Encyclopedia of the Solar System 2nd ed

404 Encyclopedia of the Solar System

165-170 K

1 bar

6300-6800 K

2 Mbar

15000-21000 K

40 Mbar

Molecular

H 2

Metallic H

Ices(?) + Rocks

~3x solar

Jupiter

helium

enriched

helium

depleted

~3x solar

1.0

0.8

0.1

FIGURE 1 Highly schematic, idealized cross section of the
interior structure of Jupiter. The numbers to the right refer to
the relative radius (r/R) of the core and the
molecular-to-metallic hydrogen phase transition. On the left are
listed the approximate temperatures and pressures at which
these interfaces occur. Arrows indicate convection. Boxes denote
approximate enhancement of elements other than H and He
over the abundance found in the Sun. The core mass is
uncertain, but likely has a mass between 0 and 10 Earth masses.
The real Jupiter is undoubtedly more complex. It is likely that
interfaces are gradual and the composition of the various regions
is inhomogeneous.

helium, methane, ammonia, and water. At 1barpressure
(the pressure at sea level on Earth), the temperature in
Jupiter’s atmosphere is 165 K. Near this level, the ammonia
condenses into clouds; the water condensation level is even
deeper. In the colder atmospheres of Uranus and Neptune,
methane also condenses into clouds. Deeper into the planet
the pressure of the overlying atmosphere compresses the
gas, increasing its temperature and density. This process,
adiabatic compression, is the same one responsible for
the increase in temperature with decreasing altitude on
Earth. One hundred kilometers beneath the cloud tops the
temperature has reached 350 K.
As pressures and temperatures increase, the gas begins
to take on the characteristics of a liquid. Since thecritical
pointof the dominant constituent, molecular hydrogen,
lies at 13 bars and 33 K, there is not a distinct gas–liquid
phase boundary. By several hundred thousand bars the en-
velope closely resembles a hot liquid. This characteristic
of the giant planets—they exist in the supercritical regime
of their primary constituent—leads to their most funda-
mental property: these planets have essentially bottomless
atmospheres.

Deeper into the planet, the temperature and pressure

continue to increase steadily. By 20,000 km beneath the

cloud tops, the temperature reaches 7000 K, and the pres-

sure is 2 Mbar. Recent experiments suggest that by this

point hydrogen, previously present as molecules of H 2 , has

undergone a phase transition to a liquid, metallic state. Most

of the mass of Jupiter consists of thismetallic hydrogen:

protons embedded in a sea of electrons. Helium and other

constituents exist as impurities in the hydrogen soup. For

the remaining 50,000 km to Jupiter’s core, the pressure

and temperature continue to rise, reaching 40 Mbar and

20,000 K in the deep interior. Near the center of the planet,

the composition changes, perhaps gradually, from a pre-

dominantly hydrogen–helium mixture to a combination of

rock and ice. The density of this rock and ice core is 10,000–

20,000 kg m−^3 , higher than the metallic hydrogen density

of about 1000 kg m−^3 (uncompressed water, like that which

comes out of a tap, also has a density of 1000 kg m−^3 ).

Throughout most of the interior, the transport of energy

by radiation is severely hampered by the high opacity of

compressed hydrogen. Other constituents such as methane

and water effectively block energy transport by radiation

in those regions of the spectrum where the hydrogen is a

less powerful absorber. Because conduction of heat by the

thermal motion of molecules is also inefficient,convection

is the prevailing energy transport mechanism throughout

the interior. It had been suggested in the 1990s that in a

thin zone in Jupiter’s interior at temperatures of 1000 to

3000 K energy transport by radiation was in fact dominant.

However, more recent studies suggest this is not the case.

The rising and sinking convective cells in the interior move

slowly, at velocities of just centimeters per second or less.

Because of the continuous nature of the atmosphere, the

wind patterns seen in the belts and zones of Jupiter and

Saturn may have roots that reach into the deep, convective

interior of the planet. Indeed the winds measured by the

Galileospacecraft’s atmosphere probe continued to blow

steadily at the deepest levels reached by the probe, about

20 bars.

The interior of Saturn is much like that of Jupiter. Sat-

urn’s lower mass and consequently lower pressures produce

a smaller metallic hydrogen region. Uranus and Neptune

lack a metallic hydrogen region; instead, at about 80% of

their radius, the abundance of methane, ammonia, and

water increases markedly. In this region, temperatures of

over 5000 to 10,000 K produce an ocean of electrically

charged water, ammonia, and methane molecules, along

with more complex compounds. Most of the mass of Uranus

and Neptune exists in such a state. Deep in their interiors,

all the planets likely have cores of primarily rocky material.

This picture of the interiors of the jovian planets has been

painstakingly pieced together since the 1930s. This chapter

discusses the components of observation, experiment, and

theory that are combined to reach these conclusions.