Venus

Venus  The Venusian symbol, a circle with a small equal-armed cross beneath it
Venus in real color, a nearly uniform pale cream. The planet's disk is about three-quarters illuminated. Almost no variation or detail can be seen in the clouds.
Venus in real color
Designations
Pronunciation /en-us-Venus.oggˈviːnəs/
Adjective Venusian or (rarely) Cytherean, Venerean
Epoch J2000
Aphelion 108,942,109 km
0.728 231 28 AU
Perihelion 107,476,259 km
0.718 432 70 AU
Semi-major axis 108,208,930 km
0.723 332 AU
Eccentricity 0.006 8
Orbital period 224.700 69 day
0.615 197 0 yr
1.92 Venus solar day
Synodic period 583.92 days[1]
Average orbital speed 35.02 km/s
Inclination 3.394 71° to Ecliptic
3.86° to Sun’s equator
2.19° to Invariable plane[2]
Longitude of ascending node 76.670 69°
Argument of perihelion 54.852 29°
Satellites None
Physical characteristics
Mean radius 6,051.8 ± 1.0 km[3]
0.949 9 Earths
Flattening 0[3]
Surface area 4.60 × 108 km2
0.902 Earths
Volume 9.38 × 1011 km3
0.857 Earths
Mass 4.868 5 × 1024 kg
0.815 Earths
Mean density 5.204 g/cm3
Equatorial surface gravity 8.87 m/s2
0.904 g
Escape velocity 10.46 km/s
Sidereal rotation
period		 −243.018 5 day
Equatorial rotation velocity 6.52 km/h (1.81 m/s)
Axial tilt 177.3°[1]
North pole right ascension 18 h 11 min 2 s
272.76°[4]
North pole declination 67.16°
Albedo 0.65 (geometric) or 0.75 (Bond)[1]
Surface temp.
   Kelvin
   Celsius
min mean max
735 K[1][6][7]
460 °C
Apparent magnitude up to −4.6[1] (crescent)
−3.8[5] (full)
Angular diameter 9.7"–66.0"[1]
Atmosphere
Surface pressure 93 bar (9.3 MPa)
Composition ~96.5% Carbon dioxide
~3.5% Nitrogen
0.015% Sulfur dioxide
0.007% Argon
0.002% Water vapor
0.001 7% Carbon monoxide
0.001 2% Helium
0.000 7% Neon
trace Carbonyl sulfide
trace Hydrogen chloride
trace Hydrogen fluoride

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. 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 has been known as the Morning Star or Evening Star.

Venus is classified as a terrestrial planet and it is sometimes called Earth's "sister planet" due to the similar 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. Venus has the densest atmosphere of all the terrestrial planets in our solar system, consisting mostly of carbon dioxide. Venus has no carbon cycle to lock carbon back into rocks and surface features, nor does it seem to have any organic life to absorb it in biomass. A younger Venus is believed to have possessed Earth-like oceans,[8] but these evaporated as the temperature rose, leaving a dusty dry desertscape with many slab-like rocks. The water has most likely dissociated, and, because of the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind.[9] The atmospheric pressure at the planet's surface is 92 times that of the Earth.

The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the twentieth century. It was finally mapped in detail by Project Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate that there have been some recent eruptions.[10][11] However, the absence of evidence of lava flow accompanying any of the visible caldera remains an enigma. The planet has few impact craters, demonstrating that the surface is relatively young, approximately half a billion years old.[12] There is no evidence for plate tectonics, possibly because its crust is too strong to subduct without water to make it less viscous. Instead, Venus may lose its internal heat in periodic massive resurfacing events.[13]

Contents

Physical characteristics

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 Earth's "sister" or "twin".[14] 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% being nitrogen.[15]

Internal structure

Without seismic data or knowledge of its moment of inertia, there is little direct information about the internal structure and geochemistry of Venus.[16] However, the similarity in size and density between Venus and Earth suggests that they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.[17] 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.[18]

Geography

About 80% of the Venusian surface is covered by smooth volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains.[19] 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 the Venusian 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. A network of fractures and faults covers much of this area.[20]

