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Venus (planet)

I INTRODUCTION

Venus (planet), one of the planets in the solar system, the second in distance from the Sun. Except for the Sun and the Moon, Venus is the brightest object in the sky. The planet was named for the Roman goddess of beauty. It is often called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because the planet orbits closer to the Sun than Earth does, Venus seems to either precede or trail the Sun in the sky. Venus is never visible more than three hours before sunrise or three hours after sunset.

II OBSERVATION

When viewed through a telescope, Venus exhibits phases like the Moon. Maximum brilliance (a stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when the planet is closer to Earth. Venus's full phase appears smaller and dimmer because it occurs when the planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeat every 1.6 years (see Time; Year). Transits of Venus (when the planet moves across the face of the Sun as seen from Earth) are rare, occurring in pairs at intervals of a little more than a century. The most recent transit occurred in 2004. The second transit of the pair will be in 2012. The next pair of transits will occur in 2117 and 2125.

Venus is only slightly smaller and less dense than Earth. Its radius is 6,052 km (3,760 mi) and its average density is 5.2 g/cm. The planet's surface gravity is nine-tenths as strong as surface gravity on Earth; an object that weighs ten kilograms on Earth would weigh 9 kilograms on Venus. Venus circles the Sun once every 224.7 days. The planet rotates very slowly, once every 243 days. Venus's rotation is retrograde, which means that the planet turns clockwise as seen looking down on its north pole. Earth turns counterclockwise.

III EXPLORATION

Venus's complete cloud cover and deep atmosphere make it difficult to study from Earth. 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 former Union of Soviet Socialist Republics developed several entry probes, some combined with flybys or orbiters: Venera 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 United States 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, measured the upper atmosphere for 14 years. The Magellan probe, launched toward Venus in 1989, transmitted radar images of the planet from 1990 to 1994.

IV ATMOSPHERE

The atmosphere of the planet consists of 97 percent carbon dioxide (CO₂) and is so thick that the surface pressure is 96 bars (compared with 1 bar on Earth). The surface temperature on Venus varies little from place to place and is extremely hot, about 462C (736 K/864F). The high surface temperature is explained by an intense greenhouse effect. Even though only a small percentage of the solar energy that falls on Venus reaches the surface, the planet stays hot because the thick atmosphere prevents the energy from escaping.

That nearly all of Venus's atmosphere is CO₂ is not as strange as it might seem; in fact, the crust of Earth contains almost as much CO₂ chemically bound 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.

Cloud particles on Venus mostly consist of concentrated sulfuric acid. Earth's atmosphere also contains a very thin haze of sulfuric acid particles in the stratosphere. On Earth, however, sulfuric acid does not build up because rain carries it down to react with surface materials. On Venus the acid evaporates at the cloud base, which lies about 50 km (31 mi) above the surface, and so remains 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 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 that can be discerned in the cloud tops give some information about wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph). These winds cover the planet completely, blowing toward the east at virtually every latitude from equator to pole. The motions of descending probes, however, have shown that the bulk of Venus's tremendously dense atmosphere, closer to 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/h (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 were studied in great detail by Pioneer Venus 1, which passed 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, on the night side of Venus the upper atmosphere is extremely cold. (Day-side temperatures are 40C/104F, compared to night-side temperatures of -170C/-274F.) Scientists believe 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 around the planet, probably by a dynamo action involving its own magnetic field.

V SURFACE FEATURES

Curiously, the periods of Venus's orbit and rotation cause the same side of the planet to always face Earth when the two planets are closest. At such times, the side facing Earth can be viewed and mapped by Earth-based radar.

In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and 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 volcanoes, large solidified lava flows, and a large array of meteorite craters. The largest impact crater is almost 300 km (190 mi) across—the smallest about 5 km (3 mi). 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. It is believed that all craters older than about 500 million years have been obliterated, while the more recent ones show little sign of modification.

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 1,400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes sent back photographs of the areas in which they set down, and 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.

Contributed By:

Donald M. Hunten

Venus

Venus is the brightest object in our sky, after the sun and moon. Swirling clouds of sulfur and sulfuric acid obscure Venus's surface and inhibited study of the planet from Earth until technology permitted space vehicles, outfitted with probes, to visit it. These probes determined that Venus is the hottest of the planets, with a surface temperature of about 460 C (about 860 F). Scientists believe that a greenhouse effect causes the extreme temperature, hypothesizing that the planet's thick clouds and dense atmosphere trap energy from the sun.

Photo Researchers, Inc./NASA/Science Source