Interiors of the Giant Planets 413
A clear trend of Jupiter modeling over the past 30 years
is that as we have gained better knowledge of the EOS of
hydrogen, the calculated mass of the core has shrunk.
Surrounding the core is an envelope of hydrogen and
helium. The temperature and pressure at the bottom of the
hydrogen–helium envelope is near 20,000 K and 40 Mbar
for typical models. The gravitational harmonics require the
envelope to be denser at each pressure level than a model
that has only a solar mixture of elements. Thus, the envelope
must be enriched in heavy elements compared to a purely
solar composition. The total mass of heavy elements is
constrained between 10 and 40 Earth masses. If Jupiter had
only a solar abundance of heavy elements, this value would
be 6 Earth masses. This means that, averaged throughout
the planet, Jupiter is enriched in heavy elements over solar
abundances by a factor of 1.5 to 6.
Jupiter’s atmospheric abundance of helium,Y=0.238±
0.007, is less than the solar abundance of about 0.28. This
depletion is likely an indication that the process of helium
differentiation, described more fully in Section 6, may have
recently begun on Jupiter. The interior models do not pro-
vide a sufficiently clear view into the interior structure to
determine if this is the case. The inferred interior structure
is, however, compatible with limited helium differentiation.
Hydrogen and helium compose about 90% of Jupiter’s
mass. Most of the hydrogen exists in the form of metallic hy-
drogen. Jupiter is the largest reservoir of this material in the
solar system. Convection in the metallic hydrogen interior
is likely responsible for the generation of Jupiter’s magnetic
field. The transition from molecular to metallic hydrogen
takes place about 10,000 km beneath the cloud tops, com-
pared to about 30,000 km at Saturn. The exceptionally large
volume of metallic hydrogen is likely responsible for the
great strength of Jupiter’s magnetic field. The relative prox-
imity of the electrically conductive region to the surface
may explain why Jupiter’s magnetic field is more complex
than Saturn’s.
5.3 Saturn
The observational constraints for Saturn are listed in Ta-
ble 1. Although Saturn has less than one-third of Jupiter’s
mass, it has almost the same radius. This is a consequence of
the relative insensitivity of radius to mass for hydrogen plan-
ets in Jupiter and Saturn’s mass range (see Fig. 6). Saturn’s
atmosphere, like Jupiter’s, is enriched in methane and am-
monia. The atmosphere’s carbon enrichment (in the form
of methane) was recently determined to be 7 times the solar
abundance. There is also evidence that Saturn’s atmosphere
has less helium than Jupiter’s but the uncertainties are large
because there has never been a Saturn entry probe. Since
there is no known process by which Saturn could have ac-
creted less helium than Jupiter, another process must be at
work. As noted in Section 2.1, Saturn’s true rotation rate is
uncertain; the following discussion is based on interior mod-
FIGURE 8 The inferred distribution of heavy elements in the
interiors of Jupiter and Saturn. The masses of these planets’
cores (Mcore) are shown as a function of the masses of heavy
elements in their hydrogen–helium envelopes,MZ. The hashed
regions show current preferred models based on new
experimental shock data, which shows that hydrogen is less
compressible than previously thought. However, all models
within the larger boundary are viable at this time. In general,
Saturn has more heavy elements in the central core and less in
the envelope than does Jupiter.els computed by assuming the previously accepted rotation
period of 10 hours, 39 minutes, and 22.4 seconds.
Saturn’s interior is grossly similar to Jupiter’s. The biggest
difference is that it is clear that Saturn has a core of 10
to 20 Earth masses. A sample Saturn model is shown in
Fig. 7. Temperatures inside Saturn are also cooler. In the
model shown in Fig. 7, the temperature and pressure at
the base of the metallic hydrogen envelope are 9000 K and
10 Mbar. There is strong evidence that Saturn’s envelope,
like Jupiter’s, is enriched in heavy elements over solar abun-
dance. The mass of the core and the heavy elements in
the hydrogen–helium envelope, are also shown in Fig. 8.
Again, all models within the solid curve are plausible, but
the hashed regions shows models that are currently pre-
ferred. The total mass fraction of heavy elements in Saturn
is about 2^1 / 2 times greater than in Jupiter. On the whole,
Saturn is enhanced in heavy elements by a factor of 6–14,
relative to the Sun. This may be an indication that more
condensed icy material was available to be incorporated
into Saturn at its location in the solar nebula. Neverthe-
less, as at Jupiter, hydrogen and helium are the dominant
component of Saturn’s mass (∼75%).
Saturn’s somewhat low atmospheric helium abundance
implies that the process of helium differentiation (see Sec-
tion 6) has begun inside the planet. This process results
in removal of helium from the outer molecular hydrogen
envelope of the planet and enhancement of helium in the
deep interior. Thus, the helium fraction should increase