Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko
1 University of Bern, Physics Institute, Sidlerstrasse 5, 3012 Bern, Switzerland
2 UPMC (Sorbonne Univ.); UVSQ (UPSay); CNRS/INSU; LATMOS-IPSL, BC 102, 4 place Jussieu, 75005 Paris, France
3 Aix–Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, Campus universitaire de Saint-Jérôme, avenue Escadrille-Normandie-Niemen, 13013 Marseille, France
4 Observatoire de Paris, LERMA, 61 avenue de l’Observatoire, 75014 Paris, France
5 University of Southern California, Viterbi School of Engineering, 1042 Downey Way, Los Angeles, CA 90089-1112, USA
6 UJF-Grenoble 1/CNRS-INSU, IPAG, UMR 5274, 38041 Grenoble, France
Received: 15 March 2015
Accepted: 15 July 2015
Aims. Permittivity measurements on porous samples of volcanic origin have been performed in the 0.05–190 GHz range under laboratory conditions in support of the Rosetta mission to comet 67P/Churyumov–Gerasimenko, specifically with the MIRO radiometric experiment and CONSERT radar experiment.
Methods. The samples were split into several subsamples with different size ranges covering a few μm to 500 μm. Bulk densities of the subsamples were estimated to be in the 800 to 1500 kg/m3 range. The porosities were in the range of 48% to 65%. From 50 MHz to 6 GHz and at 190 GHz, permittivity has been determined with a coaxial cell and with a quasi-optical bench, respectively.
Results. Without taking into account the volume-scattering effect at 190 GHz, the real part of the permittivity, normalized by the bulk density, is in the range of 2.1 to 2.6. The results suggest that the real part of the permittivity of an ice-free dust mantle covering the nucleus is in the 1.5−2.2 range at 190 GHz. From these values, a lower limit for the absorption length for the millimeter receiver of MIRO has been estimated to be between 0.6 and 2 cm, in agreement with results obtained from MIRO in September 2014. At frequencies of interest for CONSERT experiment, the real part of the permittivity of a suspected ice-free dust mantle should be below 2.2. It may be in the range of 1.2 to 1.7 for the nucleus, in agreement with first CONSERT results, taking into account a mean temperature of 110 K and different values for the dust-to-ice volumetric ratio. Estimations of contributions of the different parameters to the permittivity variation may indicate that the porosity is the main parameter.
Key words: comets: individual: 67P/Churyumov-Gerasimenko / techniques: radar astronomy / methods: laboratory: solid state / comets: general / methods: data analysis
© ESO, 2015