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

Maurizio Pajola 1, 2 Nilda Oklay 3 Fiorangela La Forgia 4 Lorenza Giacomini 5 Matteo Massironi 5 Ivano Bertini 2 M. R. El-Maarry 6 Francesco Marzari 4 Frank Preusker 7 Frank Scholten 7 Sebastian Höfner 3 Jui-Chi Lee 8 Jean-Baptiste Vincent 3 Olivier Groussin 9 Giampiero Naletto 2, 10, 11 Monica Lazzarin 4 Cesare Barbieri 2, 4 Holger Sierks 3 Philippe Lamy 9 Rafael Rodrigo 12, 13 Detlef Koschny 14 Hans Rickman 15, 16 Horst U. Keller 17 Jessica Agarwal 3 Michael F. A'Hearn 18 Maria A. Barucci 19 Jean-Loup Bertaux 20 Gabriele Cremonese 21 Vania Da Deppo 11 Björn Davidsson 22 Mariolino de Cecco 23 Stefano Debei 24 Francesca Ferri 2 Sonia Fornasier 19 Marco Fulle 25 Carsten Güttler 3 Pedro J. Gutiérrez 26 Stubbe F. Hviid 7 Wing-Huen Ip 8 Laurent Jorda 9 Jörg Knollenberg 7 J.-Rainer Kramm 3 Michael Küppers 27 Ekkehard Kührt 7 Luisa M. Lara 26 Zhong-Yi Lin 8 Jose J. Lopez Moreno 26 Sara Magrin 4 Harald Michalik 28 Stefano Mottola 7 Nicholas Thomas 6 Cecilia Tubiana 3 
Abstract : 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.
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Maurizio Pajola, Nilda Oklay, Fiorangela La Forgia, Lorenza Giacomini, Matteo Massironi, et al.. Aswan site on comet 67P/Churyumov-Gerasimenko: Morphology, boulder evolution, and spectrophotometry. Astronomy and Astrophysics - A&A, EDP Sciences, 2016, 592, A69 (17 p.). ⟨10.1051/0004-6361/201527865⟩. ⟨insu-01353737⟩



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