An interpretation of the CONSERT and SESAME-PP results based on new permittivity measurements of porous water ice and ice-basaltic/organic dust mixtures suggests an increase of porosity with depth in 67P

The CONSERT bistatic radar on Rosetta and Philae sounded the interior of the small lobe of 67P/C-G at 90 MHz and determined the average of the real part of the complex permittivity (hereafter e') to be equal to 1.27±0.05 [1,2]. The permittivity probe (PP) of the SESAME package sounded the near-surface in the 400–800 Hz range and derived a lower limit of e' equal to 2.45±0.20 [3,4]. At the time of the measurements, the temperature was found to be below 150 K at Philae's location and expected to be close or below 100 K inside the nucleus [4-6]. The complex permittivity depends of the frequency, the composition, the porosity and the temperature of the material [7,8,9]. These parameters have to be taken into account to interpret the permittivity values. The non-dispersive behavior of e' below 150 K [9], allows us to compare the CONSERT and SESAME-PP results and to interpret their difference in terms of porosity and/or composition. For this purpose we use a semi-empirical formula obtained from reproducible permittivity measurements performed in the laboratory at 243 K on water ice particles and ice-basaltic dust mixtures [10], with a controlled porosity in the 26–91% range and dust-to-ice volumetric ratios in the 0.1–2.8 range. The influence of the presence of organic materials on e' is also discussed based on new measurements of analogues of complex extraterrestrial organic matter [11]. Our results suggest an increase of the porosity of the small lobe of 67P with depth [11], in agreement Lethuillier et al. [4]'s conclusion using a different method. [1]Kofman et al., 1998. Adv. Space Res., 21, 1589. [2]Ciarletti et al., 2015. A&A, 583, A40. [3]Seidensticker et al., 2007. Space Sci. Rev., 128, 301. [4]Lethuillier et al., 2016. A&A, 591, A32. [5]Spohn et al., 2015. Science, 349, aab0464. [6]Festou et al. (Eds.), Comets II. Univ. of Arizona Press. [7]Campbell and Ulrichs, 1969. J. Geophys. Res., 74, 5867. [8]Brouet et al., 2015. A&A, 583, A39. [9]Mattei et al., 2014. Icarus, 229, 428. [10]Brouet et al., 2016. J. Geophys. Res., under review. [11]Brouet et al., 2016. MNRAS, Rosetta special issue, submitted.