Venus, the second planet out from the Sun, is similar to the Earth in density and size, yet its atmosphere and surface environment are quite different. Seen from the Earth, Venus is usually the brightest planet in the sky.
Its radius is 6,050 km and its mean density suggests that the planet's interior is generally similar to that of the Earth. The surface of Venus is always obscured by thick yellowish-white clouds, but radar observations of the surface reveal that the planet spins backwards once every 243 days. Since the planet orbits the Sun in a nearly circular orbit once every 225 days, the length of the apparent day on Venus is 116 days.
The planet's cloudy atmosphere is mostly made of carbon dioxide, with small amounts of nitrogen and water vapour. Other minor constituents include hydrofluoric acid and hydrochloric acid. The temperature averages 1020 degrees and changes little with latitude or from day to night.
The high surface temperatures are believed to be maintained by a greenhouse effect. Some sunlight filters through the thick clouds. The escape of heat rays is hindered by the thick atmosphere and clouds, allowing the surface to heat up to the high temperatures observed. Evidently liquid water cannot exist at the planet's surface.
Little is known about the interior of Venus. It may resemble that of the Earth, with a metallic core extending out to about 50 percent of the radius. The planet has no magnetic field, but due to the very slow spin rate, this need not mean that the planet's ore is not molten.
Much attention has focused on the remarkable difference between the atmosphere of Venus and Earth. It is generally believed that Venus, being closer to the Sun, was always too hot to permit water to exist in the liquid state. The present atmospheres of the Earth-like planets are believed to be the result of gradual outgassing from the interior, with carbon dioxide and water as the principle constituents. On Venus, the carbon dioxide released from the interior remains in the atmosphere. If Venus ever outgassed anything like an ocean of water, some efficient removal process must be sought. The water vapour may have dissociated into hydrogen and oxygen. The hydrogen escaped from the planet and the oxygen was being used up in chemical weathering of the surface.
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