Quantitative analysis of isolated boulder fields 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: pamela.cambianica@inaf.it
² INAF Astronomical observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴ Department of Geosciences, University of Padova, Via Giovanni Gradenigo 6, 35131 Padova, Italy
⁵ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁶ CNR-IFN UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy
⁷ Max Planck Institute for Solar System Research, Justusvon-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & Université de Versailles Saint-Quentin-en-Yvelines, Guyancourt, France
⁹ Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain
¹⁰ International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
¹¹ Scientific Support Office, European Space Research and Technology Center/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk, The Netherlands
¹² Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹³ LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Sorbonne Paris Cité UPMC Université Paris 06, Sorbonne Universités, 5 place Jules Janssen, 92195 Meudon, France
¹⁴ Physics Department, Allison Laboratory, Auburn University, Auburn AL 36849, USA
¹⁵ Department of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy
¹⁶ Faculty of Engineering, University of Trento, Via Mesiano 77, 38121 Trento, Italy
¹⁷ INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 38121 Trieste, Italy
¹⁸ Instituto de Astrofísica de Andalucía (CSIS), c/Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹⁹ Deutches Zentrum für Luft- und Raumfahrt (DLR), Institüt für Planetenforschung, Rutherfordstraße 26, 12489 Berlin, Germany
²⁰ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, Chung-Li 32054, Taiwan
²¹ Space Science Institute, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau
²² Insitut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
²³ Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary

Received: 4 December 2018
Accepted: 7 June 2019

Abstract

Aims. We provide a detailed quantitative analysis of isolated boulder fields situated in three different regions of comet 67P/Churyumov-Gerasimenko: Imhotep, Hapi, and Hatmehit. This is done to supply a useful method for analyzing the morphology of the boulders and to characterize the regions themselves.

Methods. We used OSIRIS Narrow Angle Camera images with a spatial scale smaller than 2 m px⁻¹ and analyzed the size-frequency distribution and the cumulative fractional area per boulder population. In addition, we correlated shape parameters, such as circularity and solidity, with both the spatial and the size-frequency distribution of the three populations.

Results. We identified 11 811 boulders in the Imhotep, Hapi, and Hatmehit regions. We found that the Hatmehit and Imhotep areas show power indices in the range of −2.3/−2.7. These values could represent a transition between gravitational events caused by thermal weathering and sublimation, and material formed during collapses that has undergone sublimation. The Hapi area is characterized by a lower power index (−1.2/−1.7), suggesting that those boulders have a different origin. They can be the result of material formed during gravitational events and collapses that has undergone continuous fragmentation. We calculated the cumulative fractional area (CFA) in order to investigate how the area is covered by boulders as a function of their sizes. The Hatmehit and Imhotep regions show a CFA that is well fit by a power law. In contrast, the Hapi area does not show the same trend. We analyzed the fractal distributions, finding that the populations seem to be fractal at all dimensions, except for the Hapi distribution, which shows a possible fractal behavior for small dimensions only. Finally, the average values of the shape parameters reveal solid and roundish boulders in all populations we studied.