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Venus


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(♀) The second planet from the Sun. It has the most circular orbit of all the planets. Its mean geometric albedo, 0.65, is the highest of all the planets, a result of its unbroken cover of white clouds. At its brightest it reaches magnitude −4.7, far brighter than any other planet. Its rotational axis is tilted at nearly 180° to the upright, so that its rotation is retrograde. It rotates on its axis every 243 days, and so presents the same face to the Earth every time the two planets are at their closest.

Venus has a dense atmosphere composed (by volume) of about 96.5% carbon dioxide and 3.5% nitrogen, with traces of sulphur dioxide, water vapour, argon, carbon monoxide, and helium. The pressure of the atmosphere at the surface is about 92 bar (i.e. 92 times the Earth's sea-level pressure). Surface temperatures average about 460°C due to the greenhouse effect in the planet's atmosphere. There is inconclusive evidence for lightning activity. A thick cloud layer at 45–65 km altitude, composed of sulphuric acid and water droplets, permanently obscures the surface. Through an optical telescope Venus appears virtually featureless, but at ultraviolet wavelengths streamers of cloud can be seen extending directly from the equator to the poles, indicating that there is a single Hadley circulation cell in each hemisphere carrying hot air directly from the equator to the poles, where atmospheric vortices are seen. The temperature difference in the atmosphere between equator and pole is no more than about 10°. At the equator the upper atmosphere rotates in only four Earth days in a retrograde direction, a phenomenon known as super-rotation. At the surface, wind speeds are only around 1 m/s, as measured by lander craft.

Radar maps of the surface have been made, both from the Earth and from space probes, which indicated the presence of highland areas such as Aphrodite Terra and Ishtar Terra and extensive rolling plains. In the early 1990s the Magellan spacecraft revealed that much of Venus is intensely fractured, and there are vast assemblages of graben and wrinkle ridges, suggesting both crustal expansion and contraction. Volcanism is an important surface process, and eruptions may be going on at present. There are many circular structures hundreds of kilometres across, called coronae, which appear to be super-volcanoes that have undergone phases of uplift, faulting, and degradation; the largest, Artemis Corona, is 2600 km across. Volcanic features more like those on Earth are also found, such as steep-sided circular volcanic domes, volcanic cones, large shield volcanoes, calderas, and lava flows. There are also volcanic features more like those on the Moon, including vast plains-forming lavas of low viscosity, and many sinuous rilles, one of which, Baltis Vallis, at 6000 km long, is the longest lava channel in the Solar System.

The dense atmosphere protects the surface from all but the largest meteoroids, so impact craters are scarce compared with Mercury or Mars, although more frequent than on Earth. About one thousand impact craters have so far been identified on Venus, with diameters greater than 1 km; the largest, Mead, is 270 km across. Many appear remarkably fresh, with central peaks or inner rings, but most are flooded or partially covered by subsequent lavas. All the best-preserved ones have ejecta outflows somewhat similar to those associated with rampart craters on Mars, but the flows are longer, sometimes winding away from the crater for hundreds of kilometres like lava. Greater impact melt due to the higher temperatures of Venus, and also debris flow due to the thick atmosphere, may both be responsible.

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Subjects: Astronomy and Astrophysics.


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