Map of Venus, showing the elevated "continents" in yellow: Ishtar Terra at the top and Aphrodite Terra just below the equator to the right

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 100–1,000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spider webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.[21]

Most Venusian surface features are named after historical and mythological women.[22] Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. The former three features were named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.[23]

The longitudes of physical features on Venus are expressed relative to its prime meridian. The original prime meridian passed through the radar-bright spot at the center of the oval feature Eve, located south of Alpha Regio.[24] After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne.[25][26]

Surface geology

A false color image of Venus. Ribbons of lighter color stretch haphazardly across the surface. Plainer areas of more even colouration lie between.
Global view of the surface (from Magellan radar imaging)

Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it possesses some 167 massive volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii.[21] However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years,[27] while the Venusian surface is estimated to be about 500 million years old.[21]

Several lines of evidence point to ongoing volcanic activity on Venus. During the Soviet 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. The European Space Agency's Venus Express recorded abundant lightning in the high atmosphere.[28] While rainfall drives thunderstorms on Earth, there is no rainfall on the surface of Venus (though it does rain sulfuric acid in the upper atmosphere that evaporates around 25 km above the surface). One possibility is that ash from a volcanic eruption was generating the lightning. Another 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.[29]

Impact craters on the surface of Venus (image reconstructed from radar data)

There are almost a thousand impact craters on Venus evenly distributed across its surface. On other cratered bodies, such as the Earth and the Moon, craters show a range of states 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 global resurfacing event about 500 million years ago,[12] followed by a decay in volcanism.[30] 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.[21]

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.[31] Incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the ground.[32]

Atmosphere and climate

Main article: Atmosphere of Venus
Cloud structure in The Venusian atmosphere, revealed by ultraviolet observations

Venus has an extremely dense 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 CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System, creating surface temperatures of over 460 °C (860 °F).[33] This makes the Venusian surface hotter than Mercury's which has a minimum surface temperature of −220 °C and maximum surface temperature of 420 °C,[34] even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. The surface of Venus is often said to resemble Hell.[35]

Studies have suggested that several billion years ago the Venusian 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.[36]

Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of the Venusian 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 the Venusian surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat were not a problem.[37]

Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[38][39] These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of the Venusian 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 lit. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days.[40] Venusian winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to 20% rotation speed.[41]

The surface of Venus is effectively isothermal; it retains a constant temperature not only between day and night but between the equator and the poles.[1][42] The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth), also minimizes seasonal temperature variation.[43] The only appreciable variation in temperature occurs with altitude. In 1995, the Magellan probe imaged a highly reflective substance at the tops of the 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 lead sulfide (galena).[44]

The clouds of Venus are capable of producing lightning much like the clouds on Earth.[45] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. However, in 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[45] In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.[46][47]

Magnetic field and core

In 1980, the Pioneer Venus Orbiter found that the Venusian magnetic field is much weaker than that of Earth's. This magnetic field is induced by an interaction between the ionosphere and the solar wind,[48] rather than by an internal dynamo in the core like the one inside the Earth. Venus's small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.[49]

The lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show that it is adequate to produce a dynamo.[50][51] This implies that the dynamo is missing because of a lack of convection in the Venusian 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. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, there is not an internal geodynamo that can drive a magnetic field. Instead the heat energy from the core is being used to reheat the crust.[52]

Venus has no solid inner core,[53] or 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. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.[52]

Orbit and rotation

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

Venus orbits the Sun at an average distance of about 108 million kilometers (about 0.7 AU), 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 0.01.[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 an average distance of 41 million km during inferior conjunction.[1] The planet reaches inferior conjunction every 584 days, on average.[1] Due to the decreasing eccentricity of both orbits of Earth and Venus, the minimum distances will become greater. From the year 1 to 5383, there are 526 approaches less than 40 million km; then there are none for about 60,200 years.[54] During periods of greater eccentricity Venus can come as close as 38.2 million km.[1]

