Atmospheres of the Giant Planets 389
FIGURE 4 Vertical profiles of photochemical species in the
Neptunian stratosphere. (From P. Romani et al., 1993,Icarus
106 , 442–462. Copyright by Academic Press.)
Tropical Zones, are persistently bright, whereas others,
like the South Equatorial Belt, are sometimes bright and
sometimes dark. On Jupiter, there is a correlation between
visible albedo and temperature, such that bright zones are
usually cool regions and dark belts are usually warm near
the tropopause. Cool temperatures are associated with adi-
abatic cooling of upwelling gas, and the correlation of cool
temperatures with bright clouds points to enhanced con-
densation of ice particles as condensable gases flow up-
ward and cool. This correlation does not hold completely
on Jupiter and almost not at all on the other giant plan-
ets. The mechanisms responsible for producing reflectivity
contrasts and color remain largely mysterious, although a
number of proposals have been advanced. These will be
discussed in more detail.
Our understanding of aerosols and clouds is rooted in
thermochemical equilibrium models that predict the tem-
perature (and hence pressure and altitude) of the bases of
condensate clouds. The cloud base occurs where the vapor
pressure of a condensable gas equals its partial pressure.
Model predictions for the four giant planets are shown in
Fig. 6. The deepest cloud to form is a solution of water and
ammonia on Jupiter and Saturn, with dissolved H 2 S as well
on Uranus and Neptune. At higher altitudes, an ammonium
hydrosulfide cloud forms, and its mass depends on both the
amounts of H 2 S and NH 3 available and the ratio of S to N.
At still higher altitudes, an ammonia or hydrogen sulfide
cloud can form if the S/N ratio is less than or greater than
1, respectively. If the ratio is greater than 1, all the N will
be taken up as NH 4 SH, with the remaining sulfur avail-
able to condense at higher altitudes. This seems to be the
situation on Uranus and Neptune, but the reverse is true
for Jupiter and Saturn. Only the atmospheres of UranusFIGURE 5 Voyagerimages of
Jupiter, Saturn, Uranus, and
Neptune, scaled to their relative
sizes. Earth and Venus are also
sown scaled to their relative sizes.