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Venus (planet)

From Academic Kids

Venus, the second planet from the Sun, is named after the Roman goddess Venus. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are very similar in size and bulk composition. Although all planets' orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 1%.

Because Venus is closer to the Sun than the Earth, it always appears in roughly the same direction from Earth as the Sun (the greatest elongation is 47.8?), so on Earth it can usually only be seen a few hours before sunrise or a few hours after sunset. However, when at its brightest, Venus may be seen during the daytime, making it one of only two heavenly bodies that can be seen both day and night (the other being the Moon). It is sometimes referred to as the "Morning Star" or the "Evening Star", and when it is visible in dark skies it is by far the brightest star-like object in the sky.

Venus was known to ancient Babylonians around 1600 BC, and to the Mayan civilization (the Mayans developed a religious calendar based on Venus's motion) and must have been known long before in prehistoric times, given that it is the third brightest object in the sky after the Sun and Moon. Its symbol is a stylized representation of the goddess Venus's hand mirror: a circle with a small cross underneath (Unicode: ♀).

The adjective Venusian is commonly used for Venus, but it is etymologically incorrect. The true adjective coming from Latin, Venereal, is avoided because of its modern association with sexually transmitted diseases. Some astronomers use Cytherean, which comes from Cythera. Other less common adjectives include Venerean and Veneran.

The Chinese, Korean, and Japanese cultures refer to the planet as the Metal Star, based on the Five Elements.

Contents

Physical characteristics

Atmosphere

Venus has an atmosphere consisting mainly of carbon dioxide and a small amount of nitrogen, with a pressure at the surface about 90 times that of Earth (a pressure equivalent to a depth of 1 kilometre under Earth's oceans). This enormously CO2-rich atmosphere results in a strong greenhouse effect that raises the surface temperature more than 400°C above what it would be otherwise, causing temperatures at the surface to reach extremes as great as 500°C in low elevation regions near the planet's equator. This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and only receives 25% of the solar irradiance (2613.9 W/m? in the upper atmosphere, and just 1071.1 W/m? at the surface). Owing to the thermal inertia and convection of its dense atmosphere, the temperature does not vary significantly between the night and day sides of Venus despite its extremely slow rotation (less than one rotation per Venerean year; at the equator, Venus's surface rotates at a mere 6.5 km/h). Winds in the upper atmosphere circle the planet in only 4 days, helping to distribute the heat.

The solar irradiance is so much lower at the surface of Venus because the planet's thick cloud cover reflects the majority of the sunlight back into space. This prevents most of the sunlight from ever heating the surface. Venus's bolometric albedo is approximately 60%, and its visual light albedo is even greater. Thus, despite being closer to the Sun than Earth, the surface of Venus is not as well heated and even less well lit by the Sun. In the absence of any greenhouse effect, the temperature at the surface of Venus would be quite similar to Earth. A common conceptual misunderstanding regarding Venus is the mistaken belief that its thick cloud cover traps heat, as the opposite is actually true. The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO2 in the atmosphere is what traps the heat by the greenhouse mechanism.

There are strong 350-kilometre-per-hour winds at the cloud tops, but winds at the surface are very slow, no more than a few kilometres per hour. However, owing to the high density of the atmosphere at Venus' surface, even such slow winds exert a significant amount of force against obstructions. The clouds are mainly composed of sulfur dioxide and sulfuric acid droplets and cover the planet completely, obscuring any surface details from the human eye. The temperature at the tops of these clouds is approximately −45°C. The official mean surface temperature of Venus, as given by NASA, is 464°C. The minimal value of the temperature, listed in the table, refers to cloud tops — the surface temperature is never below 400°C.

Surface features

 image of the surface of Venus, centered at 180 degrees east longitude
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Radar image of the surface of Venus, centered at 180 degrees east longitude

Venus has slow retrograde rotation, meaning it rotates from east to west, instead of west to east as most of the other major planets do. (Pluto and Uranus also have retrograde rotation, though Uranus's axis, tilted at 97.86 degrees, almost lies in its orbital plane.) It is not known why Venus is different in this manner, although it may be the result of a collision with a very large asteroid at some time in the distant past. In addition to this unusual retrograde rotation, the periods of Venus's rotation and of its orbit are synchronized in such a way that it always presents the same face toward Earth when the two planets are at their closest approach (5.001 Venusian days between each inferior conjunction). This may be the result of tidal locking, with tidal forces affecting Venus's rotation whenever the planets get close enough together, or it may simply be a coincidence.

