Geologic analysis of the Rosetta NavCam, Osiris and ROLIS images of the comet 67P/Churyumov-Gerasimenko nucleus

This paper is based on geologic analysis of the surface morphology of nucleus of the Jupiter family comet 67P. This comet was visited by the ESA mission Rosetta, which escorted the comet since May 2014 till the end of September 2016 and studied it by 11 instruments of the mission orbiter and 10 instruments of the lander. The nucleus is ~4km in diameter, has a bilobate shape with the smaller (Head) and larger (Body) lobes, and the narrow neck between them. For the analysis, primarily images taken by the Rosetta Navigation camera (NavCam) were used and then complemented by selected images from the ROLIS and OSIRIS cameras. Two major types of the nucleus material are distinguished by us and other researchers: 1) the consolidated nucleus material and 2) the loose material, a kind of cometary regolith, covering the nucleus consolidated material. On the surface of the consolidated material rather long (up to hundreds meters) straight lineaments are distinguishable. They probably correspond to fractures and in some cases to strata. Their presence suggests that the consolidated material is rather compact and lacks voids larger than tens of meters across. Surfaces of consolidated nucleus material typically show knobby appearance at the scales from tens of meters and meters to centimeters and millimeters. This suggests that this material is grainy, consisting of more and less resistant (to surface weathering) “particles” on the scale of the visible knobs. The geometric analysis of steep slopes based on the nucleus shape model allowed us to estimate a tensile, shear and compressive strength of the consolidated material. It was shown that the 67P consolidated nucleus material is very fragile, and taking into account the scale effect one can conclude that it is as fragile as fresh fallen snow and maybe even more fragile. In addition, estimates of the compressive strength of the surface material were considered at the sites of the first and the last contacts of the Philae lander with the surface. Observations also showed evidence of various downslope and lateral movements of rather large material masses (landslide? avalanche?) as well as boulders and “fines”, which are driven primarily by gravity and then by the acquired inertia, but in some cases a material transport by dust-gas jets/outbursts could play a role. The latter could also be responsible for formation of the eolian-type ripples.