396 Encyclopedia of the Solar System
and months. Jupiter has an abundance of small features and
the zonal winds are well mapped. Saturn has fewer features,
and they are of less contrast than those on Jupiter, but there
is still a large enough number to provide detail in the wind
field. Only a few features were seen inVoyagerimages of
the Uranus atmosphere, and all but one of these were be-
tween latitudes 20◦S and 40◦S. More recent images from
theHubble Space Telescopeshow new features at many
other latitudes. TheVoyager 2radio occultation provided
an additional estimate for wind speed at the equator. Nep-
tune has more visible features than Uranus, but most of
them are transitory and difficult to follow long enough to
gauge wind speed.
Figure 11 reveals a great diversity in the zonal flow
among the giant planet atmospheres. Wind speed is rel-
ative to the rotation rate of the deep interior as revealed by
the magnetic field and radio emissions. Jupiter has a series
of jets that oscillate with latitude and are greatest in the pro-
grade direction at latitude 23◦N, and near± 10 ◦. The pat-
tern of east–west winds is approximately symmetric about
the equator except at high latitude. Saturn has a very strong
prograde jet at low latitudes (within the region± 15 ◦). It
also has alternating but mostly prograde jets at higher lati-
tudes, with the scale of latitudinal variation being about 10◦.
Uranus appears to have a single prograde maximum near
60 ◦S, and the equatorial region is retrograde. Neptune has
an enormous differential rotation, mostly retrograde except
at high latitude. Various theories have been advanced to ex-
plain the pattern of zonal jets. None of them can account
for the great variety among the four planets.
The zonal jets are stable over long time periods (obser-
vations span many decades for Jupiter and Saturn), despite
the many small-scale features that evolve with much shorter
life times. An interesting exception to this rule occurred
at equatorial latitudes on Saturn between the time of the
Voyagerobservations (around 1981) and observations in the
1990s and later by theHubble Space Telescopeand begin-
ning in 2004 by theCassinicameras. Current equatorial jet
speeds are significantly less than those measured on Saturn
byVoyager. It is difficult to understand how such a large
change of momentum could occur, and another explana-
tion has been sought. Possibly the equatorial atmosphere
was clearer (less haze) during theVoyagerepoch, permit-
ting observations to deeper levels where the wind speed
is higher. Detailed analyses of haze altitudes show that the
haze is thicker and higher in more recent times than it was in
1981, but probably not enough to account for the difference
in wind speed.
Some of the key observations that any dynamical theory
must address include: (1) the magnitude, direction, and lati-
tudinal scale of the jets; (2) the stability of the jets, at least for
Jupiter and Saturn, where observations over long periods
show little or no change except for Saturn’s equatorial jet,
which was mentioned earlier; (3) the magnitude and latitu-
dinal gradients of heat flux; and (4) the interactions of the
mean zonal flow with small spots and eddies. One of the con-
troversies during the past two decades concerns how deep
the flow extends into the atmosphere. It is possible to con-
struct shallow-atmosphere models that have approximately
correct jet scales and magnitudes. A shallow-atmosphere
model is one in which the jets extend to relatively shallow
levels (100 bar or less), and the deeper interior rotates as a
solid body, or at least as one whose latitudinal wind shear
is not correlated with the wind shear of the jets. The facts
that the jets and some spots on Jupiter are very stable, that
there is approximate hemispheric symmetry in the zonal
wind pattern between latitudes± 60 ◦, and that the Jovian
interior has no density discontinuities down to kilobar levels
suggested to some investigators that the jets extend deep
into the atmosphere. A natural architecture for the flow in
a rotating sphere with no density discontinuities is one in
which the flow is organized on rotating cylinders (Fig. 12).
Apart from the stability and symmetry noted here, there
is little evidence to suggest that the zonal wind pattern
really does extend to the deep interior. The conductivity
of Jupiter’s atmosphere at depth is probably too high to al-
low the type of structure depicted in Fig. 12 to exist. The
strength of the zonal jet at the location where theGalileo
probe entered (6.5◦N) increased with depth, consistent with
the idea of a deeply rooted zonal wind field on Jupiter. One
way to test that hypothesis is to make highly precise mea-
surements of the gravity field close to the planet. There are
density gradients associated with the winds, and these pro-
duce features in the gravity field close to the planet. The
largest signature is produced by Neptune’s remarkable dif-
ferential rotation. TheVoyager 2spacecraft flew just above
Neptune’s atmosphere and provided the first evidence that
the differential rotation cannot extend deep into the atmo-
sphere. Gravity-field tests of the deep-wind hypothesis for
the other giant planets are more difficult because the dif-
ferential rotation is much weaker. No spacecraft have come
close enough to make the measurements but one is planned
for Jupiter
What process maintains the zonal wind pattern?Voyager
measurements shed some light on this question, but pro-
vided some puzzles as well. The ultimate energy source
for maintaining atmospheric motions is the combination of
internal thermal and solar energy absorbed by the atmo-
sphere. Jupiter, Saturn, and Neptune all have significant
internal energy sources, whereas Uranus has little or none.
A measure of the amount of energy available for driving
winds is the escaping radiative energy per square meter of
surface area. Twenty times as much energy per unit area is
radiated from Jupiter’s atmosphere as from Neptune’s, yet
the wind speeds (measured relative to the interior as deter-
mined from the magnetic field rotation rate) on Neptune
are about three times higher than those on Jupiter. Rather
than driving zonal winds, the excess internal energy may go