Venus has two major continent-like highlands on its surface, rising over vast plains. The northern highland is named Ishtar Terra and has Venus's highest mountains, named the Maxwell Montes (roughly 2 km taller than Mount Everest) after James Clerk Maxwell, which surround the plateau Lakshmi Planum. Ishtar Terra is about the size of Australia. In the southern hemisphere is the larger Aphrodite Terra, about the size of South America. Between these highlands are a number of broad depressions, including Atalanta Planitia, Guinevere Planitia, and Lavinia Planitia. With only the exception of Maxwell Montes, all surface features on Venus are named after real or mythological females. Venus's thick atmosphere causes meteors to decelerate as they fall toward the surface, and even large meteors will strike the surface at too low a speed to form an impact crater if they have less than a certain threshold kinetic energy. Because of this, no impact crater smaller than about 3.2 km in diameter can form.

Nearly 90% of Venus's surface appears to consist of recently (in the geological sense) solidified basaltic lava, with very few meteorite craters. The oldest features present on Venus seem to be only around 800 million years old, with most of the terrain being considerably younger (though still not less than several hundred million years for the most part). This suggests that Venus underwent a major resurfacing event in the not too distant geological past. The interior of Venus is probably similar to that of Earth: an iron core about 3000 km in radius, with a molten rocky mantle making up the majority of the planet. Recent results from the Magellan gravity data indicate that Venus's crust is stronger and thicker than had previously been assumed. It is theorized that Venus does not have mobile plate tectonics as Earth does, but instead undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava. Other recent findings suggest that Venus is still volcanically active in isolated geological hotspots.

Venus's intrinsic magnetic field has been found very weak compared to other planets in the solar system. This may be due to its slow rotation being insufficient to drive an internal dynamo of liquid iron. As a result, solar wind strikes Venus's upper atmosphere without mediation. It is thought that Venus originally had as much water as Earth, but that under the Sun's assault water vapor in the upper atmosphere was split into hydrogen and oxygen, with the hydrogen escaping into space owing to its low molecular mass; the ratio of hydrogen to deuterium (a heavier isotope of hydrogen which doesn't escape as quickly) in Venus's atmosphere seems to support this theory. Molecular oxygen is thought to have combined with atoms in the crust (large amounts of oxygen, however, remain in the atmosphere in the form of carbon dioxide). Because of their dryness, Venus's rocks are much harder than Earth's, which leads to steeper mountains, cliffs and other features.

Venus was once thought to possess a moon, named Neith after the chief goddess of Sais, Egypt, (whose veil no mortal raised), first observed by Giovanni Domenico Cassini in 1672. Sporadic sightings of Neith by astronomers continued until 1892, but these sightings have since been discredited (they were mostly faint stars that happened to be in the right place at the right time) and Venus is now known to be moonless.

Observations and explorations of Venus

Historical observations

Venus is the most prominent astronomical feature in Earth's morning and evening sky (other than the Sun and Moon), and has been known since before recorded history. One of the oldest surviving astronomical documents, from the Babylonian library of Ashurbanipal around 1600 BC, is a 21-year record of the appearances of Venus (which the early Babylonians called Nindaranna). The ancient Sumerians and Babylonians called Venus Dil-bat or Dil-i-pat; in Akkadia it was the special star of the mother-god Ishtar; and in Chinese it is Jīn-xīng (金星), the planet of the metal element.

Venus was considered the most important celestial body observed by the Maya, who called it Chak ek, "the Great Star", possibly more important even than the Sun. The Mayans monitored the movements of Venus closely and observed it in daytime. The positions of Venus and other planets were thought to influence life on Earth, so Maya and other ancient Mesoamerican cultures timed wars and other important events based on their observations. In the Dresden Codex, the Maya included an almanac showing Venus's full cycle, in five sets of 584 days each (approximately eight years), after which the patterns repeated (since Venus has a synodic period of 583.92 days).

At the half-full phase Venus is at greatest elongation — east of the Sun when an evening star and west of the Sun as a morning star. The precise angle the planet makes with the Sun at this time varies from approximately 45.0° to 47.8° depending on whether Earth and Venus are at perihelion or aphelion. This range is much smaller than that of Mercury because Venus' orbit is far less eccentric than Mercury's.

Early Greeks thought that the evening and morning appearances of Venus represented two different objects, calling it Hesperus when it appeared in the western evening sky and Phosphorus when it appeared in the eastern morning sky. They eventually came to recognize that both objects were the same planet; Pythagoras is given credit for this realization. In the 4th century BC, Heraclides Ponticus proposed that both Venus and Mercury orbited the Sun rather than Earth.

