When viewed through a telescope, the planet exhibits phases like the moon. Full Venus appears the smallest because it is on the far side of the sun from earth. Maximum brilliance (a stellar magnitude of -4.4, or 15 times the brightest star) is seen in the crescent phase. The phases and positions of Venus in the sky repeat with the synodic period of 1.6 years. Transits across the face of the sun are rare, occurring in pairs at intervals of a little more than a century. The next two will be in 2004 and 2012.

Exploration

Venus's complete cloud cover and deep atmosphere make it difficult to study from earth, and most knowledge of the planet has been obtained through the use of space vehicles, particularly those carrying probes that descend through the atmosphere. The first flyby was that of Mariner 2, launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner 10 in 1974. The Soviet Union has developed several entry probes, some combined with flybys or orbiters: Veņera 4 and 5 (1967), 6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15 and 16 (1983); Vega 1 and 2, sent toward Halley's comet in 1984, also flew by Venus and released descent capsules. Several of these probes successfully reached the planet's surface. The U.S. sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere. Pioneer Venus 1, an orbiter, continues to measure the upper atmosphere. The Magellan probe, launched toward Venus in 1989, began transmitting radar images of the planet in 1990.

Atmosphere

The surface temperature on Venus is highly uniform and is about 462° C (736 K/864° F); the surface pressure is 96 bars (compared with 1 bar for earth); the atmosphere of the planet consists of nearly all carbon dioxide (CO2). The cloud base is at 50 km (31 mi), and the cloud particles are mostly concentrated sulfuric acid. The planet has no detectable magnetic field.

That 97 percent of Venus's atmosphere is CO2 is not as strange as it might seem; in fact, the crust of earth contains almost as much in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas (N2). By contrast, 78 percent of earth's atmosphere is nitrogen. Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the sun, was subjected to a so-called runaway greenhouse effect, which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Another possibility is that Venus had very little water to begin with.

The sulfuric acid of the clouds also has its analogue on earth in a very thin haze in the stratosphere. On earth, sulfuric acid is carried down in the rain and reacts with surface materials; indeed, this so-called acid rain is damaging parts of the environment. On Venus the acid evaporates at the cloud base and can only remain in the atmosphere. The upper parts of the clouds, visible from earth and from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above the planet surface. The clouds contain a pale yellow impurity, better detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide content of the atmosphere may indicate active volcanism on the planet.

Certain cloud patterns and weather features can be discerned in the cloud tops that give some information about wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/hr (225 mph). These winds cover the planet completely, blowing at virtually every latitude from equator to pole. Tracking the motions of descending probes has shown that, despite the scale of these high-speed, upper-level winds, well more than half of Venus's tremendously dense atmosphere, near the planet's surface, is almost stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only about 3 to 18 km/hr (2 to 11 mph). The high-speed winds probably result from the transfer of momentum from Venus's slow-moving, massive lower atmosphere to higher altitudes where the atmosphere is less massive, so that the same momentum results in a much higher velocity.

The upper atmosphere and ionosphere have been studied in great detail by Pioneer Venus 1, which passes through them once each day. On earth this region is very hot; on Venus it is not, even though Venus is closer to the sun. Surprisingly, the night side of Venus is extremely cold. (Day-side temperatures are 40° C/104° F, compared to night-side temperatures of -170° C/-274° F.) Scientists suspect that strong winds blow from the day side toward the near vacuum that is caused by the low temperatures on the night side. Such winds would carry along light gases, such as hydrogen and helium, which are concentrated in a night-side "bulge."

On earth the ionosphere is isolated from the solar wind by the magnetosphere. Venus lacks a magnetic field of its own, but the solar wind seems to generate an induced magnetosphere, probably by a dynamo action involving its own magnetic field.

Surface Features

Venus rotates very slowly on its axis, and the direction is retrograde (opposite to that of earth). Curiously, Venus's synodic year is almost exactly five solar days long, with the result that the same side always faces earth when the two planets are closest. At such times, the side facing earth can be viewed and mapped by earth-based radio telescopes.

In contrast to the very large antenna needed for earth-based radar mapping of Venus, a modest instrument of Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes and from earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from earth at closest approach.

The more powerful radar aboard the Magellan spacecraft has revealed huge active volcanoes, large solidified lava flows, and a vast array of meteorite craters. The largest impact crater yet observed is almost 160 km (100 mi) across-the smallest about 5 km (3mi). Although the probe's radar could resolve even smaller craters, if any were present, Venus's dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet.

The global survey and other probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Such evidence includes ridges, canyons, a troughlike depression that extends across 1400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes have sent back photographs of the areas in which they set down, and have also measured the natural radioactivity of the rocks. The radioactivity resembles that of granite and strongly suggests that the material of Venus is differentiated, or chemically separated, by volcanic activity. Angular rocks that are visible in the Soviet pictures also suggest the existence of geologic activity that would counteract the forces of erosion.