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

¹ NASA Ames Research Center, Moffett Field, CA 94035, USA
e-mail: maurizio.pajola@nasa.gov
² Center of Studies and Activities for Space, CISAS, “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy ;
e-mail: maurizio.pajola@unipd.it
³ INAF Osservatorio Astronomico di Padova, Vic. Osservatorio 5, 35122 Padova, Italy
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁵ Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
⁶ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
⁷ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
⁸ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁹ Geosciences Department, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy
¹⁰ Aix-Marseille Université, CNRS LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹¹ Centro de Astrobiologa, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹² International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹³ Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹⁴ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹⁵ PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁶ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁷ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁸ LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
¹⁹ Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
²⁰ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
²¹ NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²² Department of Mechanical Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²³ UNITN, University of Trento, via Mesiano, 77, 38100 Trento, 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 Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
²⁸ Operations Department European Space Astronomy Centre/ESA, P.O. Box 78, 28691 Villanueva de la Canada, Madrid, Spain
²⁹ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany
³⁰ Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary

Received: 10 May 2016
Accepted: 18 June 2016

Abstract

Aims. 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.