Venus
Venus is the second planet from the sun, named after the Greek 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.
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 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. Due to the thermal inertia and convection of its dense atmosphere, surface 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).
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. This observation supported Copernicus's heliocentric description of the solar system.
Venus 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. This may be the result of resonance, with tidal forces affecting Venus' rotation whenver the planets get close enough together, or it may simply be a coincidence.
Transits of Venus, when the planet crosses directly between the Earth and the Sun' visible disc, are important astronomical events. 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.
Venus's surface appears to consist largely 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 to be lost; 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 mysterious goddess of 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.
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 American space program followed up the Mariner program with a series of Pioneer orbiters and finally the recent Magellan orbiter, mapping the surface of Venus by using radar to penetrate its permanent total cloud cover.
On August 10, 1990, the American 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.
Image:Venus-venera13-right thumb.jpg
Color image taken from the surface of Venus by the Soviet Venera 13 lander. click here for full-sized image
See also: Venus (goddess)
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