The Solar System

466 PART 4^ |^ THE SOLAR SYSTEM

past or orbited Venus, and over a dozen have landed on its sur-
face. Th e resulting picture of Venus is dramatically diff erent from
the murky swamps of fi ction. In fact, the surface of Venus is drier
than any desert on Earth and twice as hot as a kitchen oven set
to its highest temperature. Another startling contrast to Earth is
that Venus rotates very slowly, once in 243 Earth days, in the
retrograde (backward) direction.
If Venus is not a planet gone wrong, it is certainly a planet
gone down a diff erent evolutionary path than your home planet.
How did Earth’s twin become so diff erent?

The Atmosphere of Venus

Although Venus is Earth’s twin in size, its atmosphere is truly
unearthly. Th e composition, temperature, and density of Venus’
atmosphere make the planet’s surface entirely inhospitable. About
96 percent of its atmosphere is carbon dioxide, and 3.5 percent
is nitrogen. Th e remaining 0.5 percent is water vapor, sulfuric
acid (H 2 SO 4 ), hydrochloric acid (HCl), and hydrofl uoric acid
(HF). In fact, the thick clouds that hide the surface are composed
of sulfuric acid droplets and microscopic sulfur crystals.
Soviet and U.S. spacecraft dropped probes into the atmo-
sphere of Venus, and those probes radioed data back to Earth as
they fell toward the surface. Th ese studies show that Venus’s
cloud layers are much higher and more stable than those on
Earth. Th e highest layer of clouds, the layer visible from Earth,
extends from about 60 to 70 km (about 40 to 45 mi) above the
surface (■ Figure 22-1). For comparison, the clouds on Earth
normally do not extend higher than about 16 km (10 mi).

100

50

• 200 0 200 400 600 800

Altitude (km)

Temperature (K)

100 300 500 700

Earth Venus

Haze

Haze

Clouds

Clouds

UV image Clouds

Th ese cloud layers are highly stable because the atmospheric

circulation on Venus is much more regular than that on Earth.

Th e heated atmosphere at the subsolar point, the point on the

planet where the sun is directly overhead, rises and spreads out in

the upper atmosphere. Convection circulates this gas toward the

dark side of the planet and the poles, where it cools and sinks.

Th is circulation produces 300 km/hour jet streams in the upper

atmosphere, which move from east to west (the same direction

the planet rotates) so rapidly that the entire atmosphere rotates

with a period of only four days.

Th e details of this atmospheric circulation are not well

understood, but it seems that the slow rotation of the planet is

an important factor. On Earth, large-scale circulation patterns

are broken into smaller cyclonic disturbances by Earth’s rapid

rotation. Because Venus rotates more slowly, its atmospheric

circulation is not broken up into cyclonic storms but instead is

organized as a planetwide wind pattern.

Although Venus’s upper atmosphere is cool, the lower atmo-

sphere is quite hot (Figure 22-1b). Instrumented probes that

have reached the surface report that the temperature is 470°C

(880°F), and the atmospheric pressure is 90 times that of Earth.

Earth’s atmosphere is 1000 times less dense than water, but on

Venus the air is only 10 times less dense than water. If you could

survive the unpleasant atmospheric composition, intense heat,

and high pressure, you could strap wings to your arms and fl y.

Th e present atmosphere of Venus is extremely dry, but

there is evidence that it once had signifi cant amounts of water.

As one of the Pioneer Venus probes descended through the

■ Figure 22-1

The four main cloud layers in the atmosphere of Venus
are over 10 times higher above the surface than are
Earth clouds. They completely hide the surface. If you
could insert thermometers into the atmosphere at
different levels, you would fi nd that the lower atmo-
sphere is much hotter than that of Earth, as indicated
by the red line in the graph. (NASA)