Broadband permittivity measurements on porous planetary regoliths simulants, in relation with the Rosetta mission to 67P/C-G
Résumé
The Rosetta mission has successfully rendezvous comet 67P/Churyumov-Gerasimenko (hereafter 67P) last year and landed Philae module on its nucleus on 12 November it 2014. Among instruments onboard Rosetta, MIRO [1], composed of two radiometers, with receivers at 190 GHz and 563 GHz (center-band), is dedicated to the measurements of the subsurface and surface brightness temperatures. These values depend on the complex relative permittivity (hereafter permittivity) with ε' and ε " the real and imaginary parts. The permittivity of the material depends on frequency, bulk density/porosity, composition and temperature [2]. Considering the very low bulk density of 67P nucleus (about 450 kg.m-3 [3]) and the suspected presence of a dust mantle in many areas of the nucleus [4], investigations on the permittivity of porous granular samples are needed to support the interpretation of MIRO data, as well as of other microwave experiments onboard Rosetta, e.g. CONSERT [5], a bistatic penetrating radar working at 90 MHz. We have developed a programme of permittivity measurements on porous granular samples over a frequency range from 50 MHz to 190 GHz under laboratory conditions (e.g. [6] and [7]). We present new results obtained on JSC-1A lunar soil simulant and ashes from Etna. The samples were split into several sub-samples with different size ranges covering a few to 500 µm. Bulk densities of the sub-samples were carefully measured and found to be in the 800-1400 kg.m-3 range. Sub-samples were also dried and volumetric moisture content was found to be below 0.6%. From 50 MHz to 6 GHz and at 190 GHz, the permittivity has been determined, respectively with a coaxial cell and with a quasi-optical bench mounted in transmission, both connected to a vector network analyzer. The results demonstrate the dispersive behaviours of ε' between 50 MHz and 190 GHz. Values of ε' remain within the 3.9-2.6 range for all sub-samples. At CONSERT frequency, ε " is within the 0.01-0.09 range for all sub-samples. The single-relaxation Debye model fits relatively well the global behaviour of ε' over the frequency range, thus validating the experimental setups and measurements obtained. Furthermore, results confirm that ε' decreases quasi-linearly with the decreasing bulk density at any frequency, as expected by the mixing formulae. Taking into account possible temperature variations within 67P nucleus [8] and the linear decrease of the permittivity with the temperature, as measured by [9] on JSC-1A sample, these results indicate that, on the near-surface of 67P covered by a free-ice dust mantle at the frequencies of MIRO and CONSERT