142 Encyclopedia of the Solar System
There is, of course, plenty of oxygen in carbon dioxide, and
dissociation by sunlight liberates it in copious quantities.
It is readily detected (as is CO) by spacecraft instruments
orbiting through the upper atmosphere but is removed be-
fore it can reach the cloud level. Small quantities of O 2 are
also found below the clouds, probably liberated by the ther-
mal decomposition of the cloud particles. All these lines of
evidence point to the action of a strong mechanism in the
middle atmosphere that converts O 2 and CO back into CO 2.
The observed HCl molecules are the key; they too are bro-
ken apart by solar radiation, and the free chlorine atoms
enter acatalytic cyclethat does the job. This chemistry is
closely coupled to the sulfur chemistry (see Section 4) that
maintains the clouds.
Carbon dioxide, aided by the other molecules listed in
Table 1, makes the lower atmosphere opaque to thermal (in-
frared) radiation; it is this opacity that makes the extreme
greenhouse effect possible. Only a few percent of the inci-
dent solar energy reaches the surface, but this is enough.
Venus is a remarkable and extreme example of the large
climatic effects that can be produced by seemingly small
causes. One chlorine atom in two and a half million can
completely eliminate free oxygen from the middle atmo-
sphere, and ozone has no hope of surviving in significant
quantities. The temperature increase caused by the green-
house effect is almost 500◦C. The idea that the 30◦seen on
Earth could become 32◦or 33◦ if its atmospheric content
of CO 2 should double seems entirely probable to experts
on Venus’s atmosphere, and so does significant loss of ozone
from release of chlorinated refrigerants. It thus seems that
the obvious differences between Earth and Venus are all
traceable to the differences in their endowments of water
(vapor or liquid). Although origin and evolution are dis-
cussed in Section 6, a short preview is given here. It is plau-
sible that both planets started out with similar quantities,
but that the greater solar flux at Venus caused all its water to
evaporate (a “runaway greenhouse”). Solar ultraviolet pho-
tons could then dissociate it into hydrogen (which escaped)
and oxygen (which reacted with surface materials). Strong
evidence in favor of this scenario is the extreme enhance-
ment of heavy hydrogen (deuterium, or D), almost exactly
100 times more abundant relative to H than it is on Earth.
Such a fractionation is expected because the escape of H
is much easier than that of D. [SeeEarth as aPlanet:
Atmospheres andOceans.]
1.4 Near-Infrared Sounding
Study of the atmosphere below the clouds was revitalized in
1988 by the discovery of several narrow spectral windows in
the near infrared, where the radiation from deep layers can
be detected from above (Fig. 3). The two most prominent
ones are at 1.74 and 2.3μm (Fig. 4), and others are at 1.10,
1.18, 1.27, and 1.31μm. As we have seen, at microwave
radio wavelengths, radiation from the actual surface can es-
cape to space. At other infrared wavelengths, the emission
FIGURE 3 Near-infrared images (2.36-μm) of the night side
combined into maps for (above) December 31 to January 7,
1991, and (below) February 7 to 15, 1991. Bright areas are
thinner parts of the cloud through which thermal radiation from
deeper layers can shine. (From Crisp et al., 1991.)from the night side is characteristic of the temperature of
the cloud tops, about 240 K. In the windows, the bright-
ness, and therefore the temperature of the emitting region,
is considerably higher. Images taken in a window reveal hor-
izontally banded structures that appear to be silhouettes of
the lowest part of the cloud (around 50 km) against the
hotter atmosphere below (see Fig. 3 and Section 4). [See
InfraredViews of theSolarSystem fromSpace.]
Numerous absorption lines and bands allow inferences
about the composition to levels all the way to the surface.
One such spectrum is shown in Fig. 4. Each “window” allows
the composition to be obtained at a different level; this is
particularly important for water vapor, discussed in the next
section. The measurement of carbonyl sulfide (COS) shown
in Table 1 was obtained by this analysis. This gas has resisted
all attempts to measure it from entry probes, even though it
has long been expected to be present. Other gases include
CO, HF, HCl, and light and heavy water vapor, all in good
agreement with prior results. These results are also included
in Table 1.2. Lower Atmosphere
2.1 Temperatures
It is convenient to regard the lower atmosphere as extend-
ing from the surface to about 65 km, the level of the visible
cloud tops and also of the tropopause. This region has been
measured in detail by many descent probes, with results in
close agreement, and also by radio occultation. The temper-
ature profile (see Fig. 2) is close to theadiabat,becoming