Absorptions for the v2 band of deuterated methane (CH 3D) have been observed in the 5-micron spectrum of Saturn, obtained with a Fourier transform spectrometer. Analysis of the band yields a CH 3D abundance of 2.6 ± 0.8 centimeter-amagat and a temperature of 175 ± 30 K for the mean level of spectroscopic line formation. This temperature indicates that a substantial portion of Saturn's flux at 5 microns is due to thermal radiation, and that we are therefore looking fairly deep into its atmosphere, as is the case for the Jupiter 5-micron window. This CH 3D abundance leads to a deuterium/hydrogen ratio of about 2 × 10-5 in Saturn's atmosphere. This ratio is much lower than the terrestrial value but comparable to that determined for Jupiter and may be taken as representative of the deuterium/hydrogen ratio in the solar system at the time of its formation.
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