Encyclopedia
Venus is the second-closest
planet to the
Sun, orbiting it every 224.7 Earth days. After Earth's
Moon, it is the brightest object in the night sky, reaching an apparent magnitude of −4.6. As an inferior planet from
Earth it never appears to venture far from the Sun, and its
elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, and is often referred to as the
Morning Star or as the
Evening Star.
A
terrestrial planet, it is sometimes called Earth's "sister planet", as the two are similar in size and bulk composition. The planet is covered with an opaque layer of highly reflective
clouds and its surface cannot be seen from space in
visible light, making it a subject of great speculation until some of its secrets were revealed by planetary science in the 20th century. Venus has the densest
atmosphere of the terrestrial planets, consisting mostly of
carbon dioxide, and the
atmospheric pressure at the planet's surface is 90 times that of the Earth.
Venus' surface has been mapped in detail only in the last 20 years. It shows evidence of extensive
volcanism, and some of its volcanoes may still be active today. In contrast to the constant crustal movement seen on Earth, Venus is thought to undergo periodic episodes of
plate tectonics, in which the crust is
subducted rapidly within a few million years separated by stable periods of a few hundred million years.
The planet is named after
Venus, the Roman goddess of
love, and most of its surface features are named after famous and mythological women. The adjective
Venusian is commonly used for items related to Venus, though the
Latin adjective is the rarely used
; the now-archaic
Cytherean is still occasionally encountered.
Structure
Venus is one of the four
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 80% of the Earth's. However, conditions on the Venusian surface differ radically from those on Earth, due to its dense
carbon dioxide atmosphere.
Internal structure
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.
Geography
About 80% of Venus' 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 lies 11 km above Venus' average surface elevation; in contrast, Earth's highest mountain,
Mount Everest, rises to just under 9 km above
sea level. 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.
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.
Almost all Venusian surface features are named after historical and mythological women. 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.
Surface geology
Much of Venus' 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' surface is estimated to be about 500 million years old.
There are almost 1,000 impact craters on Venus, more or less evenly distributed across its surface. On other cratered bodies, such as 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. 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.
Atmosphere
- Main article: Atmosphere of Venus
Venus has an extremely thick
atmosphere, which consists mainly of
carbon dioxide and a small amount of
nitrogen. The pressure at the planet's surface is about 90 times that at Earth's surface—a pressure equivalent to that at a depth of 1 kilometer under Earth's
oceans. The enormously CO
2-rich atmosphere generates a strong
greenhouse effect that raises the surface temperature to over 400 °C. This makes Venus' surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and receives only 25% of the solar irradiance.
Studies have suggested that several billion years ago Venus' 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. Venus is thus an example of an extreme case of
climate change, making it a useful tool in climate change studies.
Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of Venus' 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' surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface.
Above the dense CO
2 layer are thick clouds consisting mainly of
sulfur dioxide and
sulfuric acid droplets. These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of Venus' 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.
Magnetic field and core
In 1980, The
Pioneer Venus Orbiter found that Venus'
magnetic field is both weaker and smaller than Earth's. What small magnetic field is present is induced by an interaction between the
ionosphere and the
solar wind, rather than by an internal dynamo in the
core like the one inside the Earth. Venus'
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. This implies that the dynamo is missing because of a lack of convection in Venus' 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.
Orbit and rotation
Venus orbits the Sun at an average distance of about 106 million km, and completes an orbit every 224.7 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.
Venus rotates once every 243 days – by far the slowest rotation period of any of the major planets. A Venusian day thus lasts more than a Venusian year . At the equator, Venus' surface rotates at 6.5 km/h; on Earth, the rotation speed at the equator is about 1,600 km/h. To an observer on the surface of Venus, the Sun would appear to rise in the west and set in the east every 116.75 days .
If viewed from above the Sun's north pole, all of the planets are orbiting in an anticlockwise direction; but while most planets also rotate anticlockwise, 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
solar nebula, Venus would have had a much faster, prograde rotation, but calculations show that over billions of years,
tidal effects on its dense atmosphere could have slowed down its initial rotation to the value seen today.
A curious aspect of Venus' orbit and rotation periods is that the 584-day 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.
Venus is known to be moonless, though the
asteroid 2002 VE
68 currently maintains a
quasi-satellite orbital relationship with it.
Observation
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.
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 almost impossible not to identify 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 1969, future
U.S. President Jimmy Carter reported having seen a UFO, which later analysis suggested was probably the planet, and countless other people have mistaken Venus for something more exotic.
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' 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.
Studies of Venus
Early studies
Venus is 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 'wandering star'. Several cultures historically held its appearances as a morning and evening star to be those of two separate bodies.
Pythagoras is usually credited with recognizing in the sixth century BC that the morning and evening stars were a single body, though he espoused the view that Venus orbited the Earth. When
Galileo first observed the planet in the early 17th century, he found that it showed phases like the Moon's, varying from crescent to gibbous to full and vice versa. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the solar system was concentric and centered on the Earth.
