Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations

¹ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: olivier.groussin@oamp.fr
² Laboratoire GEOPS (Géosciences Paris Sud), Bât. 509, Université Paris Sud, 91405 Orsay Cedex, France
³ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
⁴ Planetary Science Institute, Tucson, AZ 85721-0092, USA
⁵ Institute for Geophysics and Extraterrestrial Physics, 38106 TU Braunschweig, Germany
⁶ Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany
⁷ Department of Physics and Astronomy, Padova University, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ Centro de Astrobiologia (INTA-CSIC), 28691 Villanueva de la Canada, Madrid, Spain
⁹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Scientific Support Office, European Space Agency, 2201 Noordwijk, The Netherlands
¹¹ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹² PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹³ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁴ LESIA, Obs. de Paris, CNRS, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
¹⁵ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹⁶ Centro di Ateneo di Studi ed Attività Spaziali, “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy
¹⁷ Department of Mech. Engineering University of Padova, via Venezia 1, 35131 Padova, Italy
¹⁸ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹⁹ UNITN, Universit di Trento, via Mesiano, 77, 38100 Trento, Italy
²⁰ Physikalisches Institut, Sidlerstr. 5, University of Bern, 3012 Bern, Switzerland
²¹ INAF–Osservatorio Astronomico, via Tiepolo 11, 34143 Trieste, Italy
²² Instituto de Astrofisica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
²³ Institute for Space Science, Nat. Central Univ., 300 Chung Da Rd., 32054, Chung-Li, Taiwan
²⁴ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
²⁵ Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA
²⁶ INAF, Osservatorio Astronomico di Padova, 35122 Padova, Italy
²⁷ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany
²⁸ University of Padova, Department of Information Engineering, via Gradenigo 6/B, 35131 Padova, Italy
²⁹ Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary

Received: 22 April 2015
Accepted: 14 July 2015

Abstract

Aims. We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths).

Methods. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface (Imhotep, Ash, Seth, Hathor, and Agilkia), using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and Philae’s footprint) and mechanical considerations.

Results. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0−20°) are covered by a fine material and contain a few large (>10 m) and isolated boulders; ii) intermediate-slope terrains (20−45°) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1 m to 10 m for the majority of them; and iii) high-slope terrains (45−90°) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3−15 Pa (upper limit of 150 Pa), 4−30 Pa for the shear strength of fine surface materials and boulders, and 30−150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.