700 Encyclopedia of the Solar System
time constant is much greater than a solar day, and the upper
atmosphere (altitudes
̃
120 km), which has a low heat ca-
pacity. In contrast to the lower atmosphere, a strong day-to-
night gradient in temperature exists above the mesosphere,
which leads to strong winds from the day to the night side.
This is very different from the retrograde zonal winds ob-
served in the visible cloud layers. These mesospheric winds
likely carry CO, formed on the day side upon photodisso-
ciation of CO 2 , to the night side of the planet. Therefore,
contrary to expectations, the spectra in Fig. 5 show the night
side line to be much deeper and narrower than the day side
line, suggestive of a large concentration of CO at high alti-
tudes on the night side of the planet.
On Mars, the CO mixing ratio, CO/CO 2 ∼ 10 −^3 , is much
less than expected from theories on photolysis of CO 2 and
subsequent recombination of CO and O. This recombina-
tion proceeds faster in the presence of chemistry involving
hydroxyl radicals (OH), derived from water vapor. Regular
photolysis of water in the martian atmosphere may be too
slow, however. New ideas being pursued include the cre-
ation of OH by electric fields in martian dust storms. The
reaction CO+OH→CO 2 +H frees up H, which eventu-
ally may lead to the formation of hydrogen peroxide (H 2 O 2 ),
a strong oxidizer. Dust storms prevail in Mars’ lower atmo-
sphere, and the formation of OH in such storms occurs
close to the surface where the water abundance is highest.
Hence, for this mechanism to efficiently remove CO from
the upper atmosphere, the mixing of CO throughout the
atmosphere should be an efficient process.
2.5 Giant Planets
2.5.1 RADIO SPECTRA
At millimeter to centimeter wavelengths, one typically
probes altitudes in the giant planet atmospheres from within
to well below (pressure of tens of bars) the cloud layers.
Representative microwave spectra are shown in Figs. 6a
(Jupiter) and 6b (Uranus). They generally show an increase
in brightness temperature with increasing wavelength be-
yond 1.3 cm, due to the combined effect of a decrease in
opacity at longer wavelengths, and an increase in temper-
ature at increasing depth in the planet. The main source
of opacity is ammonia (NH 3 ) gas, which has a broad ab-
sorption band at 1.3 cm. At longer wavelengths (typically
10 cm) absorption by water vapor and droplets becomes
important, while at short millimeter wavelengths the contri-
bution of collision induced absorption by molecular hydro-
gen becomes noticeable. On Uranus and Neptune, there is
additional absorption by hydrogen sulfide (H 2 S) and (per-
haps) phosphine (PH 3 ) gas.
The composition of all four giant planets is dominated by
H 2 and He gases, while the condensable gases CH 4 ,NH 3 ,
H 2 S, and H 2 O constitute only a small fraction of the total.
These gases, however, determine much of the “weather”
on these planets. Although only cloud tops are seen “visu-
ally,” thermochemical equilibrium calculations reveal the
presence of a number of cloud layers deeper in the atmo-
sphere, as depicted in Figs. 6c and 6d: an aqueous ammonia
solution cloud, water ice, a cloud of ammonium hydrosul-
fide particles (NH 3 +H 2 S→NH 4 SH around 250 K), am-
monia and/or hydrogen sulfide ice, and methane ice. The
“visible” cloud layers on Jupiter and Saturn are composed of
ammonia ice, while Uranus and Neptune are cold enough
to allow condensation of methane gas.
To first approximation, the spectra of both Jupiter and
Saturn resemble those expected for a solar composition at-
mosphere, while the spectra of Uranus and Neptune indi-
cate a depletion of ammonia gas compared to the solar value
by∼two orders of magnitude. As shown in Fig. 6d, this de-
pletion has been explained via formation of an extensive
NH 4 SH cloud, which is discussed in more detail later.
The thermal emission from all four giant planets has
been imaged with the VLA. To construct high signal-to-
noise images, the observations are integrated over several
hours, so that the maps are smeared in longitude and only
reveal brightness variations in latitude. The observed vari-
ations have typically been attributed to spatial variations in
opacity (NH 3 ,H 2 S gases), as caused by a combination of at-
mospheric dynamics and condensation at higher altitudes.
Below we briefly discuss findings for each planet individu-
ally.2.5.2 JUPITER
In situ observations by theGalileoprobe revealed that the
NH 3 and H 2 S abundances in Jupiter’s deep atmosphere
(P
̃>8 bar) are 3–4 times solar, while radio spectra (Fig. 6)
show a subsolar abundance of NH 3 gas at pressuresP<2
bar. The apparent decrease in the NH 3 abundance at higher
altitudes may be caused by dynamical processes, but the jury
is still out on this.
Radio images of Jupiter clearly show bright zonal bands
across the disk (Fig. 7a), which coincide with the brown
belts seen at visible wavelengths. These bands have a higher
brightness temperature, likely due to a lower opacity in the
belts relative to the zonal regions, so deeper warmer lay-
ers are probed in the belts. This phenomenon is sugges-
tive of gas rising up in the zones; when the temperature
drops below∼140 K, ammonia gas condenses out. In the
belt regions, the air, now depleted in ammonia gas (i.e., dry
air), descends. This general picture agrees with that sug-
gested from analyses of visible and infrared data. Note,
though, that the radio data probe the gas from which
the clouds condense, while visible and infrared data are
sensitive primarily to the cloud particles. Thus, the base
level of the clouds is determined through radio observa-
tions, whereas the cloud tops are probed at optical and in-
frared wavelengths.