Schematic representation of the interiors of Jupiter, 
Saturn, Uranus and Neptune. The hashed region indicate a possible
radiative zone (in
Jupiter, it corresponds to P~0.15 to 0.6 GPa, T~1450 to
1900 K, and R~0.990 to 0.984 Rjup; in Saturn, it is
located around P~0.5 GPa, T~1700 K, R~0.965 Rsat, but it is probably very
marginal. The range of temperatures for Jupiter and Saturn
is for models neglecting the presence of the inhomogeneous
region. Helium mass mixing ratios Y are indicated. In the case of
Saturn, it is assumed that Y/(X+Y)=0.16 in the molecular region.
The size of the central rock and ice cores of Jupiter and Saturn is
very uncertain.
Two representative models of Uranus and Neptune are shown, but their
actual interior structure may be significantly different.
(From Guillot 1999; Science 286, 72).