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The southern hemisphere of 67P/Churyumov-Gerasimenko: Analysis of the preperihelion size-frequency distribution of boulders ≥7 m

Maurizio Pajola 1, 2 Alice Lucchetti 3 Jean-Baptiste Vincent 4 Nilda Oklay 4 Mohamed R. El-Maarry 5 Ivano Bertini 2 Giampiero Naletto 2, 6, 7 Monica Lazzarin 8 Matteo Massironi 9 Holger Sierks 4 Cesare Barbieri 2, 8 Philippe Lamy 10 Rafael Rodrigo 11, 12 Detlef Koschny 13 Hans Rickman 14, 15 Horst U. Keller 16 Jessica Agarwal 4 Michael F. A'Hearn 17 Maria A. Barucci 18 Jean-Loup Bertaux 19 Steve Boudreault 4 Gabriele Cremonese 3 Vania Da Deppo 7 Björn Davidsson 20 Stefano Debei 21 Mariolino de Cecco 22 Jakob Deller 4 Sonia Fornasier 18 Marco Fulle 23 Adeline Gicquel 4 Olivier Groussin 10 Pedro J. Gutiérrez 24 Carsten Güttler 4 Marc Hofmann 4 Sebastian Höfner 4 Stubbe F. Hviid 25 Wing-Huen Ip 26 Laurent Jorda 10 Jörg Knollenberg 25 J.-Rainer Kramm 4 Ekkehard Kührt 25 Michael Küppers 27 Fiorangela La Forgia 8 Luisa M. Lara 24 Jui-Chi Lee 26 Zhong-Yi Lin 26 Jose J. Lopez Moreno 24 Francesco Marzari 8 Harald Michalik 28 Stefano Mottola 25 Frank Preusker 25 Frank Scholten 25 Nicholas Thomas 5 Imre Toth 29, 10 Cecilia Tubiana 4 
Abstract : We calculate the size-frequency distribution of the boulders on the southern hemisphere of comet 67P Churyumov-Gerasimenko (67P), which was in shadow before the end of April 2015. We compare the new results with those derived from the northern hemisphere and equatorial regions of 67P, highlighting the possible physical processes that lead to these boulder size distributions. Methods. We used images acquired by the OSIRIS Narrow Angle Camera (NAC) on 2 May 2015 at a distance of 125 km from the nucleus. The scale of this dataset is 2.3 m/px; the high resolution of the images, coupled with the favorable observation phase angle of 62◦, provided the possibility to unambiguously identify boulders ≥7 m on the surface of 67P and to manually extract them with the software ArcGIS. We derived the size-frequency distribution of the illuminated southern hemisphere. Results. We found a power-law index of −3.6 ± 0.2 for the boulders on the southern hemisphere with a diameter range of 7−35 m. The power-law index is equal to the one previously found on northern and equatorial regions of 67P, suggesting that similar boulder formation processes occur in both hemispheres. The power-law index is related to gravitational events triggered by sublimation and/or thermal fracturing causing regressive erosion. In addition, the presence of a larger number of boulders per km2 in the southern hemisphere, which is a factor of 3 higher with respect to the northern hemisphere, suggests that the southernmost terrains of 67P are affected by a stronger thermal fracturing and sublimating activity, hence possibly causing larger regressive erosion and gravitational events.
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Maurizio Pajola, Alice Lucchetti, Jean-Baptiste Vincent, Nilda Oklay, Mohamed R. El-Maarry, et al.. The southern hemisphere of 67P/Churyumov-Gerasimenko: Analysis of the preperihelion size-frequency distribution of boulders ≥7 m. Astronomy and Astrophysics - A&A, EDP Sciences, 2016, 592, pp.L2. ⟨10.1051/0004-6361/201628887⟩. ⟨insu-01353704⟩



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