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Permittivity probes are non-destructive geophysical prospecting instruments that give access to the low frequency (10 Hz – 10 kHz) electrical properties of the close subsurface. This provides us with information on the composition, porosity, temperature, and heterogeneity of the first meters of the subsurface.Using 4 electrodes, the technique consists in injecting a sinusoidal current of known phase and amplitude between two electrodes (transmitting dipole) and measuring the mutual impedance (complex ratio of measured potential over injected current) between this dipole and a receiving dipole. The complex permittivity (i.e. dielectric constant and conductivity) of the subsurface material is derived from the measured phase and amplitude of the mutual impedance. The frequency range of operation of permittivity probes is low, therefore the quasi static approximation applies. To this day the electrical properties of only two extra-terrestrial surfaces have been studied by permittivity probes, the surface of Titan by the instrument PWA-MIP/HASI/Huygens/Cassini-Huygens and the surface of the nucleus of comet 67P/Churyumov–Gerasimenko by SESAME-PP/Philae/Rosetta.We present a first analysis of the data collected by SESAME-PP at the surface of the comet 67P/Churyumov–Gerasimenko. With the help of (1) precise numerical models of the instrument, (2) field measurements (in a controlled and natural environment) in order to validate the analysis method, and (3) realistic hypothesis on the close environment we were able to constrain the composition and porosity of the first meters of the comet’s nucleus, showing that the subsurface is more compact than its interior. We also reanalysed of the data collected in 2005 by PWA-MIP, offering new explanations for the abrupt change in the electrical properties observed 11 minutes after the landing of Huygens. These new scenarios were built in the light of lab measurements performed at LATMOS on samples of organic matter (tholins), possible analogue of Titan’s surface material., Les sondes de permittivité sont des instruments de prospection géophysique non destructifs qui donnent accès aux propriétés électriques, aux basses fréquences (10 Hz-10 kHz), de la proche subsurface. Ce faisant, elles renseignent sur la composition, porosité, température et éventuelle hétérogénéité des premiers mètres sous la surface.Utilisant généralement 4 électrodes, le principe des sondes de permittivité est simple : il consiste à injecter un courant sinusoïdal de phase et d’amplitude connues entre deux électrodes (dipôle émetteur) et à mesurer l'impédance mutuelle (le rapport complexe entre la tension et le courant injecté) entre ce dipôle émetteur et un dipôle récepteur. La permittivité complexe du matériau de surface, à savoir sa constante diélectrique et sa conductivité électrique, sont alors déduites de la mesure de l’amplitude et de la phase de cette impédance mutuelle. Les fréquences d’opération des sondes de permittivités sont basses là où l’approximation quasi-statique s’applique. A ce jour, les propriétés électriques de seulement deux surfaces planétaires extraterrestres ont été étudiées par des sondes de permittivité : celle de Titan par l’instrument PWA-MIP/HASI/Huygens/Cassini-Huygens et celle du noyau de la comète 67P/Churyumov–Gerasimenko par SESAME-PP/Philae/Rosetta.Nous présentons la première analyse des données obtenues par SESAME-PP à la surface de la comète 67P/Churyumov–Gerasimenko. Grâce à un travail précis (1) de modélisation numérique de l’instrument et de son fonctionnement, (2) de campagne de mesures (en laboratoire et dans des grottes de glace) afin de valider la méthode d’analyse et (3) d’hypothèses réalistes sur l’environnement proche de la sonde, nous avons pu contraindre la composition et surtout la porosité des premiers mètres du noyau cométaire montrant qu’ils étaient plus compacts que son intérieur. Nous avons également travaillé à une nouvelle analyse des données obtenues en 2005 par PWA-MIP proposant notamment de nouveaux scénarios pour le changement brutal de propriétés électriques observé 11 min après l’atterrissage de Huygens. Ces nouveaux scénarios s’appuient, entre autres, sur les mesures de caractérisation électrique menées au LATMOS sur des échantillons de composés organiques (tholins), analogues possibles des matériaux recouvrant la surface de Titan.

International audience; On November 12, 2014, the Philae module landed on the surface of the nucleus of 67P/Churyumov-Gerasimenko. Among the instruments on-board Philae, the Permittivity Probe experiment (hereafter PP-SESAME), which is part of the SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment) package, operated both during descent and on the surface. The primary scientific objective of this experiment is to measure the low frequency (10 Hz-10 kHz) complex permittivity i.e. the dielectric constant and electrical conductivity, of the first meters of the cometary nucleus. Doing so, it aims at providing insights into the composition of the mantle and in particular into the water content and porosity of the first meters below the surface.In this paper, we will present the data acquired at the final landing site of Philae known as Abydos and the approach we have developed to analyze them. We emphasize that, because the configuration of operation of PP-SESAME was far from nominal, we had to adapt our analysis method in order to account for all available constraints on Philae attitude and environment at Abydos. We also had to do without the in-flight calibration performed during the descent phase but unfortunately perturbed by the concurrent operations of the bistatic radar CONSERT.We find that the first meters of the nucleus at Abydos are made of a likely pure dielectric material (i.e. with a null conductivity) which dielectric constant is larger than 2.3. This lower bound of the dielectric constant is significantly higher than the value of 1.27 inferred from the propagation time of the CONSERT signals that propagated through the smaller lobe of the comet in the vicinity of Abydos reaching depths of a few hundreds of meters. Thus, while PP-SESAME measurements put no constraint on the dust-to-ice ratio, they strongly suggest that the first meters of the nucleus are significantly more compacted (with a porosity below 50%) than its interior as sensed by CONSERT (found to have a porosity in of 75 to 85%, consistent with the low density of the nucleus). In light of other Philae and Rosetta observations, we will discuss the implications of these findings in terms of formation and evolution of cometary mantles.

