THE D H AND O-18/O-16 RATIOS IN WATER FROM COMET P/HALLEY/

From a detailed evaluation of the ion currents measured by the Neural Mass Spectrometer (NMS) on the Giotto spaceprobe in the 17.5 amu/e to 21.5 amu/e mass range inside the contact surface of P/Halley we derive the cometary ion densities for masses 18 amu/e to 21 amu/e. From thes e we calculate the deuterium and O-18 abundances in the H3O+ ion to be (D/H)(H3O+) = (2.55 +/- 0.18) x 10(-4) and O-18/O-16 = (2.13 +/- 0.18 ) x 10(-3). The errors are 3 sigma-errors of the means of eight measur ements at different distances. These results are in agreement with the pre preliminary evaluation (Eberhardt et al. 1987a) but the errors ar e considerably smaller. In a detailed discussion we show that for deut erium the isotope fractionation between the H3O+ ion and the H2O gas i n the coma is not negligible, mainly due to the isotope exchange react ion H3O+ + HDO reversible arrow H2DO+ + H2O. Based on extensive calcul ations we obtain for the deuterium abundance in the water of the coma (D/H)H2O = (3.02 +/- 0.22) x 10(-4). We show that no significant D/H f ractionation between the H2O in the coma and the ice in the nucleus is expected. We also apply our fractionation correction to the D/H deter mination in the H3O+ ion based on the measurements of the Ion Mass Spe ctrometer (IMS) (Balsiger et al. 1995) which agree within errors with our results. From the independent results of the NMS and IMS we obtain as best value for the isotopic composition in the H2O and H2O-ice of P/Halley the values (D/H)(H2O) = (3.16 +/- 0.34) x 10(-4) and (O-18/O- 16)(H2O) = (2.03 +/- 0.15) x 10(-3). The oxygen isotopic composition i n P/Halley is identical with the average solar system value. Deuterium in P/Halley's water is enriched by a factor of 12 relative to the pro tosolar D/H ratio and by a factor of 2 relative to the terrestrial D/H . We show that this strong deuterium enrichment cannot be explained by processes in the solar nebula such as the H2O + HD reversible arrow H DO + H-2 isotope exchange reaction. The deuterium enrichment in P/Hall ey is rather similar to the enrichment observed by Jacq et al. (1990) in interstellar H2O, implying an interstellar origin of at least a siz eable fraction of the cometary water. This model and other implication s of the high D/H in P/Halley are discussed.