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

¹ Center of Studies and Activities for Space, CISAS, G. Colombo, University of Padova, via Venezia 15, 35131 Padova, Italy
e-mail: maurizio.pajola@gmail.com, maurizio.pajola@unipd.it
² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
³ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
⁴ Geosciences Department, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy
⁵ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
⁶ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁷ NAF Osservatorio Astronomico di Padova, Vic. Osservatorio 5, 35122 Padova, Italy
⁸ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
⁹ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
¹⁰ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹¹ Operations Department European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹² Aix Marseille Université, CNRS LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹³ Centro de Astrobiologa, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹⁴ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁵ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹⁶ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁷ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁸ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁹ LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
²⁰ Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
²¹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
²² UNITN, University of Trento, via Mesiano 77, 38100 Trento, Italy
²³ Department of Mechanical Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²⁴ INAF Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²⁵ Instituto de Astrofisica de Andalucia CSIC, Glorieta de la Astronomia, 18008 Granada, Spain
²⁶ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstrasse 2, 12489 Berlin, Germany
²⁷ Solar System Exploration Research, Virtual Institute, Southwest Research Institute, Boulder, CO 80302, USA
²⁸ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany

Received: 26 February 2015
Accepted: 8 June 2015

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.

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.

Results. 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.