The D/H ratio in Jupiter and Saturn from High-resolution Spectral Observations near 8.6 μ m

On December 2 and 4 2000 UT, we used the mid-infrared spectrometer Texes mounted at the NASA/Infrared Telescope Facility (IRTF) to observe Jupiter and Saturn in the 8-13 μ m region. Spectra recorded in the range 1152-1159 cm^-1 at an unprecedented resolution of ∼ 75 000 display the Q-branch of CH₃D with emission lines of stratospheric origin in its core. Emission in the cores of two CH₄ lines are also detected in the same spectral interval. The two sets of lines have similar intensities and probe a similar pressure range centered around 15 mbar on Jupiter and 1 mbar on Saturn. These features allow us to determine the D/H ratio in methane for the two planets with little sensitivity to uncertainties in the flux calibration and atmospheric model.

From a preliminary analysis, we derive D/H ratios in methane of (2.0+/-0.3)x10^-5 on Jupiter and (2.4+/-0.5)x10^-5 on Saturn. Using published fractionation factors, D/H ratios in hydrogen of (1.6+/-0.3)x10^-5 on Jupiter and (1.8+/-0.5)x10^-5 on Saturn are inferred. These results are consistent with those derived from observations with the Infrared Space Observatory (ISO) at lower spectral resolution (Lellouch et al./ 2001, Astron. Astrophys. 370, 610-622) but bear smaller error bars. The D/H ratios in Jupiter and Saturn are identical within error bars, in contrast to previous measurements which suggested that Jupiter's D/H ratio might exceed that of Saturn. The Jovian value agrees with that measured in the Local Interstellar Medium (1.5+/-0.1 x10^-5), indicating a very weak consumption of deuterium in the part of the Galaxy where the Solar System formed over the last 4.5 Gyr.