408 Encyclopedia of the Solar System
FIGURE 4 Phase diagram for hydrogen, the
main constituent of Jupiter and Saturn. The
approximate domains of liquid metallic
hydrogen and molecular hydrogen are
shown along with approximate interior
temperature profiles for the Jovian planets.
The shaded area indicates the approximate
region in the interior of Saturn and, possibly,
Jupiter where helium and metallic hydrogen
cannot coexist in equilibrium. The locations
of phase boundaries are highly uncertain
except for the liquid to solid transition of H 2.
A“+” marks the highest pressure at which
the conductivity of hydrogen has been
measured. “Laser shock” shows the
pressure–temperature curve where single
shock experiments reach. The “Z” marks the
highest pressures attained in experiments
where single shocks were created by
accelerated metal plates. Most of the interior
of Jupiter and Saturn exists at temperatures
and pressures greater than can currently be
probed in laboratory experiments.The temperatures and pressures reached in these ex-
periments are the closest that terrestrial laboratories can
come to reliably duplicating the conditions in the interiors
of the jovian planets. For Jupiter, the experiments model
conditions about 90% of the way out from the planet’s cen-
ter. The experiments can equal pressures found at about
70% of Saturn’s radius and 50% of Uranus and Neptune’s.
There is currently a controversy regarding the compress-
ibility of hydrogen at the molecular-to-metallic transition
near pressures of 1 Mbar. The measured density of shock-
compressed liquid deuterium (a heavy isotope of hydrogen)
differs by 50% between data sets using shocks produced by
intense lasers and data sets obtained using shocks produced
by projectiles. We will see later that this discrepancy is our
greatest uncertainty in understanding the interior of Jupiter,
and it is also important for Saturn.
Diamond anvils are used in another type of experiment
to squeeze microscopically small samples of planetary ma-
terials to very high pressure. These experiments are most
easily conducted at room temperature, making them less
applicable to the interiors of the jovian planets.
3.2 Hydrogen
For pressures less than about 1 Mbar, the behavior of molec-
ular hydrogen, H 2 , is understood fairly well from theory and
the shock experiments. At higher pressures such as those
encountered deeper in the interiors of Jupiter and Saturn,
the hydrogen molecules are squeezed so closely together
that they begin to lose their individual identities. Under
these conditions, the hydrogen undergoes a phase transi-
tion to a metallic, pressure-ionized state commonly called
metallic hydrogen. In giant planets, this metallic hydrogen
is fluid, not solid. A shock wave experiment suggests that
this transition occurs near 1.4 Mbar at 3000 K; however,
more work is needed to fully understand this phase transi-
tion. Some theoretical calculations show that the transition
is continuous and may not be complete until a pressure
of 10 Mbar, while others predict an abrupt, discontinuous
(first-order) transition from the molecular to the metallic
phase.
In Jupiter and Saturn, liquid metallic hydrogen con-
sists of a dense mixture of ionized protons and electrons
at temperatures over about 10,000 K. The EOS of liquid
metallic hydrogen is understood well theoretically for pres-
sures above about 10 Mbar, but the EOS is not well con-
strained from 1 to 10 Mbar, the transition region. A hy-
drogen phase diagram and temperature–pressure profiles
for each giant planet are shown in Fig. 4. Because the de-
tailed behavior of hydrogen near the phase transition itself is
not known, various simplifying assumptions must be made
when considering these regions of giant planets. The EOS
in this region is typically based on a mixture of theory and
interpolation.3.3 Helium
Helium has not been as well studied as hydrogen, but
shock wave data do provide information to several hundred
kilobars. Above that pressure, theory must guide models
of the behavior of this element. Though the equations of
state of hydrogen and helium individually are reasonably