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)

1 ARC - NASA Ames Research Center
2 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
3 MPS - Max-Planck-Institut für Sonnensystemforschung
4 Dipartimento di Fisica e Astronomia "Galileo Galilei"
5 Dipartimento di Geoscienze [Padova]
6 Physikalisches Institut [Bern]
7 DLR Institut für Planetenforschung
8 IANCU - Institute of Astronomy [Taiwan]
9 LAM - Laboratoire d'Astrophysique de Marseille
10 DEI - Department of Information Engineering [Padova]
11 IFN - CNR Institute for Photonics and Nanotechnologies
12 CAB - Centro de Astrobiologia [Madrid]
13 ISSI - International Space Science Institute [Bern]
14 ESTEC - European Space Research and Technology Centre
15 CBK - Space Research Centre of Polish Academy of Sciences
16 Department of Physics and Astronomy [Uppsala]
17 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
18 Department of Astronomy [College Park]
19 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
20 PLANETO - LATMOS
21 OAPD - INAF - Osservatorio Astronomico di Padova
22 JPL - Jet Propulsion Laboratory
23 University of Trento [Trento]
24 Department of Mechanical Engineering [Padova]
25 OAT - INAF - Osservatorio Astronomico di Trieste
26 IAA - Instituto de Astrofísica de Andalucía
27 Operations Department (ESAC)
28 IDA - Institute of Computer and Network Engineering [Braunschweig]

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.

Keywords: Comets; 67P/Churyumov-Gerasimenko

Domains: Earth and Planetary Astrophysics [astro-ph.EP]

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, 2016, 592, A69 (17 p.). ⟨10.1051/0004-6361/201527865⟩. ⟨insu-01353737⟩