Findings from the PP-SESAME experiment on board the Philae/ROSETTA lander on the surface of comet 67P

International audienceThe 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

Permittivity Probe on the Rosetta Lander Philae: Preparation for On-Comet-Phase

Mid of November 2014 the ESA Rosetta spacecraft will send its Lander Philae to the surface of the comet 67P/Churyumov-Gerasimenko. The three landing gear feet carry sensors of the Surface Electrical, Seismic and Acoustic Monitoring Experiments (SESAME), among them the Permittivity Probe (PP) [1]. Together with sensors attached to the MUPUS PEN and the APXS detector lid, PP features three transmitter electrodes and 2 receiver electrodes. Using any combination of two transmitters the quadrupole arrangement can measure the electrical properties of the comet surface material at different depths between 50 cm and about 2 m. The instrument is optimized for the detection of water ice inside the observed volume, its purity and temperature. For PP a critical mission phase is the descent towards the comet surface as after separation from the spacecraft and unfolding of the landing gear the instrument is for the first time in the real measurement configuration while the vacuum condition of space provides a known reference. This phase will be used for calibration of all signal disturbances like stray capacitances caused by cable connections, structure elements and other instruments. During the past year a special test scenario was developed and tested combining the operational requirements from several instruments into an integrated time line. Together with laboratory measurements using flight-equivalent material the electrical properties of the PP instrument network will be modeled. The results set the framework for the analysis of the on-comet observations under different electrode-combinations and geometry conditions. An observation sequence for the first days after landing was defined taking the different observational requirements into account. Reference: [1] Seidensticker, K.J., Thiel, K. and Schmidt, W., 2004: The Rosetta Lander Experiment SESAME and the new Target Comet 67P/Churyumov-Gerasimenko. Astrophys. Space Sci., 311,297-307

Numerical simulations of PP-SESAME/Philae/ROSETTA operations during the Descent Phase and at the surface of the Churyumov-Gerasimenko nucleus

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

Observations performed by the SESAME/Permittivity Probe during the descent and after the landing of Philae upon the nucleus of Comet Churyumov–Gerasimenko

International audienceThe 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

Measuring the permittivity of the surface of the Churyumov-Gerasimenko nucleus: the PP-SESAME experiment on board the Philae/ROSETTA lander

International audienceThe Permittivity Probe (PP-SESAME) on-board the Philae Lander of the ROSETTA mission will de- termine the complex permittivity of the surface of the Churyumov-Gerasimenko nucleus and monitor its variations with time. Doing so, it will provide unique insight into the composition and activity of the comet. In this paper, we present the method we have devel- oped to analyze PP-SESAME active measurements. This method will be tested in May 2014 with a replica of the instrument in the giant ice cave system of Dachstein, in Austria

First results of on-comet observations by SESAME/PP

The Permittivity Probe (PP) as part of the SESAME instrument group performed measurements directly before Philae's separation from Rosetta, during its descent and on the first day on the comet surface at Philae's final landing position. The purpose of PP is the monitoring of the electrical properties of the comet's subsurface material down to a depth of about 2 m and their variation with temperature and distance from the Sun. The used implementation is especially sensitive to the presence and properties of water ice inside the observed volume. As a secondary objective the instrument tries to observe low-frequency plasma waves below 20 kHz as generated by interaction between the comet and the solar wind. The descent measurements were mainly intended to calibrate the instrument in its final configuration with deployed landing gear and without being influenced by the vicinity of the Rosetta structure, while the environmental properties like permittivity of vacuum were known. Possible deviations from this assumption were monitored by RPC and will be taken into account during the detailed data analysis. Unfortunately these measurements were heavily distorted by saturation of the sensitive pre-amplifiers in the landing gear feet, which were for the first time confronted with CONSERT radio pulses operating with deployed antenna. While PPís own transmitter for injecting currents into the medium worked nominally and the measured data can be used for calibrating the injected currents on the comet surface, probably only a small sub-set of the receiver data can be used for the intended stray capacitance calibration after post processing to extract the few time intervals where the receivers were not in saturation. The on-surface measurements were all performed during the initial safe-mode interval while the illuminated period changed into comet night, allowing the observation of the temperature dependence of the measurement parameters. Though the second transmitter electrode attached to the MUPUS PEN and the APXS sensor, not-yet deployed at that time, could not be utilized, the first interpretation of data collected in this reduced configuration indicate the presence of cold water ice at least underneath the -Y foot of Philae. The distance between the +Y foot and the surface is either too large to see a similar effect or the subsurface material in the partly sun-lit area of the current location is depleted of ice. Once the additional transmitter electrodes at the deployed MUPUS PEN and APXS can be utilized during a later LTS mission, the initial measurements can be re-evaluated in more detail

Laboratory calibrations of the PP-SESAME instrument on Philae for measuring the cometary surface permittivity

The complex permittivity of terrestrial and planetary grounds can be derived from Mutual Impedance (MI) measurements using a four-electrode array [1]; the system is working at a fixed frequency with the electrodes not necessarily in contact with the ground and with a dedicated electronic system. This concept was used to build the Permittivity Probe (PP) as part of the SESAME experiment of the Philae Rosetta cometary lander. However severe constraints due to the payload facilities and to the particular environment lead to the actual design of the instrument. Unfortunately it was not possible to perform calibrations of the full system before lauch and the ground model consists of several parts used by various instruments. Here we report the results of basic calibration tests performed with a model of the Philae Landing Gear built in DLR. These tests involve only the three feet electrodes and a mockup of the the Philae body with very simple and well defined targets for characterizing the instrument. Further measurements on natural targets would be the next step

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

Electrical properties of the first meters of 67P/Churyumov-Gerasimenko’s nucleus as constrained by PP-SESAME/Philae/Rosetta

International audienceOn 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

Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko. As constrained by the Permittivity Probe SESAME-PP/Philae/Rosetta

International audienceComets 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 <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer