388 Encyclopedia of the Solar System
FIGURE 2 Summary of CH 4 (methane)
photochemical processes in the stratospheres of the
giant planets. Photodissociation by ultraviolet light
is indicated by+hvat the indicated wavelength.
Methane photodissociation is the starting point in
the production of a host of other hydrocarbons.
(Revised by S. K. Atreya from Fig. 5-3 from J. B.
Pollack and S. K. Atreya, 1992, in “Exobiology in
Solar System Exploration” (G. Carle et al., eds.),
NASA-SP 512, pp. 82–101.)may owe their existence to the ions created by auroras in
the upper atmosphere.
As instruments become more sensitive, new species are
detected. These include C 2 H 4 ,C 3 H 4 , and C 6 H 6 in the at-
mospheres of Jupiter and Saturn, and C 3 H 8 for Saturn. The
methyl radical CH 3 (an unstable transition molecule in the
reaction chain) has been detected on Jupiter, Saturn, and
Neptune.
Hydrogen cyanide (HCN) is present in the stratospheres
of Jupiter and Neptune, but for two very different reasons.
On Jupiter, HCN was emplaced high in the stratosphere
as a result of the 1994 impacts of comet Shoemaker–Levy
- During the 3 years after the impacts, it was observed to
spread north of the impact latitude (near 45◦S), eventually
to be globally distributed. It is expected to dissipate over
the span of a decade or so. Cometary impact may also be
responsible for HCN in Neptune’s stratosphere.
Quantitative thermochemical and photochemical mod-
els are available for many of the observed constituents and
provide predictions for many others that are not yet ob-
served. These models solve a set of coupled equations that
describe the balance between the abundances of species
that interact and include important physical processes such
as ultravioletphotolysis, condensation/sublimation, and
vertical transport. Current models heuristically lump all
the transport processes into an effective eddy mixing co-
efficient, and the value of that coefficient is derived as part
of the solution of the set of equations. As we gain more
detailed observations and more comprehensive laboratory
measurements of reaction rates, we will be able to develop
more sophisticated models. Some models are beginning to
incorporate transport by vertical and horizontal winds. Fig-
ures 3 and 4 show vertical profiles calculated from models
for a number of photochemically produced species.
3. Clouds and Aerosols
The appearance of the giant planets is determined by the
distribution and optical properties of cloud and aerosol
haze particles in the upper troposphere and stratosphere.
Cameras on theVoyagerspacecraft provided detailed views
of all the giant planets, whose general appearances can be
compared in Fig. 5. Their atmospheres show a banded
structure (which is difficult to see on Uranus) of color
and shading parallel to latitude lines. These were histori-
cally named belts and zones on Jupiter and Saturn, with
belts being relatively dark and zones relatively bright. Spe-
cific belts and zones were named in accordance with their
approximate latitudinal location (Equatorial Belt, North
and South Tropical Zones near latitudes± 20 ◦, North and
South Temperate Zones and Belts near± 35 ◦, and polar
regions).
The nomenclature should not be construed to mean that
low latitudes are relatively warmer than high latitudes, as
they are on Earth and Mars. Nor is it true that the re-
flectivities of these features remain constant with time.
Some features on Jupiter, such as the North and SouthFIGURE 3 Vertical profiles of some photochemical species in
Jupiter’s stratosphere. The mixing ratios (horizontal axis) are
plotted as a function of pressure. (From G. R. Gladstone et al.,
1996,Icarus119,1–52. Copyright Academic Press.)