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B. Bézard (Obs. Paris-Meudon), T. Greathouse, J. Lacy, M. Richter (U. Texas), C.A. Griffith (LPL, U. Arizona)
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 \mum 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)\times10-5 on Jupiter and (2.4±0.5)\times10-5 on Saturn. Using published fractionation factors, D/H ratios in hydrogen of (1.6±0.3)\times10-5 on Jupiter and (1.8±0.5)\times10-5 on Saturn are inferred. These results are consistent with those derived from observations with the Infrared Space Observatory (ISO) at lower spectral resolution (Lellouch {\em et al.\/} 2001, {\em 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 \times10-5), indicating a very weak consumption of deuterium in the part of the Galaxy where the Solar System formed over the last 4.5 Gyr.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.