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

Maurizio Pajola, Nilda Oklay, Fiorangela La Forgia, Lorenza Giacomini, Matteo Massironi, Ivano Bertini, M. R. El-Maarry, Francesco Marzari, Frank Preusker, Frank Scholten, Sebastian Hofner, Jui Chi Lee, Jean Baptiste Vincent, Olivier Groussin, Giampiero Naletto, Monica Lazzarin, Cesare Barbieri, Holger Sierks, Philippe Lamy, Rafael RodrigoDetlef Koschny, Hans Rickman, Horst U. Keller, Jessica Agarwal, Michael F. A'Hearn, Maria A. Barucci, Jean Loup Bertaux, Gabriele Cremonese, Vania Da Deppo, Bjorn Davidsson, Mariolino De Cecco, Stefano Debei, Francesca Ferri, Sonia Fornasier, Marco Fulle, Carsten Guttler, Pedro J. Gutierrez, Stubbe F. Hviid, Wing Huen Ip, Laurent Jorda, Jorg Knollenberg, J. Rainer Kramm, Michael Kuppers, Ekkehard Kurt, Luisa M. Lara, Zhong Yi Lin, Jose J. Lopez Moreno, Sara Magrin, Harald Michalik, Stefano Mottola, Nicholas Thomas, Cecilia Tubiana

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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 the different 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: fineparticle 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.

Original languageEnglish
Article numberA69
JournalAstronomy and Astrophysics
Volume592
DOIs
StatePublished - 1 Aug 2016

Keywords

  • Comets: general
  • Comets: individual: 67P/Churyumov-Gerasimenko
  • Methods: data analysis

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