Laboratory and numerical simulations of the PP-SESAME instrument onboard Philae/ROSETTA for measuring cometary surface permittivity

Measuring the complex permittivity of planetary surfaces provides insigth into their chemical composition and physical state. This can be done using a Mutual Impedance (MI) which consists in a four-electrode array generally (but not necessarily) in contact with the ground, working at a fixed frequency in the ELF-VLF (Extremely Low Frequency-Very Low Frequency) range with a dedicated electronic system [1]. The Permittivity Probe (PP) as part of the SESAME experiment onboard of the Philae/ROSETTA cometary lander relies on the MI concept. However the PP design had to be accomadated to the severe constraints of the Rosetta mission. In particular, parts of the landing gear (LG) and of other instruments are used as electrodes, introducing influences on the measurements to that need to be considered. Unfortunately, as it was not possible to perform it before launch, the calibration of the full system remains to be done. In order to prepare the analysis of PP data, a reduced size mockup of the LG built in DLR (Cologne, Germany) as well as a mockup-in-size of the instrument including a setup for Philae conductive body have been used for calibration tests in LATMOS (Guyancourt, France). Two configurations have been tested: i) tests as the instrument is in open space, as far as possible from walls, ceiling and floor, ii) Tests as the instrument is placed at several heights from a conductive floor. Such configurations aim at reproducing PP operations during the descent to the comet and the phase of first ground measurements. This would provide, together with numerical simulations under COMSOL™ a full model of the PP-SESAME instrument on Philae for the descent and first ground measurements configuration. Then slight corrections to this model would be possible using the in-flight calibration data to receive from the flight model during the descent on the comet. In this paper, we will present and discuss the results obtained with both nominal and reduced-in-size LGs in free space and in the vicinity of a conductive target (water). Such experiments and numerical simulations have never been done before and are crucial for understanding PP measurements. They also offer a unique opportunity to demonstrate the scientific interest of permittivity probes as it is anticipated that similar experiments will be proposed for future planetary missions including a lander or a rover. [1] Grard, R.: A quadrupolar array for measuring the complex permittivity of the ground: application to Earth prospection and planetary exploration, Meas. Sci. Technol.Vol. 1, p. 295, 1990.