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

Maurizio Pajola, Alice Lucchetti, Jean Baptiste Vincent, Nilda Oklay, Mohamed R. El-Maarry, Ivano Bertini, Giampiero Naletto, Monica Lazzarin, Matteo Massironi, Holger Sierks, Cesare Barbieri, Philippe Lamy, Rafael Rodrigo, Detlef Koschny, Hans Rickman, Horst U. Keller, Jessica Agarwal, Michael F. A'Hearn, Maria A. Barucci, Jean Loup BertauxSteve Boudreault, Gabriele Cremonese, Vania Da Deppo, Björn Davidsson, Stefano Debei, Mariolino De Cecco, Jakob Deller, Sonia Fornasier, Marco Fulle, Adeline Gicquel, Olivier Groussin, Pedro J. Gutierrez, Carsten Güttler, Marc Hofmann, Sebastian Höfner, Stubbe F. Hviid, Wing Huen Ip, Laurent Jorda, Jörg Knollenberg, J. Rainer Kramm, Ekkehard Kührt, Michael Küppers, Fiorangela La Forgia, Luisa M. Lara, Jui Chi Lee, Zhong Yi Lin, Jose J. Lopez Moreno, Francesco Marzari, Harald Michalik, Stefano Mottola, Frank Preusker, Frank Scholten, Nicholas Thomas, Imre Toth, Cecilia Tubiana

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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.

Original languageEnglish
Article numberL2
JournalAstronomy and Astrophysics
Volume592
DOIs
StatePublished - 1 Aug 2016

Keywords

  • Comets: general
  • Comets: individual: 67P/Churyumov-Gerasimenko
  • Methods: data analysis

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