If viewed from above the Sun's north pole, all the planets are orbiting in a counter-clockwise direction; but while most planets also rotate counter-clockwise, Venus rotates clockwise in "retrograde" rotation. The present rotation period of Venus represents an equilibrium state between gravitational tidal locking by the Sun that tends to slow the rotation rate, and an atmospheric tide created by the solar heating of the thick Venusian atmosphere. When it formed from the solar nebula, Venus may have begun with a different rotation period and obliquity, then migrated to the current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere. This change in the rotation period probably took place over the course of billions of years.[55][56]

Venus rotates once every 243 Earth days—by far the slowest rotation period of any of the major planets. At the equator, the Venusian surface rotates at 6.5 km/h; on Earth, the rotation speed at the equator is about 1,670 km/h.[57] A Venusian sidereal day thus lasts longer than a Venusian year (243 versus 224.7 Earth days). However, because of the retrograde rotation, 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 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days).[7] Additionally, the Sun would appear to rise in the west and set in the east. As a result of Venus's relatively long solar day, one Venus year is about 1.92 Venus days long.[7]

A curious aspect of the Venusian 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 tidal locking with the Earth, is unknown.[58]

Venus currently has no natural satellite,[59] though the asteroid 2002 VE68 presently maintains a quasi-orbital relationship with it.[60] In the 17th century, Giovanni Cassini reported to have seen a moon orbiting Venus, which was named Neith, after the Egyptian goddess. Over the following 200 years there were numerous reported sightings. Ultimately, it was determined that most of them could be explained by stars which had been in the vicinity of Venus. According to Alex Alemi and David Stevenson of the California Institute of Technology, their 2006 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.[61][62] About 10 million years later, according to the study, another impact reversed the planet's spin direction. This caused the Venusian moon gradually to spiral inward[63] until it collided and merged with Venus. If later impacts created moons, those also were absorbed in the same manner. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets.[59]

Observation

A photograph of the night sky taken from the seashore. A glimmer of sunlight is on the horizon. There are many stars visible. Venus is at the center, much brighter than any of the stars, and its light can be seen reflected in the ocean.
Venus is always brighter than the brightest stars

Venus is always brighter than the brightest stars, with its apparent magnitude ranging from −3.8 to −4.6.[5] 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.[5]

Venus "overtakes" the Earth every 584 days as it orbits the Sun.[1] As it does so, it changes from the "Evening star", visible after sunset, to 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". U.S. President Jimmy Carter reported having seen a UFO in 1969, which later analysis suggested was probably the planet. Countless other people have mistaken Venus for something more exotic.[64]

Phases of Venus and evolution of its apparent diameter

As it moves around its orbit, Venus displays phases in a telescopic view like those of the Moon: In the phases of Venus the planet presents a small "full" image when it is on the opposite side of the Sun. It shows a larger "quarter phase" when it is at its maximum elongations from the Sun. Venus is at its brightest in the night sky and presents a much larger "thin crescent" in telescopic views as it comes around to the near side between the Earth and the Sun. Venus is at its largest and presents its "new phase" when it is between the Earth and the Sun. Since it has an atmosphere it can be seen in a telescope by the halo of light refracted around the planet.[5]

The Venusian 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 when the planet's inferior conjunction coincides with its presence in the plane of the Earth's orbit. Transits of Venus occur in cycles of 243 years with the current pattern of transits being pairs of transits separated by eight years, at intervals of about 105.5 years or 121.5 years. The most recent transit was in June 2004; the next will be in June 2012. The preceding pair of transits occurred in December 1874 and December 1882; the following pair will occur in December 2117 and December 2125.[65] Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the astronomical unit, and hence the size 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.[66][67]

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.[68]

Studies

Early studies

Galileo's discovery that Venus showed phases proved that it orbits the Sun and not the Earth
Successive inferior conjunctions of Venus repeat very near a 13:8 orbital resonance (The Earth orbits 8 times for every 13 orbits of Venus), creating a pentagrammic precession sequence.