Phases of Venus
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Phases of Venus

Because its orbit takes it between the Earth and the Sun, Venus as seen from Earth exhibits visible phases in much the same manner as the Earth's Moon. Galileo Galilei was the first person to observe the phases of Venus in December 1610, an observation which supported Copernicus's then contentious heliocentric description of the solar system. He also noted changes in the size of Venus's visible diameter when it was in different phases, suggesting that it was farther from Earth when it was full and nearer when it was a crescent. This observation strongly supported the heliocentric model. Venus (and also Mercury) is not visible from Earth when it is full, since at that time it is at superior conjunction, rising and setting concomitantly with the Sun and hence lost in the Sun's glare.

Venus is brightest when approximately 25% of its disk is illuminated; this typically occurs 37 days both before (in the evening sky) and after (in the morning sky), its inferior conjunction. Its greatest elongations occur approximately 70 days before and after inferior conjunction, at which time it is half full; between these two intervals Venus is actually visible in broad daylight, if the observer knows specifically where to look for it. The planet's period of retrograde motion is 20 days on either side of the inferior conjunction.

On rare occasions, Venus can actually be seen in both the morning (before sunrise) and evening (after sunset) on the same day. This scenario arises when Venus is at its maximum separation from the ecliptic and concomitantly at inferior conjunction; then one hemisphere (Northern or Southern) will be able to see it at both times. This opportunity presented itself most recently for Northern Hemisphere observers within a few days on either side of March 29, 2001, and for those in the Southern Hemisphere, on and around August 19, 1999. These respective events repeat themselves every eight years pursuant to the planet's synodic cycle.

Transits of Venus, when the planet crosses directly between the Earth and the Sun's visible disc, are rare astronomical events. The first time such a transit was observed was on December 4, 1639 by Jeremiah Horrocks and William Crabtree. A transit in 1761 observed by Mikhail Lomonosov provided the first evidence that Venus had an atmosphere, and the 19th-century observations of parallax during its transits allowed the distance between the Earth and Sun to be accurately calculated for the first time. Transits can only occur either in early June or early December, these being the points at which Venus crosses the ecliptic (the orbital plane of the Earth), and occur in pairs at eight-year intervals, with each such pair more than a century apart. The previous pair of transits of Venus occurred in 1874 and 1882, and the current pair is in 2004 and 2012.

In the 19th century, many observers stated that Venus had a period of rotation of roughly 24 hours. Italian astronomer Giovanni Schiaparelli was the first to predict a significantly slower rotation, proposing that Venus was tidally locked with the Sun (as he had also proposed for Mercury). While not actually true for either body, this was still a reasonably accurate estimate. The near-resonance between its rotation and its closest approach to Earth helped to create this impression, as Venus always seemed to be facing the same direction when it was in the best location for observations to be made. The rotation rate of Venus was first measured during the 1961 conjunction, observed by radar from a 26-meter antenna at Goldstone, California, the Jodrell Bank Radio Observatory in the UK, and the Soviet deep space facility in Evpatoriia. Accuracy was refined at each subsequent conjunction, primarily from measurements made from Goldstone and Evpatoriia. The fact that rotation was retrograde was not confirmed until 1964.

Before radio observations in the 1960s, many believed that Venus contained a lush, Earth-like environment. This was due to the planet's size and orbital radius, which suggested a fairly Earthlike situation as well as to the thick layer of clouds which prevented the surface from being seen. Among the speculations on Venus were that it had a junglelike environment or that it had oceans of either petroleum or carbonated water. However, microwave observations in 1956, by C. Mayer et al, indicated a high-temperature source (600 K). Strangely, millimeter-band observations made by A. D. Kuzmin indicated much lower temperatures. Two competing theories explained the unusual radio spectrum, one suggesting the high temperatures originated in the ionosphere, and another suggesting a hot planetary surface.

Appearance

Greatest Eastern Elongation Maximum brightness Stationary, begin retrograde Inferior Conjunction Stationary, resume direct Maximum brightness Greatest Western Elongation Superior Conjunction
March 29th, 2004 46? May 3rd, 2004 May 18th, 2004 June 8th, 2004 June 29th, 2004 July 13th, 2004 August 17th, 2004 45.8? March 31st, 2005
November 3rd, 2005 47.1? December 12th, 2005 December 23rd, 2005 January 13th, 2006 February 3rd, 2006 February 14th, 2006 March 25th, 2006 46.5? October 27th, 2006
June 9th, 2007 45.4? July 14th, 2007 July 25th, 2007 August 18th, 2007 September 7th, 2007 September 23rd, 2007 October 28th, 2007 46.5? June 9th, 2008
January 14th, 2009 47.1? February 20th, 2009 March 5th, 2009 March 27th, 2009 April 15th, 2009 April 29th, 2009 June 5th, 2009 45.9? January 11th, 2010
August 20th, 2010 46? September 27th, 2010 October 7th, 2010 October 29th, 2009 November 16th, 2010 December 2nd, 2010 January 8th, 2011 47? August 16th, 2011
March 27th, 2012 46? April 30th, 2012 May 15th, 2012 June 6th, 2012 June 27th, 2012 July 10th, 2012 August 15th, 2012 45.8? March 28th, 2013

Observation by spacecraft

There have been numerous unmanned missions to Venus. Several Russian probes have included a soft landing on the surface, with up to 110 minutes of communication from the surface, all without return.

