Permittivity of porous granular matter, in relation with Rosetta cometary mission

We report measurements in laboratory conditions of the relative complex permittivity (hereafter permittivity) of porous material on a large range of frequencies from 50 MHz to 190 GHz. Such measurements, developed in preparation of the Rosetta mission to comet 67P/Churyumov-Gerasimenko, specifically for the MIRO radiometric experiment, were obtained with different instrumentations in three frequency bands: 50-500 MHz, 2.45 - 12 GHz and 190 GHz (center-band frequency of the millimeter receiver of MIRO, specially developed for our purpose). Considering the expected properties of cometary nuclei, they were carried out with porous granular materials of volcanic origin, with various sizes ranging from a few to 500μm, i.e. Etna׳s ashes and NASA JSC Mars-1 martian soil simulant. The samples were split into several sub-samples with different size ranges and bulk densities. The real part and the imaginary part of the permittivity remain respectively in the 2.1 - 4.0 range and in the 0.05 - 0.31 range. Volume scattering becomes significant for the measurements at 190 GHz when the mean grain size of sub-samples is greater than about 200μm and implies an increase of the real part and the imaginary part of the permittivity. Without this effect, for any sub-sample, the results are consistent over the frequency range. From 50 MHz to 190 GHz, evidence is provided for a slight decrease of the real part of the permittivity. Bulk densities of the sub-samples, being in the 800-1300 kg m−3 range, were determined during the measurements at 190 GHz. Taking into account the expected bulk density of the nucleus (100-370 kg m−3), as well as temperature for the surface and subsurface (in the 30-300 K range) and its composition (consisting both of silica-rich dust and ices, mostly of water), these first series of results allow an estimate of the real part and the imaginary part of the permittivity of the near-surface of the cometary nucleus: the real part is likely to be lower than 1.6 for non-icy regions and lower than 1.4 for icy regions; the imaginary part is likely to be below 0.09. These estimates represent upper limits relevant for the interpretation of the future data of MIRO.