ABUNDANCE AND ORIGIN OF THE CHN-P( IONS IN THE COMA OF COMET)HALLEY/

We analyzed data in the mass/charge group 12-16 amu/e from the High In tensity Sensor (HIS) of the Ion Mass Spectrometer IMS on board the spa cecraft Giotto during the comet Halley flyby. The most abundant ion in this mass/charge range is CH3+. Its parent molecules are either CH2, CH3 or CH4. Hereby, the methane abundance serves as an important clue as to the origin of comets, and has already been discussed several tim es. Detailed analysis of the radial density profiles from the HIS inst rument lead to an upper limit for the methane production of 1% in come t Halley during the Giotto encounter. Even no methane at all is compat ible with our data. We find, however, experimental evidence for a quas i-point source of the CH2 radical with a source strength of 0.27% comp ared to water. CH2 could be due to the presence of heavy organic molec ules which undergo very fast photodissociation or it could be present in the cometary ice as a free radical.