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CHAPTER 21

Interiors of the Giant

Planets

Mark S. Marley

Jonathan J. Fortney

NASA Ames Research Center

Moffett Field, California

1. General Overview 4. Interior Modeling 7. Future Directions


2. Constraints on Planetary Interiors 5. Planetary Interior Models Bibliography


3. Equations of State 6. Jovian Planet Evolution

T

he giant or jovian planets—Jupiter, Saturn, Uranus, and
Neptune—account for 99.5% of all the planetary mass
in the solar system. An understanding of the formation and
evolution of the solar system thus requires knowledge of the
composition and physical state of the material in their inte-
riors. But such information does not come easily. The famil-
iar faces of these planets, such as the cloud-streaked disk of
Jupiter, tell relatively little about what lies beneath. Knowl-
edge of these planetary interiors must instead be gained
from analysis of the mass, radius, shape, and gravitational
fields of the planets. For giant planets around other stars,
at best only the mass and radius can be determined. The
study of the behavior of planetary materials at high densities
and pressures further provides the experimental and the-
oretical framework upon which planetary interior models
are subsequently based. Interior models provide a window
into the internal structure of these planets and shed light on
processes that led to planet formation in our solar system
and others.

1. General Overview

Several lines of observational evidence provide informa-
tion on the composition and structure of the giant planets.
The first and most easily obtained quantities are the mass

(known from the orbits of natural satellites), radius (polar

and equatorial radii), and rotation period (obtained orig-

inally from telescopic observations, now derived from re-

mote and in situ observations of planetary magnetic fields).

By the 1940s, these fundamental observations, coupled with

the advances in understanding the high-pressure behavior

of matter in the 1920s and 1930s, constrained the com-

position of Jupiter and Saturn to be predominantly hydro-

gen. Direct measurement of the planets’ high-order grav-

ity fields, interior rotation states, and heat flow, along with

spacecraft and ground-based spectroscopic detection of at-

mospheric elemental composition, has since allowed the

construction of more detailed interior models.

These models divide the giant planets into two broad cat-

egories. Jupiter and Saturn are predominantly hydrogen–

helium gas giants with a somewhat enhanced abundance

of heavier elements and dense cores. Uranus and Neptune

are ice giants with hydrogen–helium envelopes and dense

cores. The following description of Jupiter’s interior, as il-

lustrated schematically in Fig. 1, is qualitatively valid for

Saturn and serves as a point of departure for understanding

the interiors of Uranus and Neptune. Individual planetary

interior structures are discussed in Section 5.

The interior begins at the base of the outermost atmo-

spheric envelope that we can see directly. The jovian atmo-

spheres consist of a gaseous mixture of molecular hydrogen,

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