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Venus (planet)
Venus is the second planet from the Sun, named after the Roman goddess Venus. It is a terrestrial planet, very similar in size and bulk composition to Earth; it is sometimes called Earth's "sister planet" as a result of this similarity. Sometimes (inaccurately) referred to as the "morning star" or the "evening star", it is by far the brightest "star" in the sky. Because Venus is closer to the Sun than Earth is, it is always in roughly the same direction as the Sun, so can only be seen just before sunrise or just after sunset. Physical characteristicsAtmosphereVenus 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 kilometer under Earth's ocean). This enormous CO2-rich atmosphere results in a strong greenhouse effect that raises the surface temperature approximately 400°C above what it would be otherwise, causing temperatures at the surface to reach 500°C. This makes Venus's surface hotter than Mercury's, despite being nearly twice as distant from the Sun and only receiving 75% the solar irradiance (2660 W/m2). Due 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 Venusian year). Winds in the upper atmosphere circle the planet in only 4 days, helping to distribute the heat. There are strong 350-kilometer-per-hour winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour. However, due to the high density of the atmosphere at Venus's surface, even such slow winds exert a significant amount of force against obstructions. The clouds are composed of sulfur dioxide and sulphuric acid droplets and cover the planet completely, obscuring any surface details. 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 - on surface the temperature is never below 400°C. Surface featuresVenus has slow retrograde rotation, meaning it rotates from east to west instead of west to east as all other known planets in the solar system do. It is not known for sure 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 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. Due to Venus's thick atmosphere, which causes meteors to decelerate as they fall toward the surface, no impact crater smaller than about 3.2 km in diameter can form. Nearly 90% of Venus's surface appears to consist of recently-solidified basalt lava, with very few meteor craters. This suggests that Venus underwent a major resurfacing event recently. The interior of Venus is probably very 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 like Earth does, but instead undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava; 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). Recent findings suggest that Venus is still volcanically active in isolated geological hot spots. Venus has no magnetic field, possibly due to its slow rotation being insufficient to drive an internal dynamo of liquid iron. As a result, the solar wind impacts directly on Venus's upper atmosphere. 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 due 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. Venus was once thought to possess a moon, named Neith after the mysterious goddess Sais (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. Exploration of VenusHistorical observationsVenus is the most prominent astronomical feature in the morning and evening sky on Earth, and has been known of 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 Assyrians called Venus Dil-bat or Dil-i-pat, in ancient Egypt it was the special star of the mother-god Ishtar, and the Chinese knew it as Jin xing. Venus was the most important celestial body observed by the Maya, who called it Chak ek, "the Great Star", and considered it a representation of Quetzalcoatl; they apparently did not worship any of the other planets. (See also Maya calendar.) 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 soon came to recognize that both objects represented the same planet, however; 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. The name Venus comes from the Roman goddess of love and beauty. 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 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 also strongly supported the heliocentric model. Transits of Venus, when the planet crosses directly between the Earth and the Sun' visible disc, are important astronomical events. The first such transit was observed 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. The previous set of transits of Venus occurred within the interval of 1874 - 1882, and the next set of transits will occur in the period of 2004 - 2012. In the 19th century, many observers stated that Venus had a period of rotation of roughly 24 hours. Itallian 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 finally determined with confidence in 1961, using the Goldstone Radio Telescope in California. The fact that it was retrograde was not confirmed until 1964, however. Venus-observation spacecraftOn March 1, 1966 the Venera 3 Soviet space probe crashed on Venus's becoming the first spacecraft to land on the planet's surface. The first successful Venus probe was the American Mariner 2 probe, which flew past Venus in 1962. It established that Venus has no magnetic field and confirmed the planet's rotation rate. The Soviet Union sent a number of atmospheric probes and landers, with Venera 9 and 10 each returning a single black-and-white photograph of Venus's surface in 1975 and Venera 13 and 14 returning a number of colour photographs from Venus's surface in 1982. In 1985 the Soviet Vega 1 and 2 probes each deployed a sensor-laden balloon in Venus's atmosphere in addition to placing landers on the surface. No lander survived for more than about two hours before failing under Venus's intense surface heat and pressure. On August 10, 1990, the US Magellan probe arrived at the planet and started a mission of detailed radar mapping. 98% of the surface was mapped with a resolution of approximately 100m before the craft was deliberately deorbited on October 11, 1994.
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