Venus: Atmosphere 143
FIGURE 4 Near-infrared
spectrum in the 2.3-μm window
(bottom); the spectrum above it
was calculated by making use of
laboratory data for the sic different
molecules shown. (From B ́ezard
et al., 1990.)noticeably less steep above the tropopause. As on Earth, the
tropopause is a few kilometers lower at high latitudes than
near the equator. The high surface temperature is main-
tained by the greenhouse effect, driven by the few percent
of solar energy that reaches the surface. Converted to ther-
mal infrared, this energy leaks out very slowly because of the
opacity of the atmospheric gases at such long wavelengths.
The molecules principally responsible are CO 2 ,SO 2 ,H 2 O,
and perhaps others. Quantitative calculations have shown
that the greenhouse mechanism is adequate, and that the
observed solar and infrared net fluxes can be reproduced.
These models treat the temperatures as globally uniform,
so that they can be restricted to considering vertical heat
transport only.
The surface temperature is remarkably uniform with
both latitude and longitude, largely because of a very long
radiative time constant. A very slow atmospheric circulation
is therefore adequate, but the details of how the nonuniform
solar heating is converted to a uniform surface temperature
are not understood. The ‘runaway greenhouse’ that may
have operated early in the history of the planet is discussed
in Section 6.
2.2 Water Vapor
Table 1 shows rather uncertain quantities of H 2 O, but there
is no doubt that there is a major difference in the mole
fractions below and above the clouds. This is almost cer-
tainly because the concentrated sulfuric acid of the cloud
particles is a powerful drying agent (see Section 4.3). A
summary of the many attempts to measure the abundance
below the cloud is given in Fig. 5. Direct measurements
have been made by several mass spectrometers and gas
chromatographs, but the amounts are so small and the re-
sults so divergent that there remain many questions. Indi-
rect measurements come from radiation fluxes, which areFIGURE 5 Water vapor mixing ratios or mole fractions from
various experiments, mostly onPioneer VenusandVenera
probes. (From Donahue and Hodges, 1992.)