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Size-frequency distribution of boulders >=7 m on comet 67P/Churyumov-Gerasimenko

Maurizio Pajola 1 Jean-Bapiste Vincent 2 Carsten Güttler 2 Jui-Chi Lee 3 Ivano Bertini 1 Matteo Massironi 4 E. Simioni 5 Francesco Marzari 6 Lorenza Giacomini 4 Alice Lucchetti 1 Cesare Barbieri 1, 6 Gabriele Cremonese 5 Giampiero Naletto 7 Antoine Pommerol 8 Mohamed Ramy El-Maarry 8 Sébastien Besse 9 Michael Küppers 10 Fiorangela La Forgia 6 Monica Lazzarin 6 Nicholas Thomas 8 Anne-Thérèse Auger 11 Holger Sierks 2 Philippe Lamy 11 Rafael Rodrigo 12, 13 Detlef Koschny 9 Hans Rickman 14, 15 Horst Uwe Keller 16 Jessica Agarwal 2 Michael F. A'Hearn 17 Maria Antonietta Barucci 18 Jean-Loup Bertaux 19 Vania da Deppo 20 Björn Davidsson 14 Mariolino de Cecco 21 Stefano Debei 7 Francesca Ferri 1 Sonia Fornasier 22, 18 Marco Fulle 23 Olivier Groussin 11 Pedro J. Gutiérrez 24 Stubbe F. Hviid 25 Wing-Huen Ip 26 Laurent Jorda 11 Jörg Knollenberg 25 J. Rainer Kramm 2 Ekkehard Kührt 25 Luisa-Maria Lara 24 Zhong-Yi Lin 26 Jose J. Lopez-Moreno 24 Sara Magrin 6 Simone Marchi 27 Harald Michalik 28 Richard Moissl 10 Stefano Mottola 25 Nilda Oklay 2 Frank Preusker 25 Frank Scholten 25 Cecilia Tubiana 2 
Abstract : 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. 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. We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km2), with a global number density of nearly 100/km2 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 km2 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.
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Maurizio Pajola, Jean-Bapiste Vincent, Carsten Güttler, Jui-Chi Lee, Ivano Bertini, et al.. Size-frequency distribution of boulders >=7 m on comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics - A&A, EDP Sciences, 2015, 583, A37 (17 p.). ⟨10.1051/0004-6361/201525975⟩. ⟨insu-01202388⟩



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