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[Middle English, from Old English, from Latin, love, Venus.]
The second planet in distance from the Sun. This neighbor of the Earth is very similar to it in such gross characteristics as mass, radius, and density. In other ways Venus is apparently different. Its atmospheric mass is almost a hundred times that of the Earth; its atmosphere is mostly carbon dioxide instead of nitrogen and oxygen; an extensive cloud layer of concentrated sulfuric acid is present; its surface temperature is an unbearable 850°F (730 K); and it rotates with a period of 243 days, and from east to west, in the opposite sense of most other planets. Some of these differences are due more to alternate evolutionary paths of the two planets than to totally different initial conditions.
To the naked eye, Venus is the brightest starlike object in the sky. It is usually visible during the night either soon after sunset or close to sunrise. It can sometimes be seen during the daytime.
The light seen coming from Venus is entirely due to sunlight that is reflected from a dense cloud layer whose top is located about 45 mi (70 km) above the surface and whose bottom lies within 30 mi (50 km) of the surface. In contrast to the Earth's approximately 50% cloud cover, the clouds of Venus are present over the entire planet. The clouds of Venus consist of a large number of tiny particles, about 1 micrometer in size, that are made of a water solution of concentrated sulfuric acid.
By far, the chief gas species of Venus's atmosphere is carbon dioxide, which makes up 96% of the atmospheric molecules, while nitrogen accounts for almost all the remainder. In contrast to the dominance of carbon dioxide in Venus's atmosphere, the Earth's atmosphere consists mostly of nitrogen and oxygen, with carbon dioxide being present at a level of only 340 ppm. In part, this difference may stem more from temperature differences than from intrinsic differences. Over the lifetime of the Earth, an amount of carbon dioxide comparable to that in Venus's atmosphere was vented out of the Earth's hot interior, but almost all of it participated with rain in dissolving land rocks, and rivers carried the dissolved rock and carbon dioxide into the oceans, where they subsequently precipitated to form carbonate rocks, such as limestone. Venus's surface is much too hot for oceans of water to be present, and hence its atmosphere has been able to retain essentially all of the carbon dioxide vented from its interior. See also Atmospheric chemistry.
The atmospheric temperature has a relatively cool value of −10°F (250 K) near the top of the cloud layer, which is at a pressure of about 1/20 that at the Earth's surface. The temperature gradually increases with decreasing altitude until it reaches 850°F (730 K) at the surface, where the pressure is 90 times that at the Earth's surface. The high value of Venus's surface temperature is not due to its being closer to the Sun than the Earth. Because its cloud layer reflects to space about 75% of the incident sunlight, Venus actually absorbs less solar energy than does the Earth. Rather, the high temperature is the result of a very efficient greenhouse effect that allows a small but significant fraction of the incident sunlight to penetrate to the surface (about 2.5%), but prevents all except a negligible fraction of the heat generated by the surface from escaping directly to space. See also Greenhouse effect.
The very similar mean densities of Venus and the Earth imply that Venus is made of rocks similar to those that make up the Earth. Venus's interior may be qualitatively similar to that of the Earth in having a central iron core, a middle mantle made of rocks rich in silicon, oxygen, iron, and magnesium, and a thin outer crust containing rocks enriched in silicon in comparison with the rocks of the mantle. However, in contrast to the situation for the Earth, Venus's core may now be either entirely solid or entirely liquid, which could account for the absence of a detectable magnetic field. See also Earth interior.
Venus has been more intensely explored by spacecraft than any other planet. United States and Soviet spacecraft provided data on the composition of the surface and atmosphere, and the dynamics of the upper atmosphere. The Soviet Venera landers transmitted back the first images from the surface of another planet. The United States Magellan mapped nearly 98% of the surface. See also Space probe.