Venus was known to ancient civilizations as the "morning star" or "evening star". Several cultures historically held its appearances as a morning and evening star to be those of two separate bodies. Greek philosopher Pythagoras is usually credited with recognizing that the morning and evening stars were a single body in the sixth century BC, though he thought that Venus orbited the Earth.[69]

The transit of Venus was first observed in 1032 by the Persian astronomer Avicenna, who concluded that Venus is closer to the Earth than the Sun,[70] and established that Venus was, at least sometimes, below the Sun.[71] In the 12th century, the Andalusian astronomer Ibn Bajjah observed "two planets as black spots on the face of the Sun," which was later identified as the transit of Venus and Mercury by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[72]

When the Italian physicist Galileo Galilei first observed the planet in the early 17th century, he found that it showed phases like the Moon, varying from crescent to gibbous to full and vice versa. When Venus is furthest from the Sun in the sky it shows a half-lit phase and when it is closest to the Sun in the sky it shows as a crescent or full phase. 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.[73]

The atmosphere of Venus was discovered in 1761 by Russian polymath Mikhail Lomonosov.[74][75] Venus's atmosphere was observed in 1790 by German astronomer 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, American astronomer 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.[76] The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Italian-born astronomer Giovanni Cassini and Schröter incorrectly estimated periods of about 24 hours from the motions of markings on the planet's apparent surface.[77]

Ground-based research

Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint 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.[78]

Spectroscopic observations in the 1900s gave the first clues about the Venusian 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.[79] 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.[80]

Radar observations in the 1970s revealed details of the Venusian 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.[81] These three features are now the only ones on Venus which do not have female names.[82]

Exploration

Early efforts

Mariner 2, launched in 1962

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.[83]

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 the Venusian 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 its mass and of the astronomical unit, but was unable to detect either a magnetic field or radiation belts.[84]

Atmospheric entry

The Soviet 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.[85] 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.[85]

Another probe arrived at Venus one day later on October 19, 1967 when Mariner 5 conducted a flyby at a distance of less than 4000 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 the Venusian atmosphere.[86] 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,[87] in an early example of space cooperation.[88]

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 bar 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 bar, neither was 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.[85]

Surface and atmospheric science

A stubby barrel-shaped spacecraft is depicted in orbit above Venus. A small dish antenna is located at the centre of one of its end faces
The Pioneer Venus orbiter

Venera 7 represented an 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 before 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.[85]

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 terrain 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.[89]

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 4000 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.[90]

The American Pioneer Venus project consisted of two separate missions.[91] 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 four probes which entered the atmosphere on December 9, 1978, returning data on its composition, winds and heat fluxes.[92]

A very tilted image of an orange-brown landscape in the immediate vicinity. A few flat rocks lie a few metres away. The circular saw-shaped foot of the lander is visible in the foreground
Color image taken from the surface of Venus by the Soviet Venera 13 lander. The orange color is due to atmospheric scattering removing the blue wavelengths from the surface sunlight.

Four more Venera lander missions took place over the next four years, with Venera 11 and Venera 12 detecting Venusian electrical storms;[93] 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.[93] Venera 14, though, had the misfortune to strike its own ejected camera lens cap and its probe failed to contact the soil.[93] 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.[94]

In 1985 the Soviet Union 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.[95][96]

Radar mapping

Magellan radar topographical map of Venus (false color)

The United States' Magellan probe was launched on May 4, 1989 with a mission to map the surface of Venus with radar.[23] The high-resolution images it obtained during its 4½ years of operation far surpassed all prior maps and were comparable to visible-light photographs of other planets. Magellan imaged over 98% of the Venusian surface by radar[97] and mapped 95% of its gravity field. In 1994, at the end of its mission, Magellan was deliberately sent to its destruction into the atmosphere of Venus to quantify its density.[98] Venus was observed by the Galileo and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan would otherwise be the last dedicated mission to Venus for over a decade.[99][100]

Current and future missions

The Venus Express probe was designed and built by the European Space Agency. Launched on November 9, 2005 by a Russian Soyuz-Fregat rocket procured through Starsem, it successfully assumed a polar orbit around Venus on April 11, 2006.[101] 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.[101] 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.[101]

