ancient Italian goddess associated with cultivated fields and gardens and later identified by the Romans with the Greek goddess of love, Aphrodite.

As a native Italian deity, Venus had no myths of her own. She therefore took over those of Aphrodite and, through her, became identified with various foreign goddesses. The most noteworthy result of this development is perhaps the acquisition by the planet Venus of that name. The planet was at first the star of the Babylonian goddess Ishtar and thence of Aphrodite. Because of her association with love and with feminine beauty, the goddess Venus has been a favourite subject in art since ancient times; notable representations include the statue known as the “Venus de Milo” (c. 150 BC) and the painting “The Birth of Venus” (c. 1485; see photograph-->) by Sandro Botticelli.

second planet from the Sun and sixth in the solar system in size and mass. No planet approaches closer to Earth than Venus; at its nearest it is the closest large body to Earth other than the Moon. Because Venus's orbit is nearer the Sun than Earth's, the planet is always roughly in the same direction in the sky as the Sun and can be seen only in the hours near sunrise or sunset. When it is visible, it is the most brilliant planet in the sky. Venus is designated by the symbol ♀.

Venus was one of the five planets—along with Mercury, Mars, Jupiter, and Saturn—known in ancient times, and its motions were observed and studied for centuries prior to the invention of advanced astronomical instruments. Its appearances were recorded by the Babylonians, who equated it with the goddess Ishtar, about 3000 BC, and it also is mentioned prominently in the astronomical records of other ancient civilizations, including those of China, Central America, Egypt, and Greece. Like the planet Mercury, Venus was known in ancient Greece by two different names—Phosphorus (see Lucifer) when it appeared as a morning star and Hesperus when it appeared as an evening star. Its modern name comes from the Roman goddess of love and beauty (the Greek equivalent being Aphrodite), perhaps because of the planet's luminous jewel-like appearance.

Planetary data for Venus Planetary data for VenusVenus has been called Earth's twin because of the similarities in their masses, sizes, and densities and their similar relative locations in the solar system. Because they presumably formed in the solar nebula from the same kind of rocky planetary building blocks, they also likely have similar overall chemical compositions. Early telescopic observations of the planet revealed a perpetual veil of clouds, suggestive of a substantial atmosphere and leading to popular speculation that Venus was a warm, wet world, perhaps similar to Earth during its prehistoric age of swampy carboniferous forests and abundant life. Scientists now know, however, that Venus and Earth have evolved surface conditions that could hardly be more different. Venus is extremely hot, dry, and in other ways so forbidding that it is improbable that life as it is understood on Earth could have developed there. One of scientists' major goals in studying Venus is to understand how its harsh conditions came about, which may hold important lessons about the causes of environmental change on Earth.

Planetary data for VenusVenus's mean radius is 6,051.8 km (3,760.4 miles), or about 95 percent of Earth's at the Equator, while its mass is 4.87 × 10²⁴ kg, or 81.5 percent that of Earth. The similarities to Earth in size and mass produce a similarity in density—5.25 grams per cubic centimetre for Venus, compared with 5.52 for Earth. They also result in a comparable surface gravity—humans standing on Venus would possess nearly 90 percent of their weight on Earth. Venus is more nearly spherical than most planets. A planet's rotation generally causes a bulging at the equator and a slight flattening at the poles, but Venus's very slow spin allows it to maintain its highly spherical shape. For additional orbital and physical data, see the table (Planetary data for Venus).

Venus has the most massive atmosphere of the terrestrial planets, which includes Mercury, Earth, and Mars. Its gaseous envelope is composed of more than 96 percent carbon dioxide and 3.5 percent molecular nitrogen. Trace amounts of other gases are present, including carbon monoxide, sulfur dioxide, water vapour, argon, and helium. The atmospheric pressure at the planet's surface varies with surface elevation; at the elevation of the planet's mean radius it is about 95 bars, or 95 times the atmospheric pressure at Earth's surface. This is the same pressure found at a depth of about 1 km (0.6 mile) in Earth's oceans.

