The D/H and ^18^O/^16^O ratios in water from comet P/Halley.

Details The D/H and ^18^O/^16^O ratios in water from comet P/Halley. The D/H and ^18^O/^16^O ratios in water from comet P/Halley.

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...302..301e&link_type=abstract

Astronomy and Astrophysics, v.302, p.301

Astronomy and Astrophysics

Astrophysics

96

Comets: P/Halley, Comets: General, Molecular Processes, Plasmas

Scientific paper

From a detailed evaluation of the ion currents measured by the Neutral Mass Spectrometer (NMS) on the Giotto spaceprobe in the 17.5amu/e to 21.5amu/e mass range inside the contact surface of P/Halley we derive the cometary ion densities for masses 18amu/e to 21amu/e. From these we calculate the deuterium and ^18^O abundances in the H_3_O^+^ ion to be (D/H)_H_3_O^+^_=(2.55+/-0.18)x10^-4^ and ^18^O/^16^O=(2.13+/-0.18)x10^-3^. The errors are 3σ-errors of the means of eight measurements at different distances. These results are in agreement with the preliminary evaluation (Eberhardt et al. 1987a) but the errors are considerably smaller. In a detailed discussion we show that for deuterium the isotope fractionation between the H_3_O^+^ ion and the H_2_O gas in the coma is not negligible, mainly due to the isotope exchange reaction H_3_O^+^+HDO<->H_2_DO^+^+H_2_O. Based on extensive calculations we obtain for the deuterium abundance in the water of the coma (D/H)_H_2_O_=(3.02+/-0.22)x10^-4^. We show that no significant D/H fractionation between the H_2_O in the coma and the ice in the nucleus is expected. We also apply our fractionation correction to the D/H determination in the H_3_O^+^ ion based on the measurements of the Ion Mass Spectrometer (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 H_2_O and H_2_O-ice of P/Halley the values (D/H)_H_2_O_=(3.16+/-0.34)x10^-4^ and (^18^O/^16^O)_H_2_O_=(2.03+/-0.15)x10^-3^. The oxygen isotopic composition in 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 protosolar 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 H_2_O+HD<->HDO+H_2_ isotope exchange reaction. The deuterium enrichment in P/Halley is rather similar to the enrichment observed by Jacq et al. (1990) in interstellar H_2_O, implying an interstellar origin of at least a sizeable fraction of the cometary water. This model and other implications of the high D/H in P/Halley are discussed.

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