100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 7
Venus: Atmosphere
Donald M. Hunten
University of Arizona
Tucson, Arizona- Introduction 5. General Circulation
- Lower Atmosphere 6. Origin and Evolution
- Middle and Upper Atmosphere Bibliography
- Clouds and Hazes
V
enus possesses a dense, hot atmosphere, primarily of
carbon dioxide, with a pressure of 93 bars and a glob-
ally uniform temperature of 740 K at the surface. The sur-
face is totally hidden at visible wavelengths by a cloud deck
(really a deep haze) of concentrated sulfuric acid droplets
that extends from 50 km altitude to a poorly defined top at
65 km (Fig. 1 and also Fig. 8). The clouds are thus located
in the top part of thetroposphere, which extends from
0 to 65 km. The middle atmosphere (stratosphere and
mesosphere) extends from 65 to about 95 km, and the
upper atmosphere (thermosphere and exosphere) from
95 km up. Although the rotation period of the solid planet
is 243 Earth days (sidereal), the atmosphere in the cloud
region rotates in about 4 days, and the upper atmosphere
in about 6 days, all in the sameretrogradedirection.
1. Introduction
1.1 History
The study of Venus by Earth-based telescopes has been
frustrated by the complete cloud cover. The presence of
CO 2 was established in 1932, as soon as infrared-sensitive
photographic plates could be applied to the problem. But
establishment of the abundance was impossible because
there was no way to determine the path length of the light
as it scattered among the cloud particles. Moreover, it was
assumed that nitrogen would also be abundant, as it is on
Earth, and this gas cannot be detected in the spectral range
available from the ground. Careful observation of the fee-
ble patterns detectable in blue and near-ultraviolet images
was able to establish the presence of the 4-day rotation at
the cloud tops. These patterns are shown in the much more
recent spacecraft images of Fig. 1. Radio astronomers, ob-
serving Venus’s emission at the microwave wavelength of
3.15 cm, discovered in 1958 that it appears to be much hot-
ter than expected, and this was confirmed by later results at
other wavelengths. The most likely suggested explanation
was that the radiation came from a hot surface, warmed by
an extreme version of thegreenhouse effect; but the re-
quired warming is so extreme that other hypotheses were
debated. Spacecraft measurements, as will be described, fi-
nally settled the issue in favor of the greenhouse effect and
showed that the pressure at the mean surface is 93 bars.
A large number of spacecraft experiments on 22 missions
have been devoted to study of the atmosphere; along with
that of Mars, it is better explored than that of any planet
other than the Earth. United States missions, starting in
1962, were the flybysMariner2, 5, and 10 (which went
on to Mercury);Pioneer Venus MultiprobeandOrbiterin
1978; the radar mapperMagellan; and the Jupiter-bound
Galileo. Successful Soviet ones wereVenera 4–14, which in-
cluded entry and descent probes as well as flybys or orbiters,