Most information about wind directions at the planet's surface comes from observations of wind-blown materials. Despite low surface-wind velocities, the great density of Venus's atmosphere enables these winds to move loose fine-grained materials, producing surface features that have been seen in radar images. Some features resemble sand dunes, while others are “wind streaks” produced by preferential deposition or erosion downwind from topographic features. The directions assumed by the wind-related features suggest that in both hemispheres the surface winds blow predominantly toward the equator. This pattern is consistent with the idea that simple hemispheric-scale circulation systems called Hadley cells (Hadley cell) exist in the Venusian atmosphere. According to this model, atmospheric gases rise upward as they are heated by solar energy at the planet's equator, flow at high altitude toward the poles, sink to the surface as they cool at higher latitudes, and flow toward the equator along the planet's surface until they warm and rise again. Some deviations from the equatorward flow pattern are observed on regional scales. They may be caused by the influence of topography on wind circulation.

The high atmospheric pressure, the low wind velocities, and, in particular, the extremely high temperatures create a surface environment on Venus that is markedly different from any other in the solar system. A series of landings by robotic Soviet (Venera) spacecraft in the 1970s and early '80s provided detailed data on surface composition and appearance. Views of the Venusian landscape, typified in colour images obtained by the Soviet Union's Venera 13 lander in 1982 (see figure-->), show rocky plains that stretch toward the horizon. Despite the heavy cloud cover, the surface is well illuminated by the yellow-orange light that filters through the clouds.

Earth-based observatories and Venus-orbiting spacecraft have provided global-scale information on the nature of the planet's surface. All have used radar systems to penetrate the thick Venusian clouds.

Two striking features are the continent-sized highland areas, or terrae— Ishtar Terra in the northern hemisphere and Aphrodite Terra along the equator. Ishtar is roughly the size of Australia, while Aphrodite is comparable in area to South America. Ishtar possesses the most spectacular topography on Venus. Much of its interior is a high plateau, called Lakshmi Planum, that resembles in configuration the Plateau of Tibet on Earth. Lakshmi is bounded by mountains on most sides, the largest range being the enormous Maxwell Montes on the east. These mountains soar about 11 km (7 miles) above the mean radius of Venus. The topography of Aphrodite, more complex than that of Ishtar, is characterized by a number of distinct mountain ranges and several deep, narrow troughs. In addition to the two main terrae are several smaller elevated regions, including Alpha Regio, Beta Regio, and Phoebe Regio.

Found in the terrae, Venus's mountain belts are in some ways similar to ones on Earth such as the Himalayas of Asia and the Andes of South America. Among the best examples are those that encircle Lakshmi Planum, which in addition to Maxwell Montes include Freyja, Akna, and Danu Montes. Maxwell Montes is particularly broad and comparable in size to the Himalayas.

Rifts (see rift valley) are among the most spectacular tectonic features on Venus. The best-developed rifts are found atop broad, raised areas such as Beta Regio, sometimes radiating outward from their centres like the spokes of a giant wheel. Beta and several other similar regions on Venus appear to be places where large areas of the lithosphere have been forced upward from below, splitting the surface to form great rift valleys. The rifts are composed of innumerable faults, and their floors typically lie 1–2 km (0.6–1.2 miles) below the surrounding terrain. In many ways the rifts on Venus are similar to great rifts elsewhere, such as the East African Rift (East African Rift System) on Earth or Valles Marineris on Mars; volcanic eruptions, for example, appear to have been associated with all these features. The Venusian rifts differ from Earth and Martian ones, however, in that little erosion has taken place within them owing to the lack of water.

