The deuterium abundance in Jupiter and Saturn from ISO-SWS observations

Observations with the Short Wavelength Spectrometer (SWS) onboard the Infra red Space Observatory (ISO) are used to determine the D/H ratio in Jupiter' s and Saturn's atmospheres. The D/H ratio is measured independently in hydr ogen (i.e. from the HD/H-2 ratio) and methane (from CH3D/CH4). Observations of the HD R(2) and R(3) rotational lines at 37.7 and 28.5 mum, of the H-2 S(0) and S(1) quadrupolar lines at 17.1 and 28.2 mum, of the methane nu (4) band at 7.7 mum, and of the CH3D nu (6) band at 8.6 mum are analyzed joint ly, allowing;1 retrieval of thermal profiles and molecular abundances. On e ach planet, the D/H determinations from H-2 and CH4 give consistent results , but the accuracy is not sufficient to precisely determine the enrichment factor of D/H ill methane. Combining these determinations, we obtain the fo llowing values for the D/H ratio in hydrogen: (D/H)(H2) = (2.25 +/- 0.35) 1 0(-5) in Jupiter and (1.70(-0.45)(+0.75)) 10(-5) on Saturn. These values ar e consistent with and somewhat more accurate than roost earlier values. Com paring with inferences of protosolar D/H from solar wind measurements, it i s confirmed that Jupiter is a reliable indicator of the protosolar D/H rati o. The protosolar denterium abundance inferred from the jovian value, (2.1 +/- 0.4) 10-5, indicates a minor decrease of the D/H ratio,. over the last 4.55 Gyr, in the part of the Galaxy where the Solar System was formed. Alth ough the error bars overlap, most measurements, including ours, may point t o a slightly smaller D/H ratio in Saturn's atmosphere than in Jupiter's, a surprising result which needs confirmation.