HYDROGEN ISOTOPE-EXCHANGE REACTION-RATES - ORIGIN OF WATER IN THE INNER SOLAR-SYSTEM

Rate constants for hydrogen isotope exchange between molecular hydrogen and water (k(T)H-2-HDO) and hydrogen and methane (k(T)H-2-CH3D) have been determined experimentally. The temperature dependence of the constants can be expressed by: k(T)H-2-CH3D = 6.1 . 10(-25) exp(4.38 . 10(3)/T) and k(T)H-2-HDO = 2.0. 10(-22) exp(5.17 . 10(3)/T). with k(T) in cm3/sec. Catalytic effects on charcoal, silica, phyllosilicates, and iron were found to be negligible. A model describing the evolution (with time) of the hydrogen isotopic composition of water in the solar nebula is proposed: while the temperature is decreasing in the gas, isotope exchange between H-2 and H2O yields a deuterium enrichment in water. Using Cameron's numerical models (1972, 1978, 1985) of the solar nebula, we show that the observed difference between the protosolar and the carbonaceous chondrite D/H ratios (i.e., between 30 and 150 10(-6), respectively) is well accounted for by our model, provided that (1) water was accreted as ice at a temperature of less-than-or-equal-to 160 K and (2) the hydrogen pressure was higher than 10(-5) atm somewhere between 1 and 3 AU. In such conditions, the calculations show that meteoritic phyllosilicates did not form in the gas phase, by reactions between water vapour and silicate grains, but by the subsequent circulation of liquid water, originally condensed as ice. Water on Earth (D/H = 155 . 10(-6)) also results from such a cold nebula chemistry.