International audience; The SESAME experiment (Surface Electric Sounding and Acoustic Monitoring Experiment) is part of the lander Philae of the ESA Rosetta mission that landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. SESAME is a suite of the three instruments CASSE, DIM and PP that use different measurement techniques in order to determine physical properties of the surface and the activity of 67P. SESAME was operated successfully during the descent until the end of the First Science Sequence (FSS). The Comet Acoustic Surface Sounding Experiment (CASSE) can record and generate acoustic vibrations with two kinds of sensors in the range from 50 Hz to 10 kHz. It was operated during the descent and measured completely Philae's first touchdown at Agilkia at 15:34:04 UTC with three triaxial accelerometers mounted in Philae's feet. These data, which are influenced by the properties of Philae's landing gear and by the complex surface layer at Agilkia with a regolith ranging from dust to boulders, describe the sequence of Philae's surface contacts. At the final landing site Abydos, CASSE recorded many signals generated by the hammering of the Philae experiment MUPUS-Pen, in order to determine sound velocities of the cometary surface layer and related elastic properties. Finally, CASSE conducted sounding measurements from foot to foot. The Dust Impact Monitor (DIM) used three piezoelectric sensor plates orthogonally mounted on a cube to detect impacts of sub-millimeter and millimeter-sized ice and dust particles that are emitted from the cometary surface. DIM measurements during the 7 h descent to the cometary surface were hampered by the fact that none of the sensor plates pointed to the cometary surface. Nevertheless, one millimeter- sized particle was detected at an altitude of 2.4 km. Comparison with laboratory data showed that the particle properties are compatible with a porous grain having a bulk density of approximately 250 kg m**-3. At Abydos, DIM recorded no particle impacts, which may be due either to low cometary activity at this place or shading by obstacles close to Philae or both. The SESAME Permittivity Probe (PP) is a combined quadrupole permittivity detector and plasma wave sensor. The unplanned landing in Abydos required many modifications to the FSS operation plan. Thus, PP could study the surface layer with only a three-electrode configuration reducing the resolution significantly. Nevertheless, PP's data constrain the relative permittivity of the surface material to about 2.5 at a surface temperature of about -160 °C for the position of Philae's foot –Y, which was in permanent shadow. This result suggests that the first meters of the surface of Abydos are more compact and/or richer in dust than its interior as probed by the orbiter/lander experiment CONSERT. Additionally, PP detected weak plasma wave activities at Abydos two hours after local sunset. By combining data from different FSS operation periods, we were able to determine temperature profiles of the CASSE sensors in each Philae foot over 9.2 h of a 12.4 h comet day. With proper modeling, these data could give the surface temperature variation at Abydos during FSS.

Context. Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment−Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be >2.45 ± 0.20, which is consistent with a porosity

International audience; The Permittivity Probe (PP-SESAME [1]) on-board the Philae Lander of the ROSETTA mission was designed to constrain the complex permittivity of the first 2 meters of the nucleus of comet 67P/Churyumov-Gerasimenko and to monitor its variations with time. Doing so, it is meant to provide unique insight into the composition (and activity if data could have been acquired longer) of the comet. In this paper, we present the analysis of the PP-SESAME measurements acquired during the first science sequence, on November 13, 2014, on the surface of the comet.

International audience; The Permittivity Probe (PP), a component of the SESAME instrument on board Rosetta's Lander Philae, was operated prior to the separation of Philae from Rosetta, during the descent and at the location of the final landing site. The working principle of PP consists in measuring, with a receiving dipole, the voltage induced in the medium by a current of known phase and amplitude injected by a transmitting antenna. The primary objective of PP is to analyse the electrical properties of the comet surface material down to a depth of about 2 m, and to record their variations with temperature, solar illumination and heliocentric distance. These observations are particularly sensitive to the concentration of water ice at the landing site. The second objective of the instrument is to monitor the spectrum of the electromagnetic and electrostatic waves generated by the interaction between the comet and the solar wind at frequencies of up to 20 kHz. The measurements performed during the descent were mainly devoted to the calibration of the instrument in its nominal configuration, with deployed landing gear and away from the Rosetta spacecraft influence, in an environment of known permittivity, either a vacuum or a plasma whose density and temperature would have been derived from the LAP and MIP data. This approach is unfortunately invalidated owing to the fact the PP receiver was most of the time saturated by the operation of the CONSERT radar during the descent, an interference which seemed to have been minimized during in-flight interference tests, but which was significantly stronger after separation of Philae from Rosetta. Nevertheless, it was possible to recover some information about the instrument's transmitter and receiver performances then used during the analysis of the data measured on the cometary surface.

