Interiors of the Giant Planets 417
limit heat flow from the planet. In such a case, only the out-
ermost layers could transport energy by convection to the
atmosphere and cool effectively to space, thus producing a
lower than expected heat flow. More of Neptune’s interior
than that of Uranus might be convective, thus explaining its
higher current heat flow. Of course if this hypothesis were
correct, then the existing interior models of these planets
would have to be revised because an initial assumption that
the planets are fully convective would have been violated.
Inhibition of convection in the deep interior by this mech-
anism has been proposed as one explanation for the strong
nondipole component of both planets’ magnetic fields.
Currently it is thought that the highly irradiated extra-
solar giant planets evolve in much the same way as Jupiter,
except that their interiors cool, and the planets contract,
more slowly. The incident stellar flux leads to a radiative
zone with a shallow temperature gradient to pressures of
up to∼1 kbar, 1000 times deeper than in Jupiter. This lim-
its how quickly the interior heat flux can escape from the
planet. Finding more transiting planets, with a variety of
radii, masses, and orbital separations will allow for a better
understanding of how stellar irradiation effects the cooling
of giant planets.
7. Future Directions
Models of jovian planetary interiors have constrained the
mass of each planet’s core and the approximate composition
of their envelopes. These results have provided important
constraints on the processes by which these planets form.
In turn, formation models place limits on the mass, compo-
sition, and evolution of the solar nebula. Further progress,
however, requires even tighter limits on the interior struc-
ture of these planets. Sufficiently detailed interior models
may even provide constraints on the equation of state of
hydrogen. Because Jupiter is the largest reservoir of metal-
lic hydrogen in the solar system, it may potentially resolve
issues such as the exact pressure of the transition between
molecular and metallic hydrogen.
One might expect that future, more accurate measure-
ments of each planet’s gravitational harmonics would help
to address questions such as these. The higher order mo-
ments, however, are most sensitive to the density distribu-
tion in the outer 10 or 20% of the planetary radius. Thus,
little additional information about the deep interior is likely
to be forthcoming from such observational improvements.
The higher order harmonics do, however, provide some in-
formation about the state of rotation of the outer layers and
may help address questions regarding the degree of dif-
ferential rotation in the jovian planets. For example, it is
unknown if Jupiter rotates completely as a solid body, or
if different cylindrical regions of its interior rotate at dif-
ferent rates. NASA has recently selected a New Frontiers
mission calledJunothat will travel to Jupiter to answer this
and other questions. The spacecraft will be placed into a
low polar orbit such that the spacecraft will readily be able
to measure additional higher order harmonics up toJ 12 ,
which will allow for a determination of the planet’s interior
rotation. In addition, the spacecraft will observe microwave
emission from below the “weather layer” of the planet’s at-
mosphere (100 bars) to determine the deep abundance of
water and ammonia. Also, the planet’s magnetic field will
be mapped in unprecedented detail. Together, these new
measurements should shed additional light on the structure
of the planet.
Further improvements in delineating the equations of
state of jovian planetary components will help to clarify
their interior structures. More complete knowledge of the
behavior of planetary constituents and their mixtures at
high pressure will enable more accurate interior models
to be constructed. Nevertheless, dramatic changes in un-
derstanding are unlikely to result from such improvements.
Only significantly new and different sources of information
offer the potential of providing fundamentally new insights
into the interior structure of these planets.
Jovian seismology is one promising new avenue of re-
search into these planetary interiors. Much of our knowl-
edge of the interior structure of the Earth arises from study
of seismic waves that propagate through the interior of the
planet. The speed and trajectory of these waves carry infor-
mation about the composition and structure of the Earth’s
interior. During the collisions of the fragments of comet
Shoemaker–Levy 9 with Jupiter, several experiments at-
tempted to detect seismic waves launched by the impacts. If
these waves had been detected, they would have provided
a direct probe into the interior structure of Jupiter.
Another avenue for jovian seismology is to detect reso-
nant acoustic modes trapped inside Jupiter. The frequency
of a given jovian oscillation mode depends on the interior
structure of the planet within the region in which the mode
propagates. Thus, measurement of the frequencies of a va-
riety of modes would provide information on the overall
interior structure of the planet. The study of such modes
on the Sun, a science known as helioseismology, has revo-
lutionized our knowledge of the solar interior. In the past
20 years, a number of groups have attempted to detect the
jovian oscillations with various techniques. However, in all
cases the observations and data analysis are difficult, and in-
terpretation of the results has been limited by the restricted
number of observing nights on large telescopes. Future ob-
servational advances may allow unambiguous detection of
jovian oscillations.
As they would at Jupiter, oscillations of Saturn would per-
turb the external gravitational field of the planet. Though
there is yet no way to detect such perturbations at Jupiter,
this may be possible at Saturn. Saturn’s rings are excel-
lent detectors of faint gravitational perturbations, and thus
the possibility arises of using Saturn’s rings as a seismome-
ter. There is some evidence that certain wave features in