Coronae (Latin: “garlands” or “crowns”) are landforms that apparently owe their origin to the effects of hot, buoyant blobs of material, known from terrestrial geology as diapirs (diapir), that originate deep beneath the surface of Venus. Coronae evolve through several stages. As diapirs first rise through the planet's interior and approach the surface, they can lift the rocks above them, fracturing the surface in a radial pattern. This results in a distinctive starburst of faults and fractures, often lying atop a broad, gently sloping topographic rise. (Such features are sometimes called novae, a name given to them when their evolutionary relationship to coronae was less certain.) Once a diapir has neared the surface and cooled, it loses its buoyancy. The initially raised crust then can sag under its own weight, developing concentric faults as it does so. The result is a circular-to-oval pattern of faults, fractures, and ridges. Volcanism can occur through all stages of corona formation. During the late stages it tends to obscure the radial faulting that is characteristic of the early stages.

Coronae are typically a few hundred kilometres in diameter. Although they may have a raised outer rim, many coronae sag noticeably in their interiors and also outside their rims. Hundreds of coronae are found on Venus, observed at all stages of development. The radially fractured domes of the early stages are comparatively uncommon, while the concentric scars characteristic of mature coronae are among the most numerous large tectonic features on the planet.

Tesserae (Latin: “mosaic tiles”) are the most geologically complex regions seen on Venus. Several large elevated regions, such as Alpha Regio, are composed largely of tessera terrain. Such terrain appears extraordinarily rugged and highly deformed in radar images, and in some instances it displays several different trends of parallel ridges and troughs that cut across one another at a wide range of angles. The deformation in tessera terrain can be so complex that sometimes it is difficult to determine what kinds of stresses in the lithosphere were responsible for forming it. In fact, probably no single process can explain all tessera formation. Tesserae typically appear very bright in radar images, which suggests an extremely rough and blocky surface at scales of metres. Some tesserae may be old terrain that has been subjected to more episodes of mountain building and faulting than have the materials around it, each one superimposed on its predecessor to produce the complex pattern observed.

Along with intense tectonic activity, Venus has undergone much volcanism. The largest volcanic outpourings are the huge lava fields that cover most of the rolling plains. These are similar in many respects to fields of overlapping lava flows seen on other planets, including Earth, but they are far more extensive. Individual flows are for the most part long and thin, which indicates that the erupting lavas were very fluid and hence were able to flow long distances over gentle slopes. Lavas on Earth and the Moon that flow this readily typically consist of basalts (basalt), and so it is probable that basalts are common on the plains of Venus as well.

In many locations on Venus, volcanic eruptions have built edifices similar to the great volcanoes of Hawaii on Earth or those associated with the Tharsis region on Mars. Sif Mons is an example of such a volcano; there are more than 100 others distributed widely over the planet. Known as shield volcanoes, they reach heights of several kilometres above the surrounding plains and can be hundreds of kilometres across at their base. They are made up of many individual lava flows piled on one another in a radial pattern. They develop when a source of lava below the surface remains fixed and active at one location long enough to allow the volcanic materials it extrudes to accumulate above it in large quantities. Like those found on the rolling plains, the flows constituting the shield volcanoes are generally very long and thin and are probably composed of basalt.

When a subsurface source of lava is drained of its contents, the ground above it may collapse, forming a depression called a caldera. Many volcanic calderas are observed on Venus, both atop shield volcanoes and on the widespread lava plains. They are often roughly circular in shape and overall are similar to calderas observed on Earth and Mars. The summit region of Sif Mons, for example, exhibits a caldera-like feature 40–50 km (25–30 miles) in diameter.

Along with the extensive lava plains and the massive shield volcanoes are many smaller volcanic landforms. Enormous numbers of small volcanic cones are distributed throughout the plains. Particularly unusual in appearance are so-called pancake domes, which are typically a few tens of kilometres in diameter and about 1 km (0.6 mile) high and are remarkably circular in shape. Flat-topped and steep-sided, they appear to have formed when a mass of thick lava was extruded from a central vent and spread outward for a short distance in all directions before solidifying. The lavas that formed such domes clearly were much more viscous than most lavas on Venus. Their composition is unknown, but—given the knowledge of lavas on Earth—they are likely to be much richer in silica (see silica mineral) than the basalts thought to predominate elsewhere on the planet.

