EDP Sciences
Free access
Volume 370, Number 2, May I 2001
Page(s) 610 - 622
Section The solar system
DOI http://dx.doi.org/10.1051/0004-6361:20010259

A&A 370, 610-622 (2001)
DOI: 10.1051/0004-6361:20010259

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

E. Lellouch1, B. Bézard1, T. Fouchet1, H. Feuchtgruber2, T. Encrenaz1 and T. de Graauw3

1  DESPA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon Cedex, France
2  Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany
3  Space Research Organization of the Netherlands and Kapteyn Institute PO Box 800, 9700 AV Groningen, The Netherlands

(Received 22 December 2000 / Accepted 14 February 2001)

Observations with the Short Wavelength Spectrometer (SWS) onboard the Infrared 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 hydrogen (i.e. from the HD/H2 ratio) and methane (from CH3D/CH4). Observations of the HD R(2) and R(3) rotational lines at 37.7 and 28.5 $\mu$m, of the H2 S(0) and S(1) quadrupolar lines at 17.1 and 28.2 $\mu$m, of the methane $\nu_4$ band at 7.7 $\mu$m, and of the CH3D $\nu_6$ band at 8.6 $\mu$m are analyzed jointly, allowing a retrieval of thermal profiles and molecular abundances. On each planet, the D/H determinations from H2 and CH4 give consistent results, but the accuracy is not sufficient to precisely determine the enrichment factor of D/H in methane. Combining these determinations, we obtain the following values for the D/H ratio in hydrogen: (D/H) $_{\rm H_2}= (2.25\pm 0.35) 10^{-5}$ in Jupiter and (1.70+0.75-0.45) 10-5 on Saturn. These values are consistent with and somewhat more accurate than most earlier values. Comparing with inferences of protosolar D/H from solar wind measurements, it is confirmed that Jupiter is a reliable indicator of the protosolar D/H ratio. The protosolar deuterium abundance inferred from the jovian value, ( $2.1 \pm 0.4) 10^{-5}$, 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. Although the error bars overlap, most measurements, including ours, may point to a slightly smaller D/H ratio in Saturn's atmosphere than in Jupiter's, a surprising result which needs confirmation.

Key words: planets and satellites: Jupiter -- infrared: solar system

Offprint request: E. Lellouch, emmanuel.lellouch@obspm.fr

© ESO 2001