AN EXPERIMENTAL INVESTIGATION OF ATMOSPHERIC ELECTRICITY AND LIGHTNING ACTIVITY TO BE PERFORMED DURING THE DESCENT OF THE HUYGENS PROBE ONTO TITAN

No terrestrial-like electrical activity was observed during the Voyage r 1 flyby of Titan on 12 November 1980, in spite of a predicted global lightning energy dissipation rate of 4 x 10(-6) Wm-2. This lack of ev idence does not, however, rule out the existence of electrical dischar ges with magnitudes, rates of occurrence and spectral characteristics drastically different from those known on Earth, owing to large dissim ilarities between the temperatures, chemical compositions and, especia lly, electrical conductivities of the two atmospheres. Towards the end of the year 2004, the ESA probe Huygens will be jettisoned from the N ASA Saturn orbiter, Cassini. This probe will descend onto titan and pe rform in situ measurements during a period of 3 h, from an altitude of 170 km down to the satellite surface where the atmospheric pressure r eaches 1.6 x 10(5) Pa. The Huygens scientific payload will include a s et of instruments entirely dedicated to the detection of lightning and to the characterization of the electrical properties of the atmospher e and surface. An electric antenna will search for natural emissions i n the frequency range 0-10 kHz, at altitudes lower than those of ioniz ed layers opaque to electromagnetic waves, and measure the magnitude o f static electric fields due to charge separation. The conductivity of the atmosphere and the existence of free electrons will be checked du ring the whole descent with a combination of quadrupolar and relaxatio n probes; a microphone will also record acoustic phenomena associated with electrical discharges and atmospheric processes. The impedance of the surface will be evaluated from the measurements collected with a radar during the descent and a quadrupolar array after touch down.