First results of on-comet observations by SESAME/PP

Abstract

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.