NASA's MESSENGER mission to Mercury performed two flybys of Venus in October 2006 and June 2007, to slow its trajectory for an eventual orbital insertion of Mercury in 2011. MESSENGER collected scientific data on both those flybys.[102] The European Space Agency (ESA) will also launch a mission to Mercury, called BepiColombo, which will perform two flybys of Venus in August 2013 before it reaches Mercury orbit in 2019.[103]

Artist's impression of a Stirling cooled Venus Rover devised by NASA.[104]

Future missions to Venus are planned. Japan's aerospace body JAXA devised a Venus orbiter, Akatsuki (formerly "Planet-C"), which was launched on May 20, 2010 and is expected to reach its target in December 2010. Planned investigations include surface imaging with an infrared camera and experiments designed to confirm the presence of lightning as well as the determination of the existence of current surface volcanism.[105] Under its New Frontiers Program, NASA has proposed a lander mission called the Venus In-Situ Explorer to land on Venus to study surface conditions and investigate the elemental and mineralogical features of the regolith. The probe would be equipped with a core sampler to drill into the surface and study pristine rock samples not weathered by the very harsh surface conditions. The Venera-D (Russian: Венера-Д) probe is a proposed Russian space probe to Venus, to be launched around 2016 with its goal to make remote-sensing observations around the planet Venus and deploying a lander, based on the Venera design, capable of surviving for a long duration on the planet's surface. Other proposed Venus exploration concepts include rovers, balloons, and airplanes.[106]

Manned Venus flyby

A manned Venus flyby mission, using Apollo program hardware, was proposed in the late 1960s.[107] The mission was planned to launch in late October or early November 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.[107]

In culture

The adjective Venusian is commonly used for items related to Venus, though the Latin adjective is the rarely used Venerean; the archaic Cytherean is still occasionally encountered. Venus is the only planet in the Solar System named after a female figure,[a] although three dwarf planets — Ceres, Eris and Haumea — along with many of the first discovered asteroids[108] also have feminine names.

Historic understanding

The Mayan Dresden Codex, which calculates its appearances

As one of the brightest objects in the sky, Venus has been known since prehistoric times and as such has gained an entrenched position in 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.[109] The Babylonians named the planet Ishtar (Sumerian Inanna), the personification of womanhood, and goddess of love.[110]

The Ancient Egyptians believed Venus to be two separate bodies and knew the morning star as Tioumoutiri and the evening star as Ouaiti.[111] Likewise, believing Venus to be two bodies, the Ancient Greeks called the morning star Φωσφόρος, Phosphoros (Latinized Phosphorus), the "Bringer of Light" or Ἐωσφόρος, Eosphoros (Latinized Eosphorus), the "Bringer of Dawn". The evening star they called Hesperos (Latinized Hesperus) (Ἓσπερος, the "star of the evening"). By Hellenistic times, the ancient Greeks realized the two were the same planet,[112][113] which they named after their goddess of love, Aphrodite (Phoenician Astarte).[114] Hesperos would be translated into Latin as Vesper and Phosphoros as Lucifer ("Light Bearer"), a poetic term later used to refer to the fallen angel cast out of heaven.[b] The Romans, who derived much of their religious pantheon from the Greek tradition, named the planet Venus after their goddess of love.[115] Pliny the Elder (Natural History, ii,37) identified the planet Venus with Isis.[116]

In Iranian mythology, especially in Persian mythology, the planet usually corresponds to the goddess Anahita. In some parts of Pahlavi literature the deities Aredvi Sura and Anahita are regarded as separate entities, the first one as a personification of the mythical river and the latter as a goddess of fertility which is associated with the planet Venus. As the goddess Aredvi Sura Anahita—and simply called Anahita as well—both deities are unified in other descriptions, e. g. in the Greater Bundahishn, and are represented by the planet. However, in the Avestan text Mehr Yasht (Yasht 10) there is a possible early link to Mithra. The Persian name of the planet today is "Nahid" which derives from Anahita and later in history from the Pahlavi language Anahid.[117][118][119][120]

The planet Venus was important to the Maya 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. They named it Noh Ek', the Great Star, and Xux Ek', the Wasp Star. The Maya were aware of the planet's synodic period, and could compute it to within a hundredth part of a day.[121] The Maasai people named the planet Kileken, and have an oral tradition about it called The Orphan Boy.[122]

