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Title: | Size-frequency distribution of boulders ≥7 m on comet 67P/Churyumov-Gerasimenko | Authors: | PAJOLA, MAURIZIO Vincent, Jean-Baptiste Güttler, Carsten Lee, Jui-Chi BERTINI, IVANO MASSIRONI, MATTEO SIMIONI, EMANUELE MARZARI, FRANCESCO GIACOMINI, LORENZA LUCCHETTI, ALICE Barbieri, Cesare CREMONESE, Gabriele Naletto, Giampiero Pommerol, Antoine El-Maarry, Mohamed R. Besse, Sébastien Küppers, Michael La Forgia, Fiorangela Lazzarin, Monica Thomas, Nicholas Auger, Anne-Thérèse Sierks, Holger Lamy, Philippe Rodrigo, Rafael Koschny, Detlef Rickman, Hans Keller, Horst U. Agarwal, Jessica A'Hearn, Michael F. Barucci, Maria A. Bertaux, Jean-Loup Da Deppo, Vania Davidsson, Björn De Cecco, Mariolino Debei, Stefano Ferri, Francesca FORNASIER, SONIA FULLE, Marco Groussin, Olivier Gutierrez, Pedro J. Hviid, Stubbe F. Ip, Wing-Huen Jorda, Laurent Knollenberg, Jörg Kramm, J. -Rainer Kürt, Ekkehard Lara, Luisa M. Lin, Zhong-Yi Lopez Moreno, Jose J. Magrin, Sara Marchi, Simone Michalik, Harald Moissl, Richard Mottola, Stefano Oklay, Nilda Preusker, Frank Scholten, Frank TUBIANA, Cecilia |
Issue Date: | 2015 | Journal: | ASTRONOMY & ASTROPHYSICS | Number: | 583 | First Page: | A37 | Abstract: | Aims: We derive for the first time the size-frequency distribution of boulders on a comet, 67P/Churyumov-Gerasimenko (67P), computed from the images taken by the Rosetta/OSIRIS imaging system. We highlight the possible physical processes that lead to these boulder size distributions. <BR /> Methods: We used images acquired by the OSIRIS Narrow Angle Camera, NAC, on 5 and 6 August 2014. The scale of these images (2.44-2.03 m/px) is such that boulders ≥7 m can be identified and manually extracted from the datasets with the software ArcGIS. We derived both global and localized size-frequency distributions. The three-pixel sampling detection, coupled with the favorable shadowing of the surface (observation phase angle ranging from 48° to 53°), enables unequivocally detecting boulders scattered all over the illuminated side of 67P. <BR /> Results: We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km<SUP>2</SUP>), with a global number density of nearly 100/km<SUP>2</SUP> and a cumulative size-frequency distribution represented by a power-law with index of -3.6 +0.2/-0.3. The two lobes of 67P appear to have slightly different distributions, with an index of -3.5 +0.2/-0.3 for the main lobe (body) and -4.0 +0.3/-0.2 for the small lobe (head). The steeper distribution of the small lobe might be due to a more pervasive fracturing. The difference of the distribution for the connecting region (neck) is much more significant, with an index value of -2.2 +0.2/-0.2. We propose that the boulder field located in the neck area is the result of blocks falling from the contiguous Hathor cliff. The lower slope of the size-frequency distribution we see today in the neck area might be due to the concurrent processes acting on the smallest boulders, such as i) disintegration or fragmentation and vanishing through sublimation; ii) uplifting by gas drag and consequent redistribution; and iii) burial beneath a debris blanket. We also derived the cumulative size-frequency distribution per km<SUP>2</SUP> of localized areas on 67P. By comparing the cumulative size-frequency distributions of similar geomorphological settings, we derived similar power-law index values. This suggests that despite the selected locations on different and often opposite sides of the comet, similar sublimation or activity processes, pit formation or collapses, as well as thermal stresses or fracturing events occurred on multiple areas of the comet, shaping its surface into the appearance we see today. | Acknowledgments: | We thank the anonymous referee for important and constructive comments that lead to a substantial improvement of the paper. The identification of the boulders in high-resolution images has been one of the products required by the lander team to OSIRIS and has been accomplished by using OSIRIS NAC and WAC images. The position, surface, and largest size of each boulder located on the five proposed Philae landing sites, and specifically the two finalists (Agilkia and site C), were identified by Maurizio Pajola (UPD-Padova, Italy) and Jean-Baptiste Vincent (MPS-Göttingen, Germany). The final products were delivered to the lander team on 30 September 2014. OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, in Göttingen, Germany, the CISAS-University of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the Instituto de Astrofisica de Andalucia, CSIC, Granada, Spain, the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain, the Universidad Politechnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), Italy (ASI), France (CNES), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. We thank the ESA teams at ESAC, ESOC, and ESTEC for their work in support of the Rosetta mission. The team in Taiwan was supported by the Ministry of Science and Technology of Taiwan. We made use of the Arcgis 10.2 software together with IDL, Matlab, and R software to perform the presented analysis. | URI: | http://hdl.handle.net/20.500.12386/23148 | URL: | https://www.aanda.org/articles/aa/abs/2015/11/aa25975-15/aa25975-15.html | ISSN: | 0004-6361 | DOI: | 10.1051/0004-6361/201525975 | Bibcode ADS: | 2015A&A...583A..37P | Fulltext: | open |
Appears in Collections: | 1.01 Articoli in rivista |
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