Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko

Abstract

Aims. Permittivity measurements on porous samples of volcanic origin have been performed in the 0. 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/m 3 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.

CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

“…The bulk porosity of the nucleus is thus quite high, ranging from 70 to 80% . In agreement with the bulk density dependence derived from mixing formulae (Campbell & Ulrichs 1969), measurements on porous granular samples confirmed that, as expected, the dielectric constant value (which is the real part of the relative permittivity) systematically decreases with increasing porosity (Heggy et al 2012;Brouet et al 2014) with a typical slope value of about −4.7 × 10 −2 per percent at 90 MHz (Brouet et al 2015).…”

“…Based on the findings that at the comet surface the dust/ice ratio is around 4 (Rotundi et al 2015), the average porosity is around 70-80% , and the temperature below the surface is between 40 K and 190 K (Gulkis et al 2015), the ∆ value of -0.4 can be interpreted as a increase in porosity with depth of 15% for a temperature around 110K (Brouet et al 2015) or as a decrease in the dust-to-ice ratio from 4 to 0.1 at 153 K (Heggy et al 2012). We note that a significantly higher temperature inside the nucleus would require a much lower variation of the dust-to-ice ratio.…”

“…Laboratory measurements performed at low temperature on powered LL5 chondrite show a decrease in permittivity of about 25% for temperatures that decrease from 223 to 113 K (Heggy et al 2001). For porosities increasing from 70% to 80%, an ice-free dust mantle at 110 K would have an decreasing from about 1.72 to 1.39, while a mantle with a dust-toice volumetric ratio equal to 1 would have a lower decreasing from about 1.65 to 1.33 (Brouet et al 2015). Therefore, we can consider that a potential outer layer of the nucleus could have a dielectric constant value of about 1.5 ± 0.2.…”

“…The MIRO and VIRTIS experiments at 67P confirm these assumptions (e.g., Gulkis et al 2015;Capaccioni et al 2015). Consequently, the range of the permittivity values for the interior of the nucleus considered for our parametric study are those of porous and dirty water ice taken from Heggy et al (2012) and Brouet et al (2015), namely a dielectric constant ranging between 1.2 to 2.2. The imaginary part of the permittivity is expected to be low enough that it would only influence the amplitude of the received signal, which is not considered for the present study.…”

“…Indeed, VIRTIS, OSIRIS, and COSIMA observations during the second half of 2014 suggest that the nucleus of 67P is covered by an almost ice-free layer or mantle of dust particles (Capaccioni et al 2015;Schulz et al 2015;Thomas et al 2015). For an ice-free and porous dust mantle at 300 K, the dielectric constant at the frequency of CONSERT is expected to be about two, decreasing from about 2.41 to 1.94 for porosities increasing from 70% to 80% (Brouet et al 2015). However, the temperature at 67P during CONSERT measurements was significantly lower than 300 K: it was in the range of 180-230 K during daytime in VIRTIS surface measurements (Capaccioni et al 2015) and in the range of 40-190 K in MIRO very shallow subsurface measurements (Gulkis et al 2015).…”