Magellan's mapping mission revealed a unique global volcanic and tectonic style on Venus. Broad volcanic plains make up about 85% of the surface of Venus. The rest is tectonically deformed, higher-standing terrains with complex systems of folds and faults. Regional tectonism is evident in the widespread compressional and extensional deformation of much of the surface material. Venus apparently has had a dynamic mantle that has driven crustal warping, which may be ongoing. However, while various regions of the planet show evidence of motion, no evidence of Earth-like plate tectonics has been found. See also Plate tectonics.
Long, narrow troughs are seen in many areas where the crust has ruptured; these linear rift zones are associated with extensive broad, domical rises and shield-volcano complexes. The planet also has some unexplained surface features, including long channels meandering across the plains. Also seen are oval to circular volcanic-tectonic structures called coronae that range in diameter from 60 to 1300 mi (100 to 2100 km). Impact craters are much less abundant on Venus than on the Moon or Mars, and currently active volcanism has not been detected on Venus. See also Planet; Planetary physics.
For more information on Venus, visit Britannica.com.
Vēnus, originally an Italian goddess of whom virtually nothing is known; her name means ‘charm’, ‘beauty’, and she seems to have presided over the fertility of vegetable gardens. In Rome at an early date she became identified with and acquired the mythology of the Greek goddess Aphroditē, the goddess of love, through the cult of Aphrodite on Mount Eryx in Sicily which, according to tradition, had been founded by Aeneas after the death of his father Anchisēs. Venus of Eryx (Venus Erycina) had a temple on the Capitoline hill in Rome dedicated in 217 BC at a dire moment in the Second Punic War, and another outside the Colline Gate. Aeneas was the son of Aphrodite/Venus; the story of his wanderings and final settlement in Italy was particularly important to the Julian family (of which Julius Caesar and the emperors down to and including Nero were members) because it was claimed that they were descended from Aeneas' son Iulus (Ascanius) and so from Venus. Julius Caesar, in gratitude to her for his success at the battle of Pharsalus, dedicated a temple to Venus Genetrix, ‘universal mother’, in 46 BC in the Forum Iulium. It is this aspect of Venus that Lucretius extols at the beginning of his poem, De rerum natura, and that is celebrated in the Pervigilium Veneris written for the spring festival. Venus was also thought of as concerned in the origin of the Roman people as the consort of Mars. ‘Venus’ was the name given to the highest throw at dice.
Roman goddess occupying a modest position in the pantheon where, together with Feronia and Flora, she symbolized spring and fruitfulness. Equated with the Greece goddess Aphrodite.
Venus revolves around the sun at a mean distance of c.67 million mi (107 million km) in a nearly circular orbit, and its period of revolution is about 225 days. It comes closer to the earth than any other planet, being c.26 million mi (42 million km) away at inferior conjunction. Venus is often referred to as the sister planet of the earth, because it is only slightly smaller in both size and mass. Several important differences, however, exist between the two planets.
Although Venus is covered with a thick blanket of clouds that hides its surface from view, much has been learned of the conditions on Venus from U.S. and Soviet space probes. These probes indicate a surface temperature of about 890°F (475°C) and an atmospheric pressure as great as 100 times that at the earth's surface. The thick atmosphere is composed mainly of carbon dioxide, with a slight amount of water vapor and a trace of nitrogen and other elements. The high surface temperature is assumed to result partly from the greenhouse effect; radiation passing through the atmosphere heats the surface, but the heat is blocked by the enveloping carbon dioxide from escaping back out through the atmosphere. The European Space Agency's Venus Express space probe began orbiting the planet in 2006; its instruments are designed primarily to study the Venusian atmosphere.
Studies also indicate that Venus rotates on its axis in a retrograde direction (opposite to the direction of revolution about the sun) with a period of about 243 days. Despite this slow rotation there is little observed temperature difference between the lighted and unlighted sides of the planet. The surface of Venus is thought to be stormy.
