Estimating the strength of the nucleus material of comet 67P Churyumov–Gerasimenko


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Abstract

Consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67P Churyumov–Gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the Philae lander with the nucleus. The strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. Based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 Pa; the shear strength, from ∼13 to ⩾30 Pa; and the compressive strength, from 30 to 150 Pa, possibly up to 1.5 kPa. These are very low strength values. Given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at –10°C. The (compressive) strength of the surface material at the site of the first touchdown of Philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft’s legs and the geometry of the impact pits as 1–3 kPa. For comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by Philae’s legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. There was an attemp to put one of the MUPUS sensors into the surface material at the site of the last contact of Philae with the nucleus. Noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 MPa4 This fairly high strength appears to indicate the presence of highly porous ice with grains “frozen” at contacts.

About the authors

A. T. Basilevsky

Vernadsky Institute of Geochemistry and Analytical Chemistry; Max Planck Institute for Solar System Research; Moscow State University of Geodesy and Cartography

Author for correspondence.
Email: atbas@geokhi.ru
Russian Federation, Moscow; Göttingen; Moscow

S. S. Krasil’nikov

Vernadsky Institute of Geochemistry and Analytical Chemistry; Max Planck Institute for Solar System Research; Moscow State University of Geodesy and Cartography

Email: atbas@geokhi.ru
Russian Federation, Moscow; Göttingen; Moscow

A. A. Shiryaev

Institute of Physical Chemistry and Electrochemistry; Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry

Email: atbas@geokhi.ru
Russian Federation, Moscow; Moscow

U. Mall

Max Planck Institute for Solar System Research

Email: atbas@geokhi.ru
Germany, Göttingen

H. U. Keller

Institute of Geophysics and Extraterrestrial Physics

Email: atbas@geokhi.ru
Germany, Braunschweig

Yu. V. Skorov

Max Planck Institute for Solar System Research

Email: atbas@geokhi.ru
Germany, Göttingen

S. Mottola

Institute of Planetary Research

Email: atbas@geokhi.ru
Germany, Berlin

S. F. Hviid

Institute of Planetary Research

Email: atbas@geokhi.ru
Germany, Berlin

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