The Venusian surface has been altered by objects from outside the planet as well as by forces from within. Impact craters dot the landscape, created by meteorites that passed through the atmosphere and struck the surface. Nearly all solid bodies in the solar system bear the scars of meteoritic impacts, with small craters typically being more common than large ones. This general tendency is encountered on Venus as well—craters a few hundred kilometres across are present but rare, while craters tens of kilometres in diameter and smaller are common. Venus has an interesting limitation, however, in that craters smaller than about 1.5–2 km (1–1.2 miles) in diameter are not found. Their absence is attributable to the planet's dense atmosphere, which causes intense frictional heating and strong aerodynamic forces as meteorites plunge through it at high velocities. The larger meteorites reach the surface intact, but the smaller ones are slowed and fragmented in the atmosphere. In fact, craters several kilometres in size—i.e., near the minimum size observed—tend not to be circular. Instead they have complex shapes, often with several irregular pits rather than a single central depression, which suggests that the impacting body broke up into a number of fragments that struck the surface individually. Radar images also show diffuse dark and bright “smudges” that may be have resulted from the explosions of small meteorites above the surface.

Perhaps the strangest property of Venusian craters is one associated with some of the youngest. In addition to the normal ejecta, these craters are partially surrounded by huge parabola-shaped regions of dark material, a feature not found elsewhere in the solar system. In every case, the parabola opens to the west, and the crater is nestled within it, toward its eastern extremity. In radar images the dark materials tend to be smooth at small scales; it is likely that these parabolas are composed of deposits of fine-grained ejecta that was thrown upward during the impact event. Apparently the material rose above the Venusian atmosphere, fell back, and was picked up by the high-speed westward-blowing winds that encircle the planet. It was then carried far downwind from the impact site, eventually descending to the surface to form a parabola-shaped pattern.

Important early telescopic observations of Venus were conducted in the 1700s during the planet's solar transits (transit) (see eclipse: Transits of Mercury and Venus (eclipse)). In a solar transit an object passes directly between the Sun and Earth and is silhouetted briefly against the Sun's disk. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. They were extremely important events to 18th-century astronomy, since they provided the most accurate method known at that time for determining the distance between Earth and the Sun. (This distance, known as the astronomical unit, is one of the fundamental units of astronomy.) Observations of the 1761 transit were only partially successful but did result in the first suggestion, by the Russian scientist Mikhail V. Lomonosov (Lomonosov, Mikhail Vasilyevich), that Venus has an atmosphere. The second transit of the pair, in 1769, was observed with somewhat greater success. Transits must be viewed from many points on Earth to yield accurate distances, and the transits of 1761 and, particularly, 1769 prompted the launching of many scientific expeditions to remote parts of the globe. Among these was the first of the three voyages of exploration by the British naval officer James Cook (Cook, James), who, with scientists from the Royal Society, observed the 1769 transit from Tahiti. The transit observations of the 1700s not only gave an improved value for the astronomical unit but also provided the impetus for many unrelated but important discoveries concerning Earth's geography. By the time the subsequent pair of transits occurred, in 1874 and 1882, the nascent field of celestial photography had advanced enough to allow scientists to record on glass plates what they saw through their telescopes. No transits took place in the 20th century; the first of the next pair was widely observed and imaged in 2004.

Successful early Venus missions undertaken by the United States involved Mariner 2, Mariner 5 (1967), and Mariner 10 (1974). Each spacecraft made a single close flyby, providing successively improved scientific data in accord with concurrent advances in spacecraft and instrument technology. After visiting Venus, Mariner 10 went on to a successful series of flybys of Mercury. In 1978 the United States launched the Pioneer Venus mission, comprising two complementary spacecraft. The Orbiter went into orbit around the planet, while the Multiprobe released four entry probes—one large probe and three smaller ones—that were targeted to widely separated points in the Venusian atmosphere to collect data on atmospheric structure and composition. The Orbiter carried 17 scientific instruments, most of them focused on study of the planet's atmosphere, ionosphere, and interaction with the solar wind. Its radar altimeter provided the first high-quality map of Venus's surface topography. Pioneer Venus Orbiter was one of the longest-lived planetary spacecraft, returning data for more than 14 years.

