  New equations of state (EOS) for hydrogen, helium, and compounds containing
heavier elements are used to construct models for the structures of the planets
Jupiter and Saturn. Good agreement with the gravitational moments J2 and J4 is
obtained with a model that uses a two-layer gas envelope, in which the inner
region is denser than the outer one, together with a small, dense core. It is
possible to match J2 with a homogeneous envelope, but an envelope with a denser
inner region is needed to match both moments. The two-layer envelope also gives
good agreement with the global oscillation data for Jupiter. In Jupiter, the
boundary between the inner and outer envelopes occurs at 319 GPa, with an 8%
density increase. In Saturn, it occurs at 227 GPa, with a 69% density increase.
The differences between the two planets show that the need for a density
increase is not due to EOS errors. It is also shown that helium enrichment
cannot be the cause of the density increase. The phenomenon can be explained as
the result of enrichment of heavy elements from planetismals that were
dispersed and mixed with the gases during planet formation. This conclusion is
consistent with planetary formation models like that of Pollack, et al.
