The Atmosphere of Venus
The atmosphere of Venus is composed of about 96 percent carbon dioxide, with most of
the remainder being nitrogen. The atmosphere appears to be relatively clear
until about 30 km above the surface. A schematic picture
of the cloud layers is shown in the adjacent image.
The clouds are composed of sulfuric acid and various other
corrosive compounds. There is no water on the surface and little in the
atmosphere (Venus has 10,000 time less water than Earth). The clouds
extend to a maximum
height of 65 km, which may be compared with a maximum height of 16 km for clouds in Earth's
troposphere.
Pressure and Temperature
The pressure of the atmosphere is about ninety times that of the Earth at the
surface, corresponding to a density only ten times less than that of water;
the surface temperatures on Venus approach 500 degrees C (773 K),
exceeding that of Mercury and hot enough to melt soft metals (this is
approximately twice the
hottest temperature
of a typical kitchen oven). The thick cloud layer means that only about 3 percent of the sunlight falling
on the upper atmosphere of Venus makes its way to the surface, with almost
80 percent being reflected and the rest absorbed. In comparison,
the corresponding fraction of sunlight making its way to the surface of the
Earth is about 30 percent, ten times larger.
Trapping Solar Radiation
For the temperatures to be so high there must be a mechanism in
the Venusian atmosphere that traps solar radiation very effectively. As we
will see shortly, these rather remarkable properties of the
atmosphere are thought to be a consequence of a
runaway greenhouse effect.
Because of the insulating atmosphere and high velocity winds in the upper atmosphere that
transport heat effectively, the night and day sides of Venus differ in temperature
by only about 10 degrees C.
However, as noted in the right panel, we do not have a completely consistent understanding of
atmospheric circulation on Venus.
The Cloud Layer
Missions to the
surface of Venus (Russian Venera spacecraft) indicate that the cloud deck
begins about 50 km above the surface.
Spectral
analysis of the thick cloud layer surrounding Venus
suggests that the clouds are composed largely of sulfuric acid
droplets in a concentrated solution with water.
(The amount of water vapor is limited, which is why
the solution is concentrated.) The cloud layer
is almost featureless in visible light, but shows structure at other
wavelengths. The adjacent image
shows the clouds
photographed by Pioneer 10 at UV wavelengths
and computer enhanced to accentuate differences.
Rotation and Weather
The stability of the cloud layers is not well understood. A partial explanation
may lie in the very slow rotation of Venus,
which implies much weaker cyclonic eddies than we find on the Earth associated with
high and low pressure systems and frontal boundaries. The weakness of these eddies makes it much
easier for Venus to organize its atmosphere on a global scale without strong modification from
Coriolis forces. We shall see later that the very fast rotation of Jupiter leads to yet
a different
modification of the basic cyclonic pattern that dominates Earth's weather. Thus, the study of
weather on other planets contributes to the understanding of weather on our own planet.
