Aswan site on comet 67P/Churyumov-Gerasimenko: Morphology, boulder evolution, and spectrophotometry

¹ Center of Studies and Activities for Space, CISAS, “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy
² NASA Ames Research Center, Moffett Field, CA 94035, USA
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁴ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁵ Geosciences Department, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy
⁶ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
⁷ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstrasse 2, 12489 Berlin, Germany
⁸ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
⁹ Aix-Marseille Universite, CNRS LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹⁰ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
¹¹ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹² Centro de Astrobiologa, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹³ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁴ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk, The Netherlands
¹⁵ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹⁶ PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁷ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁸ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁹ LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
²⁰ Univ. Paris Diderot, Sorbonne Paris Cite, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
²¹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
²² INAF Osservatorio Astronomico di Padova, Vic. Osservatorio 5, 35122 Padova, Italy
²³ NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁴ UNITN, University of Trento, via Mesiano, 77, 38100 Trento, Italy
²⁵ Department of Mechanical Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²⁶ INAF Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²⁷ Instituto de Astrofisica de Andalucia CSIC, Glorieta de la Astronomia, 18008 Granada, Spain
²⁸ Operations Department European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
²⁹ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany

Received: 1 December 2015
Accepted: 4 June 2016

Abstract

Aims. We provide a detailed morphological analysis of the Aswan site on comet 67P/Churyumov-Gerasimenko (67P). We derive the size-frequency distribution of boulders ≥2 m and correlate this distribution with the gravitational slopes for the first time on a comet. We perform the spectral analysis of this region to understand if possible surface variegation is related to thedifferent surface textures observable on the different units.

Methods. We used two OSIRIS Narrow Angle Camera (NAC) image data sets acquired on September 19 and 22, 2014, with a scale of 0.5 m/px. Gravitational slopes derived from the 3D shape model of 67P were used to identify and interpret the different units of the site. By means of the high-resolution NAC data sets, boulders ≥2.0 m can be unambiguously identified and extracted using the software ArcGIS. Coregistered and photometrically corrected color cubes were used to perform the spectral analyses, and we retrieved the spectral properties of the Aswan units.

Results. The high-resolution morphological map of the Aswan site (0.68 km²) shows that this site is characterized by four different units: fine-particle deposits located on layered terrains, gravitational accumulation deposits, taluses, and the outcropping layered terrain. Multiple lineaments are identified on the Aswan cliff, such as fractures, exposed layered outcrops, niches, and terraces. Close to the terrace margin, several arched features observed in plan view suggest that the margin progressively retreats as a result of erosion. The size-frequency of boulders ≥2 m in the entire study area has a power-law index of −3.9 +0.2/−0.3 (1499 boulders ≥2 m/km²), suggesting that the Aswan site is mainly dominated by gravitational events triggered by sublimation and/or thermal insolation weathering causing regressive erosion. The boulder size-frequency distribution versus gravitational slopes indicates that when higher gravitational slope terrains are considered, only boulders ≤10 m are identified, as well as steeper power-slope indices. In addition, no boulders ≥2 m are observed on slopes ≥50°. This may indicate that larger blocks detached from a sublimating cliff cannot rest at these slopes and consequently fall down. The spectral analysis performed on the site shows that despite different morphologic units, no spectral differences appear in the multiple textures. This may confirm a redistribution of particles across the nucleus as a consequence of airfall, whether coming from Hapi or from the southern hemisphere when it is active during perihelion.