Venus is important in many Australian aboriginal cultures, such as that of the Yolngu people in Northern Australia. The Yolngu gather after sunset to await the rising of Venus, which they call Barnumbirr. As she approaches, in the early hours before dawn, she draws behind her a rope of light attached to the Earth, and along this rope, with the aid of a richly decorated "Morning Star Pole", the people are able to communicate with their dead loved ones, showing that they still love and remember them. Barnumbirr is also an important creator-spirit in the Dreaming, and "sang" much of the country into life.[123]

Shukra is the Sanskrit name for Venus

In western astrology, derived from its historical connotation with goddesses of femininity and love, Venus is held to influence desire and sexual fertility.[124] In Indian Vedic astrology, Venus is known as Shukra,[125] meaning "clear, pure" or "brightness, clearness" in Sanskrit. One of the nine Navagraha, it is held to affect wealth, pleasure and reproduction; it was the son of Bhrgu, preceptor of the Daityas, and guru of the Asuras.[126][127] Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the “metal star” (金星), based on the Five elements.[128]

In the metaphysical system of Theosophy, it is believed that on the etheric plane of Venus there is a civilization that existed hundreds of millions of years before Earth’s[129] and it is also believed that the governing deity of Earth, Sanat Kumara, is from Venus.[130]

♀

The astronomical symbol for Venus is the same as that used in biology for the female sex: a circle with a small cross beneath.[131] The Venus symbol also represents femininity, and in Western alchemy stood for the metal copper.[131] Polished copper has been used for mirrors from antiquity, and the symbol for Venus has sometimes been understood to stand for the mirror of the goddess.[131]

Perhaps the strangest appearance of Venus in literature is as the harbinger of destruction in Immanuel Velikovsky's Worlds in Collision (1950). In this intensely controversial book, Velikovsky argued that many seemingly unbelievable stories in the Old Testament are true recollections of times when Venus nearly collided with the Earth — when it was still a comet and had not yet become the docile planet that we know today. He contended that Venus caused most of the strange events of the Exodus. He cites legends in many other cultures (such as Greek, Mexican, Chinese and Indian) indicating that the effects of the near-collision were global. The scientific community rejected his wildly unorthodox book; however, it became a bestseller.[132]

In science fiction

The impenetrable Venusian cloud cover gave science fiction writers free rein to speculate on conditions at its surface; all the more so when early observations showed that not only was it very similar in size to Earth, it possessed a substantial atmosphere. Closer to the Sun than Earth, the planet was frequently depicted as warmer, but still habitable by humans.[133] 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. Findings from the first missions to Venus showed the reality to be very different, and brought this particular genre to an end.[134] As scientific knowledge of Venus advanced, so science fiction authors endeavored to keep pace, particularly by conjecturing human attempts to terraform Venus.[135]

Colonization

Due to its extremely hostile conditions, a surface colony on Venus is out of the question with current technology. However, the atmospheric pressure and temperature approximately fifty kilometres above the surface are similar to those at the Earth's surface and Earth air (nitrogen and oxygen) would be a lifting gas in the Venusian atmosphere of mostly carbon dioxide. This has led to proposals for extensive "floating cities" in the Venusian atmosphere.[136] Aerostats (lighter-than-air balloons) could be used for initial exploration and ultimately for permanent settlements.[136] Among the many engineering challenges are the dangerous amounts of sulfuric acid at these heights.[136]

See also


Notes

  1. ^ Goddesses such as Gaia and Terra were named after the Earth, and not vice versa.
  2. ^ Jerome translated Septuagint heosphoros and Hebrew helel as lucifer, in Isaiah 14:12.

References

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  2. ↑ "The MeanPlane (Invariable plane) of the Solar System passing through the barycenter". 2009-04-03. http://home.comcast.net/~kpheider/MeanPlane.gif. Retrieved 2009-04-10.  (produced with Solex 10 written by Aldo Vitagliano; see also Invariable plane)
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