Dielectric Model of Comet 67p/churyumov-gerasimenko in Support of the Consert Radar Tomography Experiment on Board

Introduction: The mechanism of formation of the comet nucleus, and its resulting internal structure is dependent on the initial conditions in the region of comet formation. As primitive building blocks of the solar system, an understanding of the composition, structure and formation of cometary nuclei will provide crucial knowledge on the chemical and physical conditions in the early planetary nebula at the time of planet formation. In an attempt to address this objective, ROSETTA, will intercept the orbit of Comet 67P/Churyumov-Gerasimenko in 2014. At a distance of 3 AU from the Sun, the spacecraft will release the Philae lander, which will touch down on the surface of the comet nucleus. Philae’s payload of instruments will analyze the surface and interior of the comet nucleus. CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) is a Philae instrument that will transmit radiowaves through the comet to be received by ROSETTA. Coupled with the orbiter, Philae will send radar waves through the comet nucleus, and measure the time delay between the transmitted and received signals to reveal the internal structure of the comet nucleus [1]. The challenge, however, is that the internal dielectric structure of Comet 67P is poorly known. Hence, the expected inversion of the physical properties of the nucleus from the CONSERT radar tomography may be significantly indistinct. Therefore, geoelectrical models of plausible internal structures are crucial to understand and minimize ambiguities on the radar investigations of the comet’s interior. The variations of the signal attenuations to be observed in the radar tomographies to be collected in 2014 depend mainly on the variation of the dielectrical properties of the cometary material as a function of porosity, temperature, and mineralogical composition within the comet nucleus. Unfortunately, little to no observations exist on their variability in the comet structure. The potential impact of this variability controls penetration depths and structure identification. Our objective in this work is to integrate dielectric measurements of cometary analog materials under relevant cometary environmental conditions to set comprehensive dielectric models of the nucleus to support future data inversion. Dielectric properties of cometary analogs: The relation between dielectric permittivity, temperature, and dust fraction has been determined on the basis of laboratory measurements of cometary analogs. Relative dielectric permittivities have been collected from measurements of mixtures of chondrite powder (also referred as dust) and water ice. They are plotted as a function of temperature and dust fraction using Lichteneker’s mixing laws [2]. Figure 1 show an example of those measurement for the real part of the relative dielectric permittivity versus dust fraction, taken at a temperature of -60°C (213 K) and extrapolated to 75 and 45 K with an average error range of 13%. Figure 1: Relative dielectric permittivity as a function of dust fraction (real part).