From 1990 to 1992 NASA's Magellan spacecraft mapped the Venusian surface using radar, revealing details of a continentlike feature, called Aphrodite Terra, that crosses the planet's equator and is marked by geologic faults. A second such feature, Ishtar Terra, straddles the north polar region. Magellan also observed many volcanic features, including immense lava plains and large shield volcanoes, and relatively few impact craters resulting from asteroids and comets. Compared to the number of craters on other bodies of the inner solar system, this suggests that the surface of Venus is only about 800 million years old. No strong magnetic field comparable to that of the earth has been detected.
In astronomy, the second major planet from the sun, named for the Roman goddess of love. The surface of Venus is very hot and covered with clouds. Spacecraft from the former Soviet Union landed on Venus and survived long enough to send back photographs and measurements. (See solar system; see under “Mythology and Folklore.”)
Since orbits of the planets in the solar system are in virtually the same plane, Mercury and Venus can be seen as dots passing in front of the Sun. Such an event is a transit. Kepler was the first to recognize that transits must exist and to calculate when they would occur. With Kepler's calculations in hand, Pierre Gassendi was able to observe the transit of Mercury in 1631, a year after Kepler's death.
Transits of Venus have been especially important in the history of science. They occur in pairs, eight years apart, at intervals of 105.5 or 121.5 years. They can occur only near June or December -- for example, the transits of Venus on June 8, 2004, and June 6, 2012. Jeremiah Horrocks improved on Kepler's calculations and became the first to observe a transit of Venus, on November 24, 1639. It was Horrocks who realized that if a transit of Venus were observed simultaneously from several places on Earth, the information gained could be used to calculate the distance to Venus and the distance from Earth to the Sun.
The first effort to accomplish this goal was organized by Joseph-Nicolas Delisle in 1761. Astronomers were dispatched to India, St. Helena, and other good viewing spots. War prevented some of the necessary observations from being made, and clouds prevented others. A few observations were made, however. It was during this transit that astronomers discovered that Venus has an atmosphere.
The June 3, 1769, transit of Venus became the most famous. It was to observe this transit from Tahiti that Captain James Cook undertook the first of his great voyages of discovery. Leaving England on the Endeavor on August 26, 1768, Cook made the necessary observations in Tahiti and returned to England on July 17, 1771.
Cook's was not the only trip of discovery based on the 1769 transit. The Russians journeyed overland to Siberia to observe the event. Other observers were at various points around the globe.
In the mid-19th century, Johann Encke used the 1769 transit data to calculate Earth's distance to the Sun as 153,000,000 km (95,300,000 mi), the best calculation to that time.
For the transits of 1874 and 1882, George Airy organized vast expeditions to obtain data that could be used to improve on Encke's calculations. He failed, however, to recognize that the atmosphere of Venus causes enough error in the observations that no improvement can be made by this method. Later astronomers were able to arrive at more precise measurements by using the transits of asteroids, which have no atmosphere.
 Click image for description |
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| Orbital characteristics | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Epoch J2000 | ||||||||||
| Aphelion | 108,942,109 km 0.72823128 AU |
|||||||||
| Perihelion: | 107,476,259 km 0.71843270 AU |
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| Semi-major axis: | 0.723332 AU |
|||||||||
| Eccentricity: | 0.0068 | |||||||||
| Orbital period: | 224.70069 day 0.6151970 yr |
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| Synodic period: | 583.92 day | |||||||||
| Avg. orbital speed: | 35.02 km/s | |||||||||
| Inclination: | 3.39471° 3.86° to Sun's equator |
|||||||||
| Longitude of ascending node: | 76.67069° | |||||||||
| Argument of perihelion: | 54.85229° | |||||||||
| Satellites: | None | |||||||||
| Physical characteristics | ||||||||||
| Mean radius: | 6051.8 ± 1.0 km[1] 0.9499 Earths |
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| Flattening: | < 0.0002 [1] | |||||||||
| Declination of North pole: | 67.16° | |||||||||
| Albedo: | 0.65 | |||||||||
| Surface temp.: Kelvin Celsius |
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| Apparent magnitude: | up to -4.6[2] | |||||||||
| Angular size: | 9.7" — 66.0"[2] | |||||||||
| Adjectives: | Venusian or (rarely) Cytherean | |||||||||
| Atmosphere | ||||||||||
| Surface pressure: | 9.3 MPa | |||||||||
| Composition: | ~96.5% Carbon dioxide ~3.5% Nitrogen .015% Sulphur dioxide .007% Argon .002% Water vapor .0017% Carbon monoxide .0012% Helium .0007% Neon trace Carbonyl sulfide trace trace Hydrogen fluoride |
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Venus (IPA: /ˈviːnəs/) is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. It is the brightest natural object in the night sky, except for the Moon, reaching an apparent magnitude of −4.6. Because Venus is an inferior planet, from Earth it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the Evening Star.