Venus' 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. 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 apparent markings on the planet's surface.
Ground-based research
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.
Spectroscopic observations in the 1900s gave the first clues about Venus' 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. 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.
Radar observations in the 1970s revealed details of Venus' 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. 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.
Research with space probes
Early efforts
The first
unmanned space mission to Venus, and the first to any planet, began on 12 February 1961 with the launch of the
Venera 1 probe. The first craft of the 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 14 December 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' cloudtops 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' mass and of the astronomical unit, but was unable to detect either a
magnetic field or
radiation belts.
Atmospheric entry
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' 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' atmosphere. 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 the 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.
Surface science
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 15 December 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.
The American
Pioneer Venus project consisted of two separate missions. 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 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.
Radar mapping
The United States'
Magellan probe was launched on 4 May 1989 with a mission to map the surface of Venus with radar.
Venus in human culture
Historic connections
As one of the brightest objects in the sky, Venus has been known since prehistoric times and from the earliest days has had a significant impact on human culture. It is described in
Babylonian
cuneiformic texts such as the Venus tablet of Ammisaduqa, which relates observations that possibly date from 1600 BC. The Babylonians named the planet
Ishtar, the personification of womanhood, and goddess of love. The
Ancient Egyptians believed Venus to be two separate bodies and knew the morning star as
Tioumoutiri and the evening star as
Ouaiti. Likewise believing Venus to be two bodies, the
Ancient Greeks called the morning star F?sf????,
Phosphorus, the "Bringer of Light" or ??sf????,
Eosphorus, the "Bringer of Dawn"; the evening star they called
Hesperos — by
Hellenistic times, it was realized they were the same planet. Hesperos would be translated into
Latin as Vesper and Phosphorus as
Lucifer, a poetic term later used to refer to the fallen angel cast out of heaven. The
Romans would later name the planet in honor of their goddess of love,
Venus, whereas the Greeks used the name of its Greek counterpart,
Aphrodite.
To the Hebrews it was known as
Noga ,
Ayeleth-ha-Shakhar and
Kochav-ha-'Erev . Venus was important to the Mayan civilization, who developed a religious calendar based in part upon its motions, and held the motions of Venus to determine the propitious time for events such as war. The
Maasai people named the planet
Kileken, and have an oral tradition about it called
The Orphan Boy. In western
astrology, derived from its historical connotation with goddesses of femininity and love, Venus is held to influence those aspects of human life. In
Vedic astrology, where such an association was not made, Venus or
Shukra affected wealth, comfort, and attraction. Early Chinese astronomers called the body
Tai-pe, or the "beautiful white one". Modern
Chinese,
Korean,
Japanese and
Vietnamese cultures refer to the planet literally as the
metal star , based on the Five elements.
The
astronomical symbol for Venus is the same as that used in biology for the female sex, a stylized representation of the goddess Venus' hand mirror: a circle with a small cross underneath. The Venus symbol also represents
femininity, and in ancient
alchemy stood for the metal
copper. Alchemists constructed the symbol from a circle above a cross .
In fiction
Venus' impenetrable cloud cover gave
science fiction writers free rein to speculate on conditions at its surface; all the more so when early observations showed that it was very similar in size to Earth and possessed a substantial atmosphere. The planet was frequently depicted as warmer than Earth beneath the clouds, but still
habitable by humans. The
genre reached its peak between the 1930s and 1950s, at a time when science had revealed some aspects of Venus, but not yet the harsh reality of its surface conditions.
Robert Heinlein's
Future History series was set on a Venus inspired by the chemist
Svante Arrhenius's prediction of a steamy carboniferous
swamp upon which the rain dripped incessantly. It probably inspired Henry Kuttner to the subsequent depiction given in his novel
Fury.
Ray Bradbury's short stories
The Long Rain and
All Summer in a Day also depicted Venus as a habitable planet with incessant rain. Other works, such as
C. S. Lewis's 1943
Perelandra is the second book in the
Space Trilogy [i] of C. S. Lewis [i]. ...
or
Isaac Asimov's 1954
Lucky Starr and the Oceans of Venus, drew from a vision of a
Cambrian-like Venus covered by a near planet-wide
ocean filled with exotic aquatic life.
As scientific knowledge of Venus has advanced, so the authors of science fiction have endeavored to keep pace, particularly by conjecturing human attempts to
terraform Venus. In his 1997 novel
,
Arthur C. Clarke postulated humans steering
cometary fragments to impact Venus, the resulting addition of water to the Venus environment intended to lower its temperature and absorb carbon dioxide. A terraformed Venus is the setting for a number of diverse works of fiction that have included
Star Trek,
Exosquad,
Cowboy Bebop and the
manga Venus Wars, and the theme seems to be in little danger of dying out. A variation of this theme is Frederik Pohl's The Merchants of Venus , which started his celebrated Heechee Series, where Venus was colonised long ago by mysterious aliens whose abandoned dwellings and artifacts make human colonization both materially easier and provide a strong economic incentive.
See also
