Oxygen and other volatiles in the giant planets and their satellites

Giant planet atmospheric composition and satellite densities provide insights into protoplanetary disk conditions. Abundances of condensable species and noble gases in well-mixed atmospheres can distinguish among several giant planet formation scenarios, and satellite densities are first order measurements of ice:rock ratios. Recent work on protosolar abundances, relying on three-dimensional spectroscopic modeling of the solar photosphere, provides the framework for the interpretation of measurements. Model densities of protoplanetary disk condensates are shown as a function of carbon partitioning between CO, CH (sub 4) and organics. Comparison with observed satellite densities shows that Saturn"s icy satellites are inconsistent with solar composition, and must either have formed in a water-rich environment or have suffered a complex collisional history. The larger satellites of the giant planets are consistent with solar composition, with densities that speak of variation in the partitioning of carbon. Thermochemical equilibrium calculations predict water as the deepest tropospheric cloud on Jupiter, the planet with the best-constrained bulk water abundance. Yet cloud base pressure levels, remote spectroscopic water vapor measurements, and in situ mass-spectral measurements have all been unable to distinguish conclusively between subsolar and supersolar Jovian bulk water abundances, due to modeling assumptions and/or the spatially-variable water vapor distribution in Jupiter's troposphere. Modeling of images of lightning flashes is consistent with supersolar water abundances. Galileo probe measurements are consistent with an enrichment factor of 4+ or -2 over the protosolar values for most volatiles other than water (C, N, S, and the noble gases Ar, Kr, and Xe). With that of oxygen unknown, Jupiter's enrichments of other volatiles could be explained in terms of enrichment by heretofore unidentified solar composition icy planetesimals, by planetesimals containing volatiles trapped in water ice clathrates, or by enriched gas in the evolved disk. All models involving delivery of elements by planetesimals require planetesimal formation at temperatures below 40 K, to trap argon and molecular nitrogen. Although atmospheric C/H ratios have been measured for all four giant planets, a conclusive test of the competing formation scenarios cannot be made until O/H is measured on all four planets (extremely difficult on Uranus and Neptune), and abundances of the other volatiles and noble gases are measured for the outer three.