Classified as a terrestrial planet, it is sometimes called Earth's "sister planet", for the two are similar in size, gravity, and bulk composition. Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light; this was a subject of great speculation until some of its secrets were revealed by planetary science in the twentieth century. Venus has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide, as it has no carbon cycle to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. It has become so hot that the earth-like oceans the young Venus is believed to have possessed have totally evaporated, leaving a dusty dry desertscape with many slab-like rocks. The evaporated water vapor has dissociated and hydrogen has escaped into interplanetary space. The atmospheric pressure at the planet's surface is 92 times that of the Earth, the great majority of it carbon dioxide and other greenhouse gases.
Venus's surface has been mapped in detail only in the last 20 years (Project Magellan) which listed about a thousand meteor craters, a surprisingly low number compared to Earth. It shows evidence of being geologically very young with extensive volcanism, and the sulfur in the atmosphere is taken by some experts to show many of its volcanoes are still active today, but it is an enigma as to why no evidence of lava flow accompanies any of the visible caldera.
The adjective Venusian is commonly used for items related to Venus, though the Latin adjective is the rarely used Venerean; the now-archaic Cytherean is still occasionally encountered. Venus is the only planet in the Solar System named after a female figure,[5] although two dwarf planets—Ceres and Eris—also have female names.
Venus is thought to undergo periodic episodes of plate tectonics, in which the crust is subducted rapidly within a few million years, separated by periods of a few hundred million years of relative stability. This contrasts strongly with Earth's more or less steady state of ongoing subduction and continential drift, but the venusian behavior corresponds well with Earth modeled then changed by removing the lubricant—the oceans. It is believed the surface rocks of Venus are only about a half-a-billion years old as impact crater analysis suggests that its surface dynamics have exchanged its surface for a clean face (wiping out old craters) sometime in the last billion years.
Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar to the Earth, and is often described as its 'twin'. The diameter of Venus is only 650 km less than the Earth's, and its mass is 81.5% of the Earth's. However, conditions on the Venusian surface differ radically from those on Earth, due to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% composed of nitrogen.[6]
Though there is little direct information about its internal structure, the similarity in size and density between Venus and Earth suggests that it has a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid. The slightly smaller size of Venus suggests that pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of plate tectonics on Venus, likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.[7]
About 80% of Venus's surface consists of smooth volcanic plains. Two highland 'continents' make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above Venus's average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. Much of this continent is covered by a network of fractures and faults.[8]
As well as the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin.[9]
Almost all Venusian surface features are named after historical and mythological women.[10] The only exceptions are Maxwell Montes, named after James Clerk Maxwell, and two highland regions, Alpha Regio and Beta Regio. These three features were named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.[11]
Much of Venus's surface appears to have been shaped by volcanic activity. Overall, Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, while Venus's surface is estimated to be about 500 million years old.[9]
Several lines of evidence point to ongoing volcanic activity on Venus. During the Russian Venera program, the Venera 11 and Venera 12 probes detected a constant stream of lightning, and Venera 12 recorded a powerful clap of thunder soon after it landed. While rainfall drives thunderstorms on Earth, there is no rainfall on Venus. One possibility is that ash from a volcanic eruption was generating the lightning. Another intriguing piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which were found to drop by a factor of 10 between 1978 and 1986. This may imply that the levels had earlier been boosted by a large volcanic eruption.[12]
