Enrichment in volatiles in the giant planets of the Solar System

We propose an interpretation of the enrichments in volatiles observed in the four giant planets with respect to the solar abundance. it is based on the assumpption that volatiles were trapped in the form of solid clathrate hydrates and incorporated in planetesimals embedded in the feeding zones of each of the four giant planets. The mass of trapped volatiles is then held constant with time. The mass of hydrogen and of riot trapped gaseous species continuously decreased with time until the formation of the planet was completed, resulting in an increase in the ratio of the mass of trapped volatiles to the mass of hydrogen (Gautier et al., Astrophys. J. 550 (2001) L227). The efficiency of the clathration depends upon the amount of ice available in the early feeding zone. The quasi-uniform enrichment in At, Kr, Xc C, N, and S observed in Jupiter is reproduced because all volatiles were trapped. The non-uniform enrichment observed in C, N and S in Saturn is due to the fact that CH4, NH3, and H2S were trapped but not CO and N-2. The non-uniform enrichment in C, N and S in Uranus and Neptune results from the trapping of CH4, CO, NH3 and H2S, while N, was not trapped. Our scenario permits LIS to interpret the strongly oversolar sulfur abundance inferred by various modelers to be present in Saturn, Uranus and Neptune for reproducing the microwave spectra of the three planets. Abundances of Ar. Kr and Xe in these three are also predicted. Only Xe is expected to be substantially oversolar. The large enrichment in oxygen in Neptune with respect to the solar abundance, calculated by Lodders and Fegley (Icarus 112 (1994) 368) from the detection of CO in the upper troposphere of the planet, is consistent with the trapping of volatiles by clathration. The upper limit of CO in Uranus does riot exclude that this process also occurred in Uranus. (C) 2003 Elsevier Ltd. All rights reserved.