Determination of D/H Ratio on Jupiter from ISO/SWS Observations

Observations of Jupiter, performed on March 29, 1996, with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) in Fabry-Perot mode, have allowed the first detection of the rotational R(2) line of HD at 37.7 mu m (265.3 cm(-1) ). The instrument aperture (a rectangular slit of 17" x 40") was approximately aligned with the jovian polar axis and the spectral resolution was about 31000 (FWHM = 0.0012 mu m). The HD line appears as a ~ 10% deep absorption feature at this resolution. We will present an analysis of these observations in terms of the jovian D/H ratio. Complications in the modelling are due to (1) the observing geometry (the aperture encompasses all ranges of airmasses at Jupiter, and Doppler shift effects due to Jupiter's rotation must be accounted for); and (2) possible opacity effects due to the ammonia clouds (synthetic calculations suggest that the continuum is formed near cloud level and Voyager/IRIS spectra indicate significant and latitudinally-varying cloud opacities in the far-infrared range). A preliminary analysis (Encrenaz et al., submitted to Astron. Astrophys.), in which a zero cloud opacity is assumed, shows that the line profile can be satisfactorily fit with a HD/H_2 mixing ratio of 4.4 x 10(-5) (i.e., D/H = 2.2 x 10(-5) ), but adding a cloud opacity will certainly result in an increase of the D/H ratio. A more refined modelling will be presented, and implications of the derived D/H value will be discussed.