There are almost 1,000 impact craters on Venus, more or less evenly distributed across its surface. On other cratered bodies, such as the Earth and the Moon, craters show a range of states of erosion, indicating a continual process of degradation. On the Moon, degradation is caused by subsequent impacts, while on Earth, it is caused by wind and rain erosion. However, on Venus, about 85% of craters are in pristine condition. The number of craters together with their well-preserved condition indicates that the planet underwent a total resurfacing event about 500 million years ago.[13] Earth's crust is in continuous motion, but it is thought that Venus cannot sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.[9]
Venusian craters range from 3 km to 280 km in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact crater.[14]
Venus has an extremely thick atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times that of Earth's atmosphere while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1 kilometer under Earth's oceans. The density at the surface is 65 kg/m³ (6.5% that of water). The enormously CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generate the strongest greenhouse effect in the solar system, creating surface temperatures of over 460 °C.[15] This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice Mercury's distance from the Sun and receives only 25% of Mercury's solar irradiance. Because of the lack of any moisture on Venus, there is no relative humidity on the surface, creating a heat index of 450 °C to 480 °C.
Studies have suggested that several billion years ago Venus's atmosphere was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[16] Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of Venus's surface does not vary significantly between the night and day sides, despite the planet's extremely slow rotation. Winds at the surface are slow, moving at a few kilometers per hour, but because of the high density of the atmosphere at Venus's surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This would make it difficult for a human to walk through.[17] Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[18][19] These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of Venus's surface in visible light. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well heated or lit. In the absence of the greenhouse effect caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus would be quite similar to that on Earth. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days.[20]
The surface of Venus is effectively isothermal; it retains a constant temperature between day and night and between the equator and the poles.[21][22] The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth), also minimises seasonal temperature variation.[23] The only appreciable variation in temperature occurs with altitude. In 1995, the Magellan probe imaged a highly reflective substance at the tops of Venus's highest mountain peaks which bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to galena (lead sulfide).[24]
In 1980, The Pioneer Venus Orbiter found that Venus's magnetic field is both weaker and smaller (i.e. closer to the planet) than Earth's. What small magnetic field is present is induced by an interaction between the ionosphere and the solar wind,[25] rather than by an internal dynamo in the core like the one inside the Earth. Venus's magnetosphere is too weak to protect the atmosphere from cosmic radiation.
This lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected to also contain a dynamo in its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive, however. Also, while its rotation is often thought to be too slow, simulations show that it is quite adequate to produce a dynamo.[26][27] This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. Since Venus has no plate tectonics to let off heat, it is possible that it has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified.
Venus orbits the Sun at an average distance of about 108 million km, and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus is the closest to circular, with an eccentricity of less than 1%. When Venus lies between the Earth and the Sun, a position known as 'inferior conjunction', it makes the closest approach to Earth of any planet, lying at a distance of about 40 million km. The planet reaches inferior conjunction every 584 days, on average.
Venus rotates once every 243 days—by far the slowest rotation period of any of the major planets. A Venusian sidereal day thus lasts more than a Venusian year (243 versus 224.7 Earth days). However, the length of a solar day on Venus is significantly shorter than the sidereal day; to an observer on the surface of Venus the time from one sunrise to the next would be 116.75 days.[28] The Sun would appear to rise in the west and set in the east. At the equator, Venus's surface rotates at 6.5 km/h; on Earth, the rotation speed at the equator is about 1,600 km/h.
