• Lasue, Jérémie
• Levasseur-Regourd, A.
• Ciarletti, Valérie
• Herique, Alain
• Kofman, Wlodek

Abstract

The CONSERT bistatic radar onboard the Rosetta spacecraft and the Philae lander has begun to reveal the internal structure of Comet 67P/Churyumov-Gerasimenko, through radio tomographic mapping between the lander and main spacecraft. The small lobe was found to be structurally homogeneous, at the spatial scale of ten meters, corresponding to a few wavelengths of CONSERT instrument [1]. The real part of the relative permittivity has been derived from the travel time of the strongest signals obtained on 12-13 November 2014, from Philae final landing site. Since the final position of the lander was not accurately defined, numerous ray-tracing simulations were performed to constrain the ambiguities on Philae position using the known position of Rosetta and the propagation time and paths inside and outside the nucleus. A least square statistical analysis between measurements and simulations lead to deduce a bulk relative permittivity about (1.27 ± 0.1); meanwhile, the uncertainty in the lander location was reduced to an area of about 21 by 34 square meters [1].Ongoing theoretical and experimental simulations are providing more insights on the nucleus properties. Numerical ray-tracing simulations of the propagation at grazing angles have been performed for various subsurface permittivity models. They establish that a permittivity gradient in the shallow sub-surface would have a strong effect on the wave propagation. The permittivity probably decreases with depth, suggesting that a significant increase of dust/ice ratio with depth is unlikely [2]. Laboratory simulations of the permittivity of subsurface cometary analog materials [3], and of surface porous analog samples [4] have taken place. Results suggest 67P dielectric properties to be mainly controlled by porosity, the dust/ice volumetric ratio to range from 0.4 to 2.6 and the porosity to range from 75 to 85% [1]. Further on-going laboratory measurements will be discussed.Supports from CNES and NASA are acknowledged.[1] Kofman et al. Science 349, 6247 aab0639, 2015.[2] Ciarletti et al. A&A (Rosetta issue), in press, 2015.[3] E. Heggy et al. Icarus 221, 925, 2012.[4] Brouet et al. A&A (Rosetta issue), in press, 2015....

Topics

• porous
• impedance spectroscopy
• surface
• simulation
• dielectric constant
• porosity