Other
Scientific paper
Apr 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004esasp.543..169h&link_type=abstract
Proceedings of the 37th ESLAB Symposium `Tools and Technologies for Future Planetary Exploration', Noordwijk, The Netherlands (E
Other
Scientific paper
Both conductivity and dielectric constant measurements can contribute to the identification of sub-surface materials. They are of great interest in the case of water and ice possibly embedded in other materials due to the high variability with frequency of the dielectric constant of water ice, the high contrast between rocks and liquid water and also the high conductivity generally observed in wet terrains. A first instrument, Permittivity, Waves and Altimetry (PWA-HASI), on the HUYGENS probe should measure the complex permittivity of Titan after landing in January 2005. It consists of a particular mode of the Mutual Impedance (MI) probe designed mainly for atmospheric conductivity measurements. The success of the measurement depends strongly on the configuration of the probe after an uncontrolled landing and in any case the data analysis will be complex as the electrodes are very close to the probe body. A second instrument, the Permittivity Probe (PP-SESAME), on the Rosetta Lander is ready to be launched towards the GuerassimoChuryumov comet in February 2004. In this case safe landing is a major requirement of the mission. The electrode array, using the lander feet and two other hosting deployable parts, is less influenced by the lander body than in the HUYGENS case. However the perturbing influence of neighbouring sensors has to be suppressed by active methods and such a system is better but again complex. In the Netlander project to the surface of Mars, actually in pause after its phase B study, the opportunity to use long GPR electric antennas deployed on the ground as permittivity sensors has been studied and will be implemented in the design with minor modifications. Our goal is to design the future generation of permittivity probes not considered as `add on's but fully optimised for their task, making simpler the analysis and providing also the possibility to calibrate the former space pioneer instruments on selected earth targets. In addition, these future probes should be able to detect also the vertical inhomogeneity of the medium (match with a two layer model). After presenting the actual instruments and projects (on HUYGENS, ROSETTA Lander and NETLANDER), we show the particular interest to use a flat system of electrodes laying on the surface at some distance from the spacecraft body that is particularly well suited for the case of a rover. We will show the design of a prototype actually prepared in CETP to be used in common calibrations with the other instruments in selected well-known terrains. 1. PRINCIPLE AND HERITAGE The measurement of the planetary surface complex permittivity (electrical conductivity and dielectric constant) vs. frequency has a twofold interest: i) to contribute with other parameters to the identification of the close sub-surface materials without penetrating the surface; ii) to characterize the electrical properties of the planetary surface which control the boundary conditions for electromagnetic waves and fields, including possible DC atmospheric electric currents. The mutual impedance (MI) probes of today's planetary missions are the heritage of the quadrupolar probes developed in the first half of the XXth century for oil prospecting [1]. The principle is to inject an AC current I in the planar homogeneous ground of relative permittivity eg through a first dipole and to measure the induced potential by this dipole or by a second dipole to obtain respectively the self and mutual impedances.
Grard Rejean
Hamelin Michel
Laakso Harri
Ney R.
Schmidt Wolfram
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