VENUS

Venus is the second planet from the Sun and the sixth largest. Venus' orbit (108,200,000 km (0.72 AU) from Sun) is the most nearly circular of that of any planet, with an eccentricity of less than 1%. Its diameter is 12,103.6 km and mass is 4.869e24 kg.

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. It is thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but Greek astronomers knew better. The first spacecraft to visit Venus was Mariner 2 in 1962.

Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde.

In some ways Venus, is very similar to Earth and is often called its sister planet because 1) Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass). 2) Both have few craters indicating relatively young surfaces and 3) Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth.

The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces an intense greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K which is hot enough to melt lead. Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

There are strong winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes such as Sif Mons. Recently announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.

The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earlier surface including any large craters from early in Venus' history.

Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the "morning star" or the "evening star", it is by far the brightest "star" in the sky.