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

Abstract

Aims. We study the link between gravitational slopes and 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, 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 strength, shear strength and compressive strength using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, Philae’s footprint) and mechanical considerations. Results. The different regions show a similar general pattern in terms of relationship between gravitational slopes and terrain morphology: i) low slopes 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 exposing 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. Due to 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 (Skorov & Blum 2012) and tends to favour a formation of comets by pebble accretion in a region of higher concentration of particles like vortices (e.g., Johansen et al. 2014).