Cosmochemical implications of CONSERT permittivity characterization of 67P/CG

Analysis of the propagation of the CONSERT signal throughout the small lobe of 67P/CG nucleus permitted to deduce the real part of the permittivity, at a value of 1.27± 0.05 [1]. The first interpretation of this value using the dielectric properties of mixtures of ices (H2O, CO2), dusts and porosity, led to the conclusion that the comet porosity ranges between 75 - 85%. In addition, the dust to ice ratio was found to range between 0.4-2.6 and the permittivity of dust (including 30% of porosity) was determined to be lower than 2.9. This last value corresponds to a permittivity lower than 4 for a material without any porosity. This paper is intended to refine the dust permittivity estimate by taking into account the updated values of nucleus densities and dust/ice ratio, and to provide further insights into the nature of the constituents of comet 67P/CG. We adopted a systematic approach: determination of the dust permittivity as a function of the Ice / Dust and Vacuum (i.e. porosity) volume fraction and comparison with the permittivity of meteoritic, mineral and organic material from literature and laboratory measurements. Then different composition models of the nuclei corresponding to cosmochemical endmembers of 67P/CG dust are tested. For each of these models the location in the I/D/V diagram is calculated based on available dielectric measurements, and confronted to the locus of 67P/CG. The number of compliant models is small and the cosmochemical implications of each are discussed to conclude on a preferred model [2]. An important fraction of carbonaceous material is required in the dust in order to match CONSERT permittivity observations, establishing that comets represent a massive carbon reservoir. [1] Kofman et al., Science, 349, 6247, aaa0639, 2015. [2] Herique et al., MNRAS, submitted, 2016.