The Permittivity Probe shall determine the water ice content in the cometary surface layer and its variation with time.
Measuring principle of PP with transmitting (TX) and receiving
To this end PP employs the quadrupole technique, where two electrodes, connected with an AC generator, induce a variable current into the cometary matter. Two additional electrodes detect the resulting voltage variation and phase shift with respect to the injected current. There are three transmitter electrodes on the Philae cometary lander (one in a lander foot and the other two close to be integrated into the sensors of the experiments MUPUS-Pen and APXS). The two receiving electrodes are mounted into the soles of the remaining two feet (see CASSE page). Using the variable geometry of different transmitter electrode-combinations, one can infer the complex permittivity down to a depth of about 2 m.
Thus one can determine the electrical conductivity and the relative permittivity. These material parameters react quite sensitively to the amount of polar molecules, especially water, and their temperature. Using these data one can calculate the water content of the surface, which can vary during one comet day
and with the distance of the comet to the sun, and possibly also with the water-vapor production rate. From this it is possible to describe the outgassing behavior and to infer the composition of the comet.
PP electrode (brown mesh) below the hammering mechanism of the MUPUS-Pen experiment
Operating in passive mode, the PP instrument can detect plasma waves with frequencies up to 10 kHz, which originate from the interaction of the solar wind with the emitted cometary particles and gas and are a measure of the cometary activity. PP was developed and built at the Finnish Meteorological Institute (FMI), Helsinki, in collaboration with the Finnish engineering company Environics Oy and the ESA technology center ESTEC.
Properties of PP
||0.01 to 10 kHz|
||20 up to 40 kHz|
||≤ 320 mW|