Statistics – Applications
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.248l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 248
Statistics
Applications
3
Scientific paper
The present-day D/H ratio of the solar system is well known. In meteorites, water is under the form of -OH bearing minerals with D/H ratio lying in the range 140.10^-6 to 180.10^-6. On the Earth, the D/H ratio of water is close to 155.10^-6. Such a value demonstrates clearly the common origin of water on Earth and in meteorites. The giant planets, which are assumed to have kept the primordial molecular hydrogen from the protosolar nebula have a ratio close to 30.10^-6. The aim of the present work is to determine if it is possible to obtain, via isotopic exchange processes, a meteorite-like D/H ratio during the life-time of the nebula. Thus isotopic exchange rate constants for the hydrogen-water and hydrogen-methane reactions were measured: HD + H2O <--> H2 + HDO & HD + CH4 <--> H2 + CH3D D2O-H2 and CD4-H2 mixtures were prepared at several temperatures and pressures. After the reaction, molecular hydrogen is separated cryogenically from CD4 or D2O. Hydrogen is analyzed by mass spectrometry. The duration and the temperature of these experiments permit a precise determination of the rate constants. Catalytic effects were also studied, on SiO2, clays and organic polymers: exchange rates do not markedly increase relative to those determined without catalysts. Based on our present data, several numerical applications to the nebula were performed using Cameron's model [1] with the following parameters: total H2 pressure in the nebula between 10^-3 and 10^-4 atm for 1 to 3 A.U.; life-time of the nebula: 30 m.y.; maximum temperature reached by the nebula: 1300 K; cooling rate: t(sub)1/2 ranging from 3 to 6.10^6 years; initial D/H ratio in hydrogen: 30.10^-6. These calculations led us to the following conclusions: 1) Deuterium exchange with CH4 is always slower than with H2O. 2) The maximum deuterium enrichment depends markedly on the total pressure in the nebula. 3) Deuterium enrichments similar to those measured in carbonaceous chondrites can be reached for distances from the Sun >= 2 A.U. Therefore, as suggested 20 years ago [2], the origin of the deuterium enrichment in meteorites and in the terrestrial oceans is satisfactorily accounted for by a simple model of isotope exchange in the solar nebula. References: [1] Cameron, (1972) Symposium on the origin of the Solar System-Nice. [2] Geiss & Reeves, (1972) Astron. Astrophys., 18, 126-132.
Lecluse Christine
Robert Frédéric
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