172 Encyclopedia of the Solar System
an atmosphere, where molecules float off into space when
they achieve escape velocity. [SeeAtmospheres of the
Giant Planets.]
Interestingly, the top of the troposphere occurs at about
the same pressure, about 0.1–0.3 bar, on most planets
(Fig. 1). This similarity is not coincidental but instead re-
sults from the pressure dependence of the atmospheric
opacity to solar and especially infrared radiation. In the
high-pressure regime of tropospheres, the gas is relatively
opaque at infrared wavelengths, which inhibits heat loss by
radiation from the deep levels and hence promotes a profile
where temperature decreases strongly with altitude. In the
low-pressure regime of stratospheres, the gas becomes rela-
tively transparent at infrared wavelengths, which allows the
temperature to become more constant—or in some cases
even increase—with altitude. This transition from opaque
to transparent tends to occur at pressures of 0.1–0.3 bar
for the compositions of most planetary atmospheres in our
solar system.
In the first 0.1 km of a terrestrial atmosphere, the effects
of daily surface heating and cooling, surface friction, and
topography produce a turbulent region called the planetary
boundary layer, or PBL. Right at the surface, molecular
viscosity forces the “no slip” boundary condition and the
wind reduces to zero, such that even a weak breeze results
in a strong vertical wind shear that can become turbulent
near the surface. However, only a few millimeters above the
surface, molecular viscosity ceases to play a direct role in
the dynamics, except as a sink for the smallest eddies. The
mixing caused by turbulent eddies is often represented as
a viscosity with a strength that is a million times or more
greater than the molecular viscosity.
Up to altitudes of about 80 km, Earth’s atmosphere is
composed of 78% N 2 , 21% O 2 , 0.9% Ar, and 0.002% Ne
by volume, with trace amounts of CO 2 ,CH 4 , and numer-
ous other compounds. Diffusion, chemistry, and other ef-
fects substantially alter the composition at altitudes above
∼90 km.
2.1 Troposphere
The troposphere is the lowest layer of the atmosphere,
characterized by a temperature that decreases with altitude
(Fig. 1). The top of the troposphere is called the tropopause,
which occurs at an altitude of 18 km at the equator but only
8 km at the poles (the cruising altitude of commercial air-
liners is typically 10 km). Gravity, combined with the com-
pressibility of air, causes the density of an atmosphere to fall
off exponentially with height, such that Earth’s troposphere
contains 80% of the mass and most of the water vapor in
the atmosphere, and consequently most of the clouds and
stormy weather. Vertical mixing is an important process in
the troposphere. Temperature falls off with height at a pre-
dictable rate because the air near the surface is heated and
becomes light, and the air higher up cools to space and
becomes heavy, leading to an unstable configuration and
convection. The process of convection relaxes the tempera-
ture profile toward the neutrally stable configuration, called
the adiabatic temperature lapse rate, for which the decrease
of temperature with decreasing pressure (and hence in-
creasing height) matches the drop-off of temperature that
would occur inside a balloon that conserves its heat as it
moves, that is, moves adiabatically.
In the troposphere, water vapor, which accounts for up
to∼1% of air, varies spatially and decreases rapidly with
altitude. The water vapor mixing ratio in the stratosphere
and above is almost 4 orders of magnitude smaller than that
in the tropical lower troposphere.2.2 Stratosphere
The nearly adiabatic falloff of temperature with height in
Earth’s troposphere gives way above the tropopause to an
increase of temperature with height. This results in a rar-
ified, stable layer called the stratosphere. Observations of
persistent, thin layers of aerosol and of long residence times
for radioactive trace elements from nuclear explosions are
direct evidence of the lack of mixing in the stratosphere. The
temperature continues to rise with altitude in Earth’s strato-
sphere until one reaches the stratopause at about 50 km.
The source of heating in Earth’s stratosphere is the photo-
chemistry of ozone, which peaks at about 25 km. Ozone ab-
sorbs ultraviolet (UV) light, and below about 75 km nearly
all this radiation gets converted into thermal energy. The
Sun’s UV radiation causes stratospheres to form in other at-
mospheres, but instead of the absorber being ozone, which
is plentiful on Earth because of the high concentrations of
O 2 maintained by the biosphere, other gases absorb the
UV radiation. On the giant planets, methane, hazes, and
aerosols do the job.
The chemistry of Earth’s stratosphere is complicated.
Ozone is produced mostly over the equator, but its largest
concentrations are found over the poles, meaning that dy-
namics is as important as chemistry to the ozone budget.
Mars also tends to have ozone concentrated over its poles,
particularly over the winter pole. The dry martian atmo-
sphere has relatively few hydroxyl radicals to destroy the
ozone. Some of the most important chemical reactions in
Earth’s stratosphere are those that involve only oxygen. Pho-
todissociation by solar UV radiation involves the reactions
O 2 +hν→O+O and O 3 +hν→O+O 2 , wherehν
indicates the UV radiation. Three-body collisions, where a
third molecule, M, is required to satisfy conservation of mo-
mentum and energy, include O+O+M→O 2 +M and
O+O 2 +M→O 3 +M, but the former reaction proceeds
slowly and may be neglected in the stratosphere. Reactions
that either destroy or create “odd” oxygen, O or O 3 , proceed
at much slower rates than reactions that convert between
odd oxygen. The equilibrium between O and O 3 is con-
trolled by fast reactions that have rates and concentrations