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

Constraints on Titan's atmospheric conductivity and buried ocean depth, disclosed by the Schumann resonance

International audienceAfter six years of a thorough data analysis of the data collected by the Permittivity, Wave and Altimetry (PWA/HASI) experiment during the descent of the Huygens Probe through Titan's atmosphere in January 2005, we report the major findings inferred from the measurements of low frequency waves and atmospheric conductivity. The observations display a Schumann resonance trapped within Titan's atmospheric cavity. In this presentation, we describe the characteristics of the observed mode, that allow us to constrain the parameters of the ionospheric cavity and to infer the presence of a conductive water-ammonia ocean buried below the surface, at a likely depth of 70 ± 10 km

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

Comment on "An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe" by J. A. Morente et al.

International audienceComment on “An analysis of VLF electric field spectrameasured in Titan’s atmosphere by the Huygens probe”by J. A. Morente et a

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

Analytic theory of Titan's Schumann resonance: Constraints on ionospheric conductivity and buried water ocean

International audienceThis study presents an approximate model for the atypical Schumann resonance in Titan's atmosphere that accounts for the observations of electromagnetic waves and the measurements of atmospheric conductivity performed with the HASI-PWA (Huygens Atmospheric Structure and Permittivity, Wave and Altimetry) instrumentation during the descent of the Huygens Probe through Titan's atmosphere in January 2005. After many years of thorough analyses of the collected data, several arguments enable us to claim that the Extremely Low Frequency (ELF) wave observed at around 36 Hz displays all the characteristics of the second harmonic of a Schumann resonance. On Earth, this phenomenon is well known to be triggered by lightning activity. Given the lack of evidence of any thunderstorm activity on Titan, we proposed in early works a model based on an alternative powering mechanism involving the electric current sheets induced in Titan's ionosphere by the Saturn's magnetospheric plasma flow. The present study is a further step in improving the initial model and corroborating our preliminary assessments. We first develop an analytic theory of the guided modes that appear to be the most suitable for sustaining Schumann resonances in Titan's atmosphere. We then introduce the characteristics of the Huygens electric field measurements in the equations, in order to constrain the physical parameters of the resonating cavity. The latter is assumed to be made of different structures distributed between an upper boundary, presumably made of a succession of thin ionized layers of stratospheric aerosols spread up to 150 km and a lower quasi-perfect conductive surface hidden beneath the non-conductive ground. The inner reflecting boundary is proposed to be a buried water-ammonia ocean lying at a likely depth of 55 - 80 km below a dielectric icy crust. Such estimate is found to comply with models suggesting that the internal heat could be transferred upwards by thermal conduction of the crust, while convective processes cannot be ruled out

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