If viewed from above the Sun's north pole, all of the planets are orbiting in a counter-clockwise direction; but while most
planets also rotate counter-clockwise, Venus rotates clockwise in "retrograde" rotation. The question of how Venus came to have a slow, retrograde rotation
was a major puzzle for scientists when the planet's rotation period was first measured. When it formed from the
A curious aspect of Venus's orbit and rotation periods is that the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days. Whether this relationship arose by chance or is the result of some kind of tidal locking with the Earth, is unknown.[31]
Venus is currently moonless, though the asteroid 2002 VE68 presently maintains a quasi-orbital relationship with it.[32] According to Alex Alemi and David Stevenson of the California Institute of Technology, their recent study of models of the early solar system shows that it is very likely that, billions of years ago, Venus had at least one moon, created by a huge impact event.[33][34] About 10 million years later, according to Alemi and Stevenson, another impact reversed the planet's spin direction. The reversed spin direction caused the Venusian moon to gradually spiral inward[35] until it collided and merged with Venus. If later impacts created moons, those moons also were absorbed the same way the first one was. The Alemi/Stevenson study is recent, and it remains to be seen what sort of acceptance it will achieve in the scientific community.
Venus is always brighter than the brightest stars, with its apparent magnitude ranging from −3.8 to −4.6. This is bright enough to be seen even in the middle of the day, and the planet can be easy to see when the Sun is low on the horizon. As an inferior planet, it always lies within about 47° of the Sun.[36]
Venus 'overtakes' the Earth every 584 days as it orbits the Sun. As it does so, it goes from being the 'Evening star', visible after sunset, to being the 'Morning star', visible before sunrise. While Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported 'unidentified flying object'. In 1973, future U.S. President Jimmy Carter reported having seen a UFO in 1969, which later analysis suggested was probably the planet, and countless other people have mistaken Venus for something more exotic.[37]
As it moves around its orbit, Venus displays phases like those of the Moon: it is new when it passes between the Earth and the Sun, full when it is on the opposite side of the Sun, and a crescent when it is at its maximum elongations from the Sun. Venus is brightest when it is a thin crescent; it is much closer to Earth when a thin crescent than when gibbous, or full.
Venus's orbit is slightly inclined relative to the Earth's orbit; thus, when the planet passes between the Earth and the Sun, it usually does not cross the face of the Sun. However, transits of Venus do occur in pairs separated by eight years, at intervals of about 120 years, when the planet's inferior conjunction coincides with its presence in the plane of the Earth's orbit. The most recent transit was in 2004; the next will be in 2012. Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the astronomical unit, and hence of the solar system. Captain Cook's exploration of the east coast of Australia came after he had sailed to Tahiti in 1768 to observe a transit of Venus.
A long-standing mystery of Venus observations is the so-called Ashen light—an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object.[38]
 Venus and pelicans at twilight |
 Venus transits the face of the Sun on June 8 2004. |
Venus was known in the Hindu Jyotisha since early times as the planet Shukra. In the West, before the advent of the telescope, Venus was known only as a '
Venus's atmosphere was discovered as early as 1790 by Johann Schröter. Schröter found that when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised that this was due to scattering of sunlight in a dense atmosphere. Later, Chester Smith Lyman observed a complete ring around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere.[2] The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Giovanni Cassini and Schröter incorrectly estimated periods of about 24 hours from the motions of markings on the planet's apparent surface.[3]
Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint as to what its surface might be like, and it was only with the development of spectroscopic, radar and ultraviolet observations that more of its secrets were revealed. The first UV observations were carried out in the 1920s, when Frank E. Ross found that UV photographs revealed considerable detail that was absent in visible and infrared radiation. He suggested that this was due to a very dense yellow lower atmosphere with high cirrus clouds above it.[4]
Spectroscopic observations in the 1900s gave the first clues about Venus's rotation. Vesto Slipher tried to measure the Doppler shift of light from Venus, but found that he could not detect any rotation. He surmised that the planet must have a much longer rotation period than had previously been thought.[5] Later work in the 1950s showed that the rotation was retrograde. Radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period which were close to the modern value.[6]
Radar observations in the 1970s revealed details of Venus's surface for the first time. Pulses of radio waves were beamed at the planet using the 300 m radio telescope at Arecibo Observatory, and the echoes revealed two highly reflective regions, designated the Alpha and Beta regions. The observations also revealed a bright region attributed to mountains, which was called Maxwell Montes.[7] These three features are now the only ones on Venus which do not have female names.