Getting to Venus

Venus orbits closer to the Sun than Earth does, with an orbital distance only 72% that of the Earth. Because of this, a spacecraft must travel over 41 million kilometers down into the Sun's gravitational potential well, resulting in a large decrease in the spacecraft's potential energy. The liberated potential energy is mostly turned into kinetic energy, increasing the velocity of the spacecraft, so the speed and direction of the spacecraft must be altered quite radically to permit a close approach to Venus. One can imagine driving along a road at the top of a high, steep cliff with another road at the bottom; the journey from Earth to Venus is rather like swerving off the cliff, freefalling for some time, and then trying to land safely and merge with traffic on the lower road.

Early flybys

On February 12, 1961, the Soviet spacecraft Venera 1 was the first probe launched to another planet. An overheated orientation sensor caused it to malfunction, but Venera-1 was first to combine all the necessary features of an interplanetary spacecraft: solar panels, parabolic telemetry antenna, 3-axis stabilization, course-correction engine, and the first launch from parking orbit.

The first successful Venus probe was the American Mariner 2 spacecraft, which flew past Venus in 1962. A modified Ranger Moon probe, it established that Venus has no magnetic field and measured the planet's thermal microwave emissions.

The Soviet Union launched the Zond 1 probe to Venus on April 2, 1964, but it malfunctioned sometime after its May 16 telemetry session.

Early landings

Missing image
Venera-5_sketch.jpg
Sketch of Venera-5

On March 1, 1966 the Venera 3 Soviet space probe crash-landed on Venus, becoming the first spacecraft to reach the planet's surface. Its sister craft Venera 2 had failed from overheating shortly before completing its flyby mission.

The descent capsule of Venera 4 entered the atmosphere of Venus on October 18, 1967. The first probe to return direct measurements from another planet, the capsule measured temperature, pressure, density and performed 11 automatic chemical experiments to analyze the atmosphere. It showed 95% carbon dioxide, and in combination with radio occultation data from the Mariner 5 probe, it showed that surface pressures were far greater than expected (75 - 100 atmospheres).

These results were verified and refined by the Venera 5 and Venera 6 missions on May 16 and 17 of 1969. But thus far, none of these missions had reached the surface while still transmitting. Venera 4's battery ran out while still slowly floating through the massive atmosphere, and Venera 5 and 6 were crushed by high pressure 18 km above the surface.

The first successful landing on Venus was by Venera 7 on December 15, 1970. It relayed surface temperatures of 457 to 474°C. Venera 8 landed on July 22, 1972. In addition to pressure and temperature profiles, a photometer showed that the clouds of Venus formed a layer, ending over 35 km above the surface. A gamma ray spectrometer analyzed the chemical composition of the crust.

Early orbiters

Missing image
Venera_9_panorama.gif
Surface of Venus taken by Venera 9 lander

The Soviet probe Venera 9 entered orbit on October 22, 1975, becoming the first artificial satellite of Venus. A battery of cameras and spectrometers returned information about the planet's clouds, ionosphere and magnetosphere, as well as performing bistatic radar measurements of the surface.

The 660 kg descent vehicle separated from Venera 9 and landed, taking the first pictures of the surface and analyzing the crust with a gamma ray spectrometer and a densitometer. During descent, pressure, temperature and photometric measurements were made, as well as backscattering and multi-angle scattering (nephelometer) measurements of cloud density. It was discovered that the clouds of Venus are formed in three distinct layers. On October 25, Venera 10 arrived and carried out a similar program of study.

Pioneer Venus

In 1978, NASA sent two Pioneer spacecraft to Venus. The Pioneer mission consisted of two components, launched separately: an Orbiter and a Multiprobe. The Pioneer Venus Multiprobe carried one large and three small atmospheric probes. The large probe was released on November 16, 1978 and the three small probes on November 20. All four probes entered the Venus atmosphere on December 9, followed by the delivery vehicle. Although not expected to survive the descent through the atmosphere, one probe continued to operate for 45 minutes after reaching the surface. The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978. It carried 17 experiments and operated until the fuel used to maintain its orbital position was exhausted and atmospheric entry destroyed the spacecraft in August 1992.

