The Terrestrial Planets (cont.)

Venus

Second from the Sun, closest planet to the Earth, Venus has long been known as the bright morning and evening star. Called "Earth's Twin" because it is almost the size of Earth, Venus is surrounded by a thick, yellowish white, opaque atmosphere. All we can see in telescopes are fuzzy images of its perpetual cloud cover.

Probed and scanned by spacecraft, however, Venus has revealed itself as no twin of Earth, but rather as a strange and hellish inferno. Its thick atmosphere is composed mostly of carbon dioxide, with a little nitrogen and traces of water, oxygen, and sulfur dioxide. At the surface, the pressure of Venus' atmosphere is 90 times that of Earth's, equal to the pressure half a mile down in the ocean! The thick atmosphere traps the Sun's heat, producing a "greenhouse effect" that keeps the surface of Venus at a scorching 480° C, literally hot enough to fry eggs. Above the surface, entry probes of the Pioneer Venus mission detected several distinct regions, two of them, and possibly even four, constituting separate cloud layers. The top most cloud layer, revealed in photos made from spacecraft and dimly perceived from Earth, consists not of water clouds as on Earth, but rather of sulfuric acid droplets, far more potent than the mild "acid rain" that concerns us now in the United States.

A haze of aerosol particles extends upward from this layer and sometimes even veils the clouds themselves from view.

Earth's cloudy "twin".
Planet-wide swirls and waves appear in the thick atmosphere of Venus, as photographed by Pioneer Venus Orbiter. This view shows a pronounced dark band (lower left), two bright areas ("polar rings") near the north and south poles, and a complex, turbulent region at the left. These large-scale patterns are strikingly different from the numerous smaller circulation patterns of the Earth's atmosphere. The Venus patterns are remarkably stable and circle the planet in only four days, although the planet itself takes about 244 days to rotate on its axis.

The surface of Venus has been photographed at two locations by Russian Venera landers that briefly survived the hot, high-pressure environment and photographed the surrounding rocky slabs and rubble.

Another Venera lander gave us our first chemical analysis of the surface of Venus, indicating a unique composition resembling that of terrestrial granite.

Until other camera-carrying spacecraft land and survive, the rest of the surface of Venus will remain unseen. But the clouds that block telescopic investigation do not stop radar waves. Powerful radar transmitters, both on Earth and on a Pioneer Venus spacecraft in orbit around Venus, have scanned and located the major topographic features of the veiled planet's surface.

Hot Rocks of Venus.
Scorched to nearly red heat under the thick hot atmosphere of Venus, the planet's surface was photographed by two successful, but short-lived, Russian Venera landers. In this picture, jagged slabs of rock extend to the horizon (upper right). The nature of the rock is unknown. The images are curved by optical effects in the television cameras. Parts of the spacecraft are seen, out of focus, at lower center and lower right.

Venus revealed by radar apparently lacks the great number of high mountain ranges and the interlocked systems of deep oceanic trenches that run for thousands of kilometers on the Earth. But it has its own remarkable features. Among them are two great plateaus, each as large as a small continent, which rise as high as 17 kilometers (11 miles) above the surface.

One, called Ishtar Terra, is about the size of the continental United States and contains a more elevated region that is higher than Earth's Tibetan plateau and more than twice as large. Elsewhere on Venus is Beta Regio, a prominent landform that contains two huge, adjoining shield volcanoes, together more extensive than the Hawaii-Midway chain. Among the rifts found in the Venus crust are a canyon deeper and longer than the Grand Canyon of Arizona and a long valley that apparently is flanked by large mountains. Do these chasms of Venus indicate that its continental masses are separating? There also are large circular depressions, perhaps great meteorite craters like those found on other terrestrial planets.

However, these basins are unusually shallow, as though the hot crust of Venus has flowed into them like hot, soft wax.

Highs and lows of Venus.
This crude relief map of the cloud shrouded surface of Venus is based on radar observations made from Earth andfrom the Pioneer Venus Orbiter. Low elevations are shown in greens and blues, higher altitudes in yellows and reds. Most of Venus is much flatter than Earth, but a few plateaus the size of small continents rise up to 17 kilometers (11 miles) above their surroundings. There is no clear indication of anything resembling the global system of trenches and mid-ocean ridges that characterizes our geologically active Earth.

The surface of Venus seems to have been shaped by internal geological activity, but we do not know what kind of phenomena were in volved, nor how long they went on.

Is Venus now dead and quiet, as Mars appears to be? Or is it more evolved, and still geologically active, perhaps approaching Earth in its development? We have no detailed maps of Venus to guide our thinking, unlike the case of the other planets. The surface of Venus, the nearest planet to Earth, is still one of the largest unexplored lands in the solar system.