The best radar images obtainable from Earth revealed features no smaller than about 5 km across. More detailed exploration of the planet could only be carried out from space.
The first robotic space probe mission to Venus, and the first to any planet, began on February 12 1961 with the launch of the Venera 1 probe. The first craft of the otherwise highly successful Soviet Venera program, Venera 1 was launched on a direct impact trajectory, but contact was lost seven days into the mission, when the probe was about 2 million km from Earth. It was estimated to have passed within 100,000 km from Venus in mid-May.
The United States exploration of Venus also started badly with the loss of the Mariner 1 probe on launch. The subsequent Mariner 2 mission enjoyed greater success, and after a 109-day transfer orbit on December 14 1962 it became the world's first successful interplanetary mission, passing 34,833 km above the surface of Venus. Its microwave and infrared radiometers revealed that while Venus's cloud tops were cool, the surface was extremely hot—at least 425 °C, finally ending any hopes that the planet might harbor ground-based life. Mariner 2 also obtained improved estimates of Venus's mass and of the astronomical unit, but was unable to detect either a magnetic field or radiation belts.[8]
The Venera 3 probe crash-landed on Venus on March 1 1966. It was the first man-made object to enter the atmosphere and strike the surface of another planet, though its communication system failed before it was able to return any planetary data. Venus's next encounter with an unmanned probe came on October 18 1967 when Venera 4 successfully entered the atmosphere and deployed a number of science experiments. Venera 4 showed that the surface temperature was even hotter than Mariner 2 had measured at almost 500 °C, and that the atmosphere was about 90 to 95% carbon dioxide. The Venusian atmosphere was considerably denser than Venera 4's designers had anticipated, and its slower than intended parachute descent meant that its batteries ran down before the probe reached the surface. After returning descent data for 93 minutes, Venera 4's last pressure reading was 18 bar at an altitude of 24.96 km.
Another probe arrived at Venus one day later on October 19 1967 when Mariner 5 conducted a flyby at a distance of less than 4,000 km above the cloud tops. Mariner 5 was originally built as backup for the Mars-bound Mariner 4, but when that mission was successful, the probe was refitted for a Venus mission. A suite of instruments more sensitive than those on Mariner 2, in particular its radio occultation experiment, returned data on the composition, pressure and density of Venus's atmosphere.[9] The joint Venera 4–Mariner 5 data were analyzed by a combined Soviet-American science team in a series of colloquia over the following year, in an early example of space cooperation.
Armed with the lessons and data learned from Venera 4, the Soviet Union launched the twin probes Venera 5 and Venera 6 five days apart in January 1969; they encountered Venus a day apart on May 16 and May 17 that year. The probes were strengthened to improve their crush depth to 25 atmospheres and were equipped with smaller parachutes to achieve a faster descent. Since then current atmospheric models of Venus suggested a surface pressure of between 75 and 100 atmospheres, neither were expected to survive to the surface. After returning atmospheric data for a little over fifty minutes, they both were crushed at altitudes of approximately 20 km before going on to strike the surface on the night side of Venus.
Venera 7 represented a concerted effort to return data from the planet's surface, and was constructed with a reinforced descent module capable of withstanding a pressure of 180 bar. The module was pre-cooled prior to entry and equipped with a specially reefed parachute for a rapid 35-minute descent. Entering the atmosphere on December 15 1970, the parachute is believed to have partially torn during the descent, and the probe struck the surface with a hard, yet not fatal, impact. Probably tilted onto its side, it returned a weak signal supplying temperature data for 23 minutes, the first telemetry received from the surface of another planet.
The Venera program continued with Venera 8 sending data from the surface for 50 minutes, and Venera 9 and Venera 10 sending the first images of the Venusian landscape. The two landing sites presented very different visages in the immediate vicinities of the landers: Venera 9 had landed on a 20 degree slope scattered with boulders around 30–40 cm across; Venera 10 showed basalt-like rock slabs interspersed with weathered material.
