In-situ studies of dust and gas in cometary comae

E. Hadamcik, A.C. Levasseur-Regourd (Univ. Paris VI/Aeronomie), N. McBride (Univ. of Canterbury)

The OPE experimemt on-board the Giotto spacecraft had performed optical measurememts of light scattered by dust and emitted by gases in the inner comae of comets 1P/Halley and 26P/Grigg-Skjellerup. New analysis and calibration of the Halley data has provided the evolution of intensity (dust, CO+, CN, OH) and polarization along the line of sight, from 100000 km to closest approach. Drastic differences, due to extended sources, are pointed out between the gaseous species profiles. For distances smaller than 7000 km, local intensities are retrieved and compared with the dust impact detectors data. The evolution of intensity (dust, CO+, C2) and polarization is obtained for Grigg-Skjellerup, for distances to the nucleus below 80000 km, before and after closest approach. This comet, which had a much lower activity than Halley, presents a different behavior for CO+ (no significant extended source) and for dust (polarization of about 10 percent for the whole coma at 90 degrees phase angle, to be compared with 20 percent for Halley at 73 degrees phase angle). Such results are tentatively interpreted in terms of different properties of the cometary dust particles.