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)

1 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
2 MPS - Max-Planck-Institut für Sonnensystemforschung
3 Institute of Space Science [Taiwan]
4 Dipartimento di Geoscienze [Padova]
5 OAPD - INAF - Osservatorio Astronomico di Padova
6 Dipartimento di Fisica e Astronomia "Galileo Galilei"
7 Department of Industrial Engineering [Padova]
8 Physikalisches Institut [Bern]
9 RSSD - Research and Scientific Support Department, ESTEC
10 Operations Department (ESAC)
11 LAM - Laboratoire d'Astrophysique de Marseille
12 CAB - Centro de Astrobiologia [Madrid]
13 ISSI - International Space Science Institute [Bern]
14 Department of Physics and Astronomy [Uppsala]
15 CBK - Space Research Centre of Polish Academy of Sciences
16 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
17 Department of Astronomy [College Park]
18 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
19 PLANETO - LATMOS
20 IFN - CNR Institute for Photonics and Nanotechnologies
21 University of Trento [Trento]
22 UPD7 - Université Paris Diderot - Paris 7
23 OAT - INAF - Osservatorio Astronomico di Trieste
24 IAA - Instituto de Astrofísica de Andalucía
25 DLR Institute of Planetary Research
26 IANCU - Institute of Astronomy [Taiwan]
27 SSERVI - Solar System Exploration Research Virtual Institute
28 Institut für Datentechnik und Kommunikationsnetze

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.

Keywords: Data analysis; Comets; 67P/Churyumov-Gerasimenko

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

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, 2015, 583, A37 (17 p.). ⟨10.1051/0004-6361/201525975⟩. ⟨insu-01202388⟩