Venus: Atmosphere 145
to the night side and help to maintain a weak ionosphere
there. Venus lacks any detectable magnetic field, and the
day side ionosphere is therefore impacted by the solar wind,
a tenuous medium of ions (mostly H+) and electrons flow-
ing from the Sun at about 400 km/s. Electrical currents are
induced in the ionosphere, and they divert the solar wind
flow around the planet. The boundary between the two me-
dia, called the ionopause, is typically at an altitude of a few
hundred kilometers near the subsolar point, flaring out to
perhaps 1000 km above the terminators and forming a long,
tail-like cavity behind the planet. [SeeTheSolarWind.]
3.3 Winds
The thermospheric winds carry the photochemical prod-
ucts O, CO, and N from the day side to the night side,
where they are almost as abundant as they are on the day
side. However, as Fig. 7 illustrates, all gases fall off much
more rapidly on the night side because of the low tempera-
ture. They descend into the middle atmosphere in a region
perhaps 2000 km in diameter and generally centered near
the equator at 2a.m.local time. This region can be observed
by the emission of airglow emitted during the recombina-
tion of N and O atoms into NO molecules, which then radi-
ate in the ultraviolet, and O 2 molecules, which radiate in the
near infrared. The light gases hydrogen and helium are also
carried along and accumulate over the convergent point
of the flow; for these gases, the peak density is observed
at about 4a.m.These offsets are the principal evidence
that this part of the atmosphere rotates with a 6-day pe-
riod, a rotation that is superposed on the rapid day-to-night
flow.
3.4 Chemical Recombination
Oxidation of the CO back to CO 2 is much slower than the re-
combination of O and N atoms, but a very efficient process
is required. This conclusion follows from Earth-based ob-
servations of a microwave (2.6-mm wavelength) absorption
line of CO, from which a height distribution can be obtained
from 80 to 110 km. It is found that the downward-flowing
CO is substantially depleted on the night side below 95 km
(as well as on the day side). The proposed solution involves
reactions of chlorine atoms, as well as residual O atoms
descending from the thermosphere. The chlorine acts as a
catalyst, promoting reactions but not being consumed itself,
and the reaction cycle works without the direct intervention
of any solar photons other than the ones that produced the
O atoms and CO molecules half a world away.
The availability of Cl atoms is assured by the observed
presence of HCl at the cloud tops (Table 1). On Earth,
any HCl emitted into the atmosphere is rapidly dissolved
in water drops and rained out. Chlorine atoms reach the
stratosphere only as components of molecules, such as the
artificial ones CCl 4 ,CF 2 Cl 2 , and CFCl 3 and the natural
one CH 3 Cl, none of which dissolve in water. Once they
have been mixed to regions above the ozone layer, they are
dissociated by solar ultraviolet photons. Because liquid wa-
ter is absent on Venus, the abundance of HCl is large to
start with, and it is not kept away from the stratosphere.
Here again the atoms are released by solar ultraviolet. The
chlorine abundance is nearly a thousand times greater than
that on Earth, and Venus is an example and a warning of
what chlorine can do to an atmosphere. The middle at-
mosphere is also the seat of important chemistry involving
sulfur, which is discussed in the next section.4. Clouds and Hazes
4.1 Appearance and Motions
The clouds are perhaps the most distinctive feature of
Venus. They do show subtle structure in the blue and
near ultraviolet, illustrated in Fig. 1, which has been pro-
cessed to bring out the detail and flattened to remove the
limb darkening. Although the level shown in the figure is
conventionally called the “cloud top,” it is not a discrete
boundary at all. Similar cloud particles extend as a haze to
much higher altitudes, at least 80 km; the “cloud top” is
simply the level at which theoptical depthreaches unity,
and the range of visibility (the horizontal distance within
which objects are still visible) is still several kilometers.
Study of daily images, first from Earth and later from
spacecraft, reveals that the cloud top region is rotating with
a period of about 4 days, corresponding to an equatorial
east–west wind speed of about 100 m/sec. The speed varies
somewhat with latitude; in some years, but not all, the ro-
tation is almost like that of a solid body. Although there are
not nearly as many near-infrared images like Fig. 3, they
show a longer period consistent with the idea that the sil-
houettes are of the lower cloud, where entry probes have
measured wind speeds of 70 to 80 m/s.4.2 Cloud Layers
Several entry probes have made measurements of cloud
scattering as they descended, but the most detailed results
were obtained fromPioneer Venusand are shown in Fig. 8.
Three regions (upper, middle, and lower) can be distin-
guished in the main cloud, and there is also a thin haze
extending down to 30 km. Size distributions are shown in
Fig. 9; it is these, more than the gross properties of Fig. 8,
that distinguish the regions. In the upper cloud, the one
that can be studied from Earth or from orbit, most particles
(“Mode 1”) are about 1μm in diameter and should really be
considered a haze rather than a cloud; there are also larger
(“Mode 2”) particles with diameters around 2μm. The same
particles extend throughout the clouds, but the Mode 2 ones
become somewhat larger in the middle and lower clouds,