Observational constraints on models for giant planet formation.

The authors review current information about element abundances and isotope ratios in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. The observed enhancement of C/H compared with the solar value favors models for the origin of these bodies that invoke the accretion and degassing of an ice-rock core followed by the accumulation of a solar composition envelope. Titan may represent an example of a core-forming planetesimal. Observations of D/H and other isotope ratios must be accommodated by these models in ways that are not yet completely clear. The authors suggest some additional tests.