In the meantime, the United States had sent the Mariner 10 probe on a gravitational slingshot trajectory past Venus on its way to Mercury. On February 5, 1974, Mariner 10 passed within 5790 km of Venus, returning over 4,000 photographs as it did so. The images, the best then achieved, showed the planet to be almost featureless in visible light, but ultraviolet light revealed details in the clouds that had never been seen in Earth-bound observations.[10]
The American Pioneer Venus project consisted of two separate missions.[11] The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4 1978, and remained there for over thirteen years studying the atmosphere and mapping the surface with radar. The Pioneer Venus Multiprobe released a total of five probes which entered the atmosphere on December 9 1978, returning data on its composition, winds and heat fluxes.
Four more Venera lander missions took place over the next four years, with Venera 11 and Venera 12 detecting Venusian electrical storms; and Venera 13 and Venera 14, landing four days apart on March 1 and March 5 1982, returning the first color photographs of the surface. All four missions deployed parachutes for braking in the upper atmosphere, but released them at altitudes of 50 km, the dense lower atmosphere providing enough friction to allow for an unaided soft landing. Both Venera 13 and 14 analyzed soil samples with an on-board X-ray fluorescence spectrometer, and attempted to measure the compressibility of the soil with an impact probe. Venera 14, though, had the misfortune to strike its own ejected camera lens cap and its probe failed to make contact with the soil. The Venera program came to a close in October 1983 when Venera 15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with synthetic aperture radar.
The Soviet Union had not finished with Venus, and in 1985 it took advantage of the opportunity to combine missions to Venus and Comet Halley, which passed through the inner solar system that year. En route to Halley, on June 11 and June 15 1985 the two spacecraft of the Vega program each dropped a Venera-style probe (of which Vega 1's partially failed) and released a balloon-supported aerobot into the upper atmosphere. The balloons achieved an equilibrium altitude of around 53 km, where pressure and temperature are comparable to those at Earth's surface. They remained operational for around 46 hours, and discovered that the Venusian atmosphere was more turbulent than previously believed, and subject to high winds and powerful convection cells.[12][13]
A manned Venus flyby mission, using Apollo program hardware, was proposed in the late 1960s.[14] The mission was planned to launch in late October or early November of 1973, and would have used a Saturn V to send three men to fly past Venus in a flight lasting approximately one year. The spacecraft would have passed approximately 5,000 kilometres from the surface of Venus about four months later.
The United States'
The Venus Express probe was designed and built by the European Space Agency. Launched by the Russian Federal Space Agency on November 9 2005, it successfully assumed a polar orbit around Venus on April 11, 2006.
The probe is undertaking a detailed study of the Venusian atmosphere and clouds, and will also map the planet's plasma environment and surface characteristics, particularly temperatures. Its mission is intended to last a nominal 500 Earth days, or around two Venusian years.[16] One of the first results emerging from Venus Express is the discovery that a huge double atmospheric vortex exists at the south pole of the planet.
NASA's MESSENGER mission to Mercury performed two flybys of Venus in October 2006 and June 2007, in order to slow its trajectory for an eventual orbital insertion of Mercury in 2011. MESSENGER collected scientific data on both those flybys. The European Space Agency (ESA) will also launch a mission to Mercury, called BepiColombo, in August 2013 that will perform two flybys of Venus before it reaches Mercury orbit in 2019.
Future dedicated missions to Venus are also being planned. Japan's aerospace body JAXA is planning to launch its Venus climate orbiter, the PLANET-C, in 2010. Under its New Frontiers Program, NASA has proposed a lander mission called the Venus In-Situ Explorer, which would launch in 2013 and land on Venus to study surface conditions and investigate the elemental and mineralogical features of the regolith. The lander will also be equipped with a core sampler to drill into the surface to study pristine rock samples not weathered by the very harsh surface conditions of the planet.
