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

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

Corresponding author: E. Lellouch, emmanuel.lellouch@obspm.fr

Received: 22 December 2000
Accepted: 14 February 2001

Abstract

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 μm, of the H₂ S(0) and S(1) quadrupolar lines at 17.1 and 28.2 μm, of the methane band at 7.7 μm, and of the CH₃D band at 8.6 μm 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) in Jupiter and 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, (, 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.