Solar System -


Venus, the second planet out from the Sun, is very similar in size to the Earth being about 7,520 miles in diameter, about 400 miles less than the Earth, with about 88% of its surface gravity.  Unlike Earth, it has a very weak magnetic field, and no moon.  It is approximately 67.2 million miles from the sun, and orbits it in about 224.7 Earth days. Venus, like Uranus, rotates on its axis in the opposite direction to the other planets, Venus  taking 243 Earth days for one revolution; thus, a Venus "day" is longer than its year.  One theory is that Venus formed from the collision of two large  objects rotating in opposite directions such that the nature of the collision cancelled most of the angular momentum.  Its closest approach to the Earth brings it to about 41 million km (less than 26 million miles) away. 

Clouds of sulfuric acid high in the atmosphere cover the surface of Venus so it is not possible to view the surface
in visible light.  As the  clouds take only about four days to orbit Venus, the winds are very strong, approaching 200 miles per hour in the upper atmosphere, over extended areas.  Most of the atmosphere comprises carbon dioxide (~96.5%), with a little nitrogen (~3.5%).  There are minute quantities of some other chemicals, totalling less than 0.03%; for example sulfur dioxide, argon, carbon monoxide, helium and neon. 

In its early days, Venus probably had oceans similar to Earth, but these evaporated as the  temperature increased, and moved into the upper atmosphere
where sunlight broke down the water vapor into hydrogen and  oxygen.  The solar wind then swept the hydrogen into space.  At the surface, atmospheric pressure is more than 90 times that of  the Earth with temperatures around 500 degrees C.  This is almost certainly the result of a runaway greenhouse effect.

It is likely that Venus undergoes periodic cataclysmic volcanism that, essentially, resurfaces the planet with material drawn  from within itself.  There are relatively few impact craters lending credence to the fact that the exposed surface is geologically young; probably only 300 to 500 million years old.  The surface also appears to be a single crustal plate rather than the series of tectonic plates here on Earth.  Some planetary scientists believe this is evidence that Venus is not as geologically evolved as the Earth, and that the surface will, perhaps, break up into several tectonic plates in the future.  

Venus is easily seen as it is the second brightest object in the night sky after the Moon, appearing soon after sunset as a very bright star.  You can also see it a few hours before sunrise when it is known as the Morning Star.  On June 6th 2012, Venus will transit the sun.  This is a very rare event, as they occur every 8, 121½, 8 and 105½ years.  The next one after 2012 is due in December 2117.  During the transit, Venus will appear as a black dot crossing the sun's face. 
While Venus does not have any true moons, like the Earth it has what is known as a quasi-moon called 2002 VE68, which is a few hundred miles across.  It orbits the Sun with the same orbital period as Venus, known as a 1:1 orbital resonance, but on a highly eccentric orbit taking it inside Mercury's orbit, and outside Earth's.  This arrangement has probably existed for only the last 7,000 years or so, and is so unstable that Venus is likely to loose the satellite in about 500 years time.  A typical example of this type of orbit is shown in the diagram to the left and on this animation, which actually shows the Earth and its quasi-satellite 3753 Cruithne, but the principle is the same. 

Description of Venus