Further Soviet successes

Color image taken from the surface of Venus by the Soviet Venera 13 lander
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Color image taken from the surface of Venus by the Soviet Venera 13 lander

Also in 1978, Venera 11 and Venera 12 flew past Venus, dropping descent vehicles on December 21 and December 25 respectively. The landers carried color cameras and a soil drill and analyzer, which unfortunately malfunctioned. Each lander made measurements with a nephelometer, mass spectrometer, gas chromatograph, and a cloud-droplet chemical analyzer using X-ray fluorescence that unexpectedly discovered a large proportion of chlorine in the clouds, in addition to sulfur. Strong lightning activity was also detected.

Venera 13 and Venera 14 carried out essentially the same mission, arriving at Venus on March 1 and March 5, 1982. This time, color camera and soil-drilling/analysis experiments were successful. X-ray fluorescence analysis of soil samples showed results similar to potassium-rich basalt rock.

On October 10 and October 11, 1983, Venera 15 and Venera 16 entered polar orbits around Venus. Venera 15 analyzed and mapped the upper atmosphere with an infrared Fourier spectrometer. From November 11 to July 10, both satellites mapped the northern third of the planet with synthetic aperture radar. These results provided the first detailed understanding of the surface geology of Venus, including the discovery of unusual massive shield volcanoes such as coronae and arachnoids. Venus had no evidence of plate tectonics, unless the northern third of the planet happened to be a single plate.

Missing image
Vega_lander.jpg
Vega lander

The Soviet Vega 1 and Vega 2 probes encountered Venus on June 11 and June 15 of 1985. Landing vehicles carried experiments focusing on cloud aerosol composition and structure. Each carried an ultraviolet absorption spectrometer, aerosol particle-size analyzers, and devices for collecting aerosol material and analyzing it with a mass spectrometer, a gas chromatograph, and an X-ray fluorescence spectrometer. The upper two layers of the clouds were found to be sulfuric acid droplets, but the lower layer is probably composed of phosphoric acid solution. The crust of Venus was analyzed with the soil drill experiment and a gamma ray spectrometer. As the landers carried no cameras on board, no images from surface were returned.

The Vega missions also deployed balloon-borne aerostat probes that floated at about 53 km altitude respectively for 46 and 60 hours, traveling about 1/3 of the way around the planet. These measured wind speed, temperature, pressure and cloud density. More turbulence and convection activity than expected was discovered, including occasional plunges of 1 to 3 kilometers in downdrafts. The Vega spacecraft continued to rendezvous with Halley's Comet nine months later, bringing an additional 14 instruments and cameras for that mission.

Magellan

On August 10, 1990, the US Magellan probe arrived at its orbit around the planet and started a mission of detailed radar mapping. 98% of the surface was mapped with a resolution of approximately 100m. After a four-year mission, Magellan, as planned, plunged into the atmosphere on October 11, 1994, and partly vaporized; some sections are thought to have hit the planet's surface.

Recent flybys

Missing image
Venus_Galileo.jpg
Image of Venus in visible light taken by Galileo probe

Several space probes en route to other destinations have used flybys of Venus to increase their speed via the gravitational slingshot method. These include the Galileo mission to Jupiter and the Cassini-Huygens Mission to Saturn (two flybys). Rather curiously, during Cassini's examination of the radiofrequency emissions of Venus with its radio and plasma wave science instrument during both the 1998 and 1999 flybys, it saw absolutely no high-frequency radio waves (0.125 to 16 MHz), which are commonly associated with lightning. This is in direct opposition to the findings of the Soviet Venera missions 20 years earlier. It is postulated that perhaps if Venus does have lightning, it might be some type of low-frequency electrical activity, due to the fact that radio signals cannot penetrate the ionosphere at frequencies below about 1 megahertz. An examination by physicist Donald Gurnett of the University of Iowa of radio emissions of Venus by the Galileo spacecraft during its gravity assist flyby in 1990 did reveal what were interpreted at the time to be indicative of lightning. However the Galileo probe was over 60 times as distant to Venus as was Cassini during its flyby, making its observations substantially less significant. To this day it remains a mystery as to whether or not Venus does in fact have lightning in its atmosphere.

Future missions

Venus Express is a mission prepared by the European Space Agency which will study the atmosphere and surface characteristics of Venus from orbit. The nominal mapping mission is planned to start in 2006 and is expected to last for two Venusian days (about 500 Earth days).

Future flybys en route to other destinations include the MESSENGER and BepiColombo missions to Mercury.


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