The D/H ratio in Saturn's atmosphere from Voyager IRIS spectra

A selection of 1656 spectra recorded between 180 and 2300 mm in the 20 to 40 N latitude range by the Voyager interferometer IRIS were analyzed. The CH3D/H2 and CH4/H2 ratios are determined from a best fit of the radiances measured in the nu6 and nu4 fundamental bands, respectively centered at 1161 and 1304 mm. Results are CH3D/H2 = 3.9 + or - 2.5 times 10 to the minus 7th power and CH4/H2 = 4.5 + 2.4, -1.9 x 0.001. The uncertainty includes random errors due to instrumental noise and possible systematic errors in the assumed temperature profile and spectroscopic parameters. The D/H isotopic ratio is determined from abundance ratios from the expression D/H = 1/4f (CH3D/H2)/(CH4/H2) where f accounts for equilibrium deuterium fractionation between the molecular hydrogen and methane phases. Beer and Taylor (1973, 1978) estimated f = 1.37 + or - 0.07 for Jupiter assuming a wide range of convective velocities and neglecting catalytic effects from dust particles. The same value was adopted for Saturn and yields D/H = 1.6 +1.3, -1.2 times 10 to the minus 5th power.