A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues

The Rosetta spacecraft made a rendezvous with comet 67P/Churyumov-Gerasimenko (67P) in 2014 August, soon after the Philae module landed on the small lobe of the nucleus on 2014 November 12. The CONSERT instrument, onboard Rosetta and Philae, sounded the upper part of the interior of 67P with radiowaves at 90 MHz and determined an average of the real part of the permittivity (hereafter epsilon') equal to about 1.27. The SESAME-PP instrument, onboard Philae, sounded the near-surface of the small lobe in the 400-800 Hz range and determined a lower limit of epsilon' equal to 2.45. We use a semi-empirical formula obtained from measurements of epsilon' performed in the laboratory at 243 K on water ice and ice-basaltic dust mixtures, with a controlled porosity in the 31-91 per cent range and a dust-to-ice volumetric ratio in the 0.12.8 range, to interpret the results of the two instruments, taking into account the temperature and frequency dependences. A graphical method is proposed to derive ranges of porosity and dust-mass fraction from a value of epsilon' derived from observations. The non-dispersive behaviour of epsilon' below 175 K, allows us to compare the values of epsilon'obtained by CONSERT and SESAME-PP. We show that the porosity of the small lobe of 67P increases with depth. Based on new measurements of analogues of complex extraterrestrial organic matter, the so-called tholins, we also suggest that, for the dust component in the cometary material, the presence of silicates has more effect on epsilon' than organic materials.