Earth as a Planet: Atmosphere and Oceans 173
that are altitude-dependent. Other reactions that are im-
portant to the creation and destruction of ozone involve
minor constituents such as NO, NO 2 , H, OH, HO 2 , and Cl.
An important destruction mechanism is the catalytic cycle
X+O 3 →XO+O 2 followed by XO+O→X+O 2 , which
results in the net effect O+O 3 →2O 2. On Earth, human
activity has led to sharp increases in the catalysts X=Cl and
NO and subsequent sharp decreases in stratospheric ozone,
particularly over the polar regions. The Montreal Protocol
is an international treaty signed in 1987 that is designed to
stop and eventually reverse the damage to the stratospheric
ozone layer; regular meetings of the parties, involving some
175 countries, continually update the protocol.
2.3 Mesosphere
Above Earth’s stratopause, temperature again falls off with
height, although at a slower rate than in the troposphere.
This region is called the mesosphere. Earth’s stratosphere
and mesosphere are often referred to collectively as the
middle atmosphere. Temperatures fall off in the meso-
sphere because there is less heating by ozone and emission
to space by carbon dioxide is an efficient cooling mecha-
nism. The mesopause occurs at an altitude of about 80 km,
marking the location of a temperature minimum of about
130 K.
2.4 Thermosphere
As is the case for ozone in Earth’s stratosphere, above the
mesopause, atomic and molecular oxygen strongly absorb
solar UV radiation and heat the atmosphere. This region is
called the thermosphere, and temperatures rise with alti-
tude to a peak that varies between about 500 and 2000 K
depending on solar activity. Just as in the stratosphere, the
thermosphere is stable to vertical mixing. At about 120 km,
molecular diffusion becomes more important than turbu-
lent mixing, and this altitude is called the homopause (or
turbopause). Rocket trails clearly mark the homopause—
they are rapidly mixed below this altitude but linger rel-
atively undisturbed above it. Molecular diffusion is mass-
dependent and each species falls off exponentially with its
own scale height, leading to elemental fractionation that
enriches the abundance of the lighter species at the top of
the atmosphere.
For comparison with Earth, the structure of the thermo-
spheres of the giant planets has been determined fromVoy-
agerspacecraft observations, and the principal absorbers of
UV light are H 2 ,CH 4 ,C 2 H 2 , and C 2 H 6. The thermospheric
temperatures of Jupiter, Saturn, and Uranus are about 1000,
420, and 800 K, respectively. The high temperature and low
gravity on Uranus allow its upper atmosphere to extend out
appreciably to its rings. [SeeAtmospheres of the Giant
Planets.]
2.5 Exosphere and Ionosphere
At an altitude of about 500 km on Earth, the mean free path
between molecules grows to be comparable to the density
scale height (the distance over which density falls off by
a factor ofe≈2.7128). This defines the exobase and the
start of the exosphere. At these high altitudes, sunlight can
remove electrons from atmospheric constituents and form a
supply of ions. These ions interact with a planet’s magnetic
field and with the solar wind to form an ionosphere. On
Earth, most of the ions come from molecular oxygen and
nitrogen, whereas on Mars and Venus most of the ions come
from carbon dioxide. Because of the chemistry, however,
ionized oxygen atoms and molecules are the most abundant
ion for all three atmospheres.
Mechanisms of atmospheric escape fall into two cate-
gories, thermal and nonthermal. Both processes provide
the kinetic energy necessary for molecules to attain es-
cape velocity. When escape velocity is achieved at or
above the exobase, such that further collisions are unlikely,
molecules escape the planet. In the thermal escape process,
some fraction of the high-velocity wing of the Maxwellian
distribution of velocities for a given temperature always has
escape velocity; the number increases with increasing tem-
perature. An important nonthermal escape process is dis-
sociation, both chemical and photochemical. The energy
for chemical dissociation is the excess energy of reaction,
and for photochemical dissociation, it is the excess energy
of the bombarding photon or electron, either of which is
converted into kinetic energy in the dissociated atoms. A
common effect of electrical discharges of a kilovolt or more
is “sputtering,” where several atoms can be ejected from
the spark region at high velocities. If an ion is formed very
high in the atmosphere, it can be swept out of a planet’s at-
mosphere by the solar wind. Similarly at Io, ions are swept
away by Jupiter’s magnetic field. Other nonthermal escape
mechanisms involve charged particles. Charged particles
get trapped by magnetic fields and therefore do not readily
escape. However, a fast proton can collide with a slow hy-
drogen atom and take the electron from the hydrogen atom.
This charge–exchange process changes the fast proton into
a fast, hydrogen atom that is electrically neutral and hence
can escape.
Nonthermal processes account for most of the present-
day escape flux from Earth, and the same is likely to be true
for Venus. They are also invoked to explain the 62±16%
enrichment of the^15 N/^14 N ratio in the martian atmosphere.
If the current total escape flux from thermal and nonthermal
processes is applied over the age of the solar system, the loss
of hydrogen from Earth is equivalent to only a few meters
of liquid water, which means that Earth’s sea level has not
been affected much by this process. However, the flux could
have been much higher in the past, since it is sensitive to
the structure of the atmosphere. [SeeMars Atmosphere:
History and Surface Interaction.]