Complex Permittivity of Porous Cometary Nuclei, in Relation with the Rosetta Mission
Abstract
Detailed information about cometary nuclei is still to be revealed by a rendezvous mission, such as Rosetta. Two on-board radiometric experiments, MIRO and CONSERT, will respectively determine the brightness temperature of the surface and subsurface near 190 and 563 GHz, and investigate the deep interior by radar tomography through radio waves at 90 MHz [1,2]. To support mission preparation and data analysis, the dielectric properties of the nucleus of comet 67P/Churyumov-Gerasimenko have to be estimated. They are expected to strongly depend, not only upon the composition and temperature, but also upon the porosity of the different regions encountered by electromagnetic waves. Modelling non-gravitational forces suggests that the nucleus has a density of about 370 kg m-3 [3,4]. Such a low bulk density may result from macro-porosity (gravitational aggregates, impact fractures) and micro-porosity (e.g. fluffy dust aggregates). To study the critical relationship between the dielectric constant and the porosity, we have systematically measured the relative complex permittivity over different frequencies, encompassing CONSERT and MIRO frequencies, for 7 samples of highly porous granular materials of volcanic origin (pyroclastic deposits from Etna and NASA JSC-Mars1 simulant), with various sizes ranges between a few tens and hundreds of micrometers. The results, as a function of size (i.e. surface/volume ratio) and frequency (with, e.g., a decrease of the real part of the permittivity with increasing frequency) will be presented and discussed. Additional measurements, under development for other samples of interest for cometary nuclei, will also be analyzed. Support from CNES and NASA is acknowledged. [1] Gulkis et al., Space Sci. Rev., 128, 561, 2007. [2] Kofman et al., Space Sci. Rev., 128, 413, 2007. [3] Davidson and Guttiérez, Icarus 176, 453, 2005. [4] Lamy et al., Space Sci. Rev., 128, 23, 2007.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #44
- Pub Date:
- October 2012
- Bibcode:
- 2012DPS....4451009L