Venus was also a major target of the Soviet Union's planetary exploration program during the 1960s, '70s, and '80s, which achieved several spectacular successes. After an early sequence of failed missions, in 1967 Soviet scientists launched Venera 4, comprising a flyby spacecraft as well as a probe that entered the planet's atmosphere. Highlights of subsequent missions included the first successful soft landing on another planet (Venera 7 in 1970), the first images returned from the surface of another planet (Venera 9 and 10 landers in 1975), and the first spacecraft placed in orbit around Venus (Venera 9 and 10 orbiters).

In 1990, on its way to Jupiter, the U.S. Galileo spacecraft flew by Venus. Among its more notable observations were images at near-infrared wavelengths that viewed deep into the atmosphere and showed the highly variable opacity of the main cloud deck.

The most ambitious mission yet to Venus, the U.S. Magellan spacecraft, was launched in 1989 and the next year entered orbit around the planet, where it conducted observations until late 1994. Magellan carried a radar system capable of producing images with a resolution better than 100 metres (330 feet). Because the orbit was nearly polar, the spacecraft was able to view essentially all latitudes on the planet. On each orbit the radar system obtained an image strip about 20 km (12 miles) wide and typically more than 16,000 km (almost 10,000 miles) long, extending nearly from pole to pole. The image strips were assembled into mosaics, and high-quality radar images of about 98 percent of the planet were ultimately produced. Magellan also carried a radar altimeter system that measured the planet's surface topography as well as some properties of its surface materials. After the main radar objectives of the mission were completed, the spacecraft's orbit was modified slightly so that it passed repeatedly through the upper fringes of the Venusian atmosphere. The resulting drag on the spacecraft gradually removed energy from its orbit, turning an initially elliptical orbit into a low, circular one. This procedure, known as aerobraking, has since been used on other planetary missions to conserve large amounts of fuel by reducing the use of thrusters for orbital reshaping. From its new circular orbit, the Magellan spacecraft was able to make the first detailed map of Venus's gravitational field.

Ladislav E. Roth and Stephen D. Wall (eds.), The Face of Venus: The Magellan Radar-Mapping Mission (1995), is a good post-Magellan popular-level book on the Venusian surface, with many excellent illustrations. Still informative pre-Magellan popular-level treatments include Garry E. Hunt and Patrick Moore, The Planet Venus (1982); and Eric Burgess, Venus, an Errant Twin (1985).The scientific understanding of Venus is definitively and comprehensively summarized in S.W. Bougher, D.M. Hunten, and R.J. Phillips (eds.), Venus II (1997), a collection of papers written after the Magellan and Galileo missions. Mikhail Ya. Marov and David H. Grinspoon, The Planet Venus (1998), provides an excellent post-Magellan treatment of Venusian geology and most other aspects of the planet. A pre-Magellan overview of Venus's surface is given by Alexander T. Basilevsky and James W. Head III, “The Geology of Venus,” Annual Review of Earth and Planetary Sciences, 16:295–317 (1988). An interesting collection of papers by Soviet scientists in English on Venus is V.L. Barsukov et al. (eds.), Venus Geology, Geochemistry, and Geophysics (1992). The results of major spacecraft missions to Venus are reported in several journals: on Pioneer Venus, Journal of Geophysical Research, 85:7573–8337 (1980); V.L. Barsukov et al., “The Geology and Geomorphology of the Venus Surface as Revealed by the Radar Images Obtained by Veneras 15 and 16,” Journal of Geophysical Research, pt. B, Solid Earth and Planets, 91(B4):D378–D398 (March 30, 1986); Science, 253:1457–1612 (Sept. 27, 1991), an issue devoted to the Galileo flyby of Venus; and two issues of Journal of Geophysical Research, pt. E, Planets, vol. 97, devoted to detailed Magellan mission results: no. 8 (Aug. 25, 1992); and no. 10 (Oct. 25, 1992).Steven W. Squyres