of the efficient dust-transport processes acting on 67P. High-activity regions might be the original sources for smooth fine-grained materials that then covered Agilkia as a consequence of airfall of residual material. More observations performed by OSIRIS as the comet approaches the Sun would help interpreting the processes that work at shaping the landing site and the overall nucleus.