International audience

International audience; Ten years after the successful landing of the HUYGENS probe on the surface of Titan, we reassess the complex permittivity measurements of the surface materials performed by the PWA-HASI experiment (Permittivity, Waves and Altimetry - Huygens Atmospheric Structure Instrument).The complex permittivity is inferred from the mutual impedance of a classical quadrupolar probe, ie. the ratio of the voltage measured by a receiving dipole over the current emitted by another dipole. Using a simple model of the quadrupole configuration, the dielectric constant of the material at the landing site was first estimated to be of the order of 1.8. A more realistic numerical model that took into account the influence of the HUYGENS gondola yielded a dielectric constant in the range 2-3 and a conductivity in the range 0.4 - 0.8 nS/m. due to uncertainties about the system geometry ( Grard et al., 2006). However, a puzzling experimental fact remains to be explained, namely a sudden variation of the amplitude and phase of the received voltage 11 mn after landing that cannot be associated with any lander mechanical disturbance. Permittivity estimations were based on the first 11 mn sequence.The present analysis takes advantage of a recent analysis of the landing process that provided more realistic final position and attitude for the HUYGENS lander (Schroder et al., 2012). The new results lie within former estimated ranges and attention is paid to their sensitivity to geometry and to the reference measurements collected immediately before landing. This point is particularly critical for the estimation of the conductivity.The complete data set has been analysed, including the sequence collected after the first 11 mn. We consider various scenarios that may explain the observed phase and amplitude discontinuity. We tested two layers ground models in order to investigate the possibility that the upper layer may have experienced a fast physical change due to deliquescence or outgasing. Unfortunately a rigid quadrupolar array measure the average electric properties of the ground and cannot detect any inhomogeneity.We present in addition the measurements made last May in the Dachstein ice cave in Austria, with a mockup of HUYGENS-PWA and a replica of the PP-SESAME instrument onboard the PHILAE lander of ROSETTA

The ROSETTA probe has never been so close to its target; the comet Churyumov-Gerasimenko that it will reach later this year. Among the instruments on board the lander, Philae, the Permittivity Probe (PP) experiment, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) package, will measure the low frequency complex permittivity (i.e. dielectric constant and electrical conductivity) of the first 2 meters of the subsurface of the cometary nucleus. At frequencies below 10 kHz, the electrical signature of the matter is especially sensitive to the presence of water ice and its temperature behavior. PP will thus allow to determine the water ice content in the near-surface and to monitor its diurnal and orbital variations thus providing essential insight on the activity and evolution of the cometary nucleus. The PP instrument is based on the quadrupole array technique, which employs a set of transmitter and receiver electrodes for emitting alternating currents into a medium of interest. The complex permittivity of the cometary surface material is determined by measuring the magnitude and phase shift of both the emitted currents and the resulting potential difference at a pair of receiver electrodes. This technique has been used for many decades on Earth and recently helped to determine the electrical properties of the Huygens landing site on Titan (PWA/HASI experiment on Cassini-Huygens). In the case of PP, 5 electrodes can be used: 2 receiver electrodes are integrated into the lander feet while the transmitter electrodes are mounted on the third foot and on 2 other instruments. In this paper we will present results from numerical simulations performed in order to model PP operations and prepare the scientific return of this experiment. Though simple in theory, the inference of the complex permittivity from PP measurements is not straightforward in practice. In particular, the actual environment of the electrodes (lander body, feet, harpoons...) must be accounted for since the presence of nearby conducting objects will affect the data. We have thus developed a numerical model of the electrodes in their environment using COMSOL Multiphysics®. A simple version of this model was validated by comparison to laboratory measurements and analytical calculations. This model was then used to simulate PP operations during the Descent Phase of the lander (i.e. in the void and as the ground gets closer) and once at the surface of the nucleus considering different types of surfaces. The first set of simulations will be very useful to better understand the calibration data that will be acquired after separation from the ROSETTA Orbiter while the second will illustrate the idealistic sensitivity of PP to the ground electrical properties.

International audience; Almost 10 years ago, the Huygens probe successfully landed on the surface of Titan, Saturn's biggest moon. The Permittivity, Waves and Altimetry (PWA-HASI) instrument onboard then investigated during 32 min the electrical properties at very low frequencies of the top meters of the surface. Preliminary results reported a relative dielectric constant of ~2 and a low electric conductivity. Since then, much has been learnt on the position of the Huygens probe after landing and on the surface characteristics of Titan. Here we describe the refined analysis of the PW A-HASI data and discuss composition scenarios.

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.