Application of Implicit 3D Modelling to Reconstruct the Layered Structure of the Comet 67P

We provide details about the procedure employed for the three-dimensional geological modelling of the lobes of comet Churyumov-Gerasimenko (67P). The two lobes of 67P are characterized by well visible terraces and elongated cliffs that revealed a pervasive layering arranged in an onion-like fashion. None of the layers can be traced for a continuity large enough to provide ameans of stratigraphic correlation. Therefore, an explicit modelling approach of the layered structure is not easily applicable.We show how a very simplemodelling strategy based on implicitly-defined analytical surfaces (spheres or ellipsoids), and requiring very limited operator decision-making, can be successfully applied to produce a geological model that easily fits the available scattered attitude observations. Our formulation has the advantage of providing a small set of parameters with a precise geometricalmeaning that can be compared with other parameters of the lobes (i.e. center of ellipsoids and center ofmass). The presentedmethod originates outside commercial geologicalmodelling software packages and required devising some ad-hoc solutions for the visualization of the resulting models in comparison with observations made on OSIRIS images. Some extracts of the visualization code, helpful for applying this procedure in similar operational contexts, are also presented.