venus False-colour, ultraviolet image of Venus, showing faint bands of cloud in the atmosphere. (Image credit NSSDC/NASA.)

Venus is the second closest planet to the Sun. It has no moon. With a diameter of 12,104 kilometres it is the closest in size to the Earth. Its orbit about the Sun takes 224.7 days with its mean distance from the Sun being almost three-quarters that of the Earth. The planet rotates retrogradely in 243 days with respect to the stars (117 days with respect to the Sun, the Venusian day).

From the Earth the planet's surface is never seen as it is always covered by very dense layers of clouds. The upper clouds rotate with a period of four days at speeds of 350 km/hr.

Because the size and mass of Venus are close to those of the Earth it was supposed by many that Venus might be Earthlike and might even have life-forms on its surface. The truth is that Venus is very different from the Earth and it is extremely unlikely that there is any possibility that life has formed on Venus.

Venus has an atmosphere which, at the surface, has a pressure 90 times that of the Earth's. Unlike the Earth's atmosphere, which is mainly composed of nitrogen and oxygen, Venus's atmosphere is made up of 97% carbon dioxide with most of the remainder being nitrogen and argon. One consequence of the preponderance of carbon dioxide in the atmosphere is that Venus suffers from the severe effects of `greenhouse effect'. This means that the carbon dioxide in the atmosphere is transparent to the light and heat coming from the Sun but is opaque to the long wavelength infra-red radiation coming from the hot planet. This means that the surface of Venus is heated to a temperature of 470°C.

The clouds in the atmosphere of Venus which obscure our view of the surface are not composed of water droplets, as on the Earth, but are believed to be composed of droplets of sulphuric acid and particles of sulphur – thus making Venus a very unpleasant place for humans to contemplate visiting!

The surface of Venus

The surface of Venus can only be seen by space probes which have parachuted down through the atmosphere to the surface. These can not survive the hostile environment for long but have given us glimpses of a stony terrain with no great weathering and no great range of heights.

The most modern information about the surface comes from the Magellan Venus orbiter. This uses radar to map the surface. It has revealed mountains, valleys, cliffs, craters and huge volcanoes, at least one of which is believed to be active.

The interior of Venus is believed to be similar to that of the Earth with a metallic core and silicate mantle. Unlike the Earth, Venus has a very small magnetic field apart from that induced by the effect of the solar wind.

Seeing Venus

Solar image in the early part of the transit of Venus (2004) - less magnifiedThe transit of Venus in progress, 8 June 2004, captured through a telescope and solar filter. Venus is the black dot silhouetted against the Sun Venus can be very easily seen from the Earth. It is often called the Evening or Morning Star and is often the brightest object visible in the sky, with the exception of the Sun and Moon.

Because its orbit lies inside that of the Earth, Venus appears to move out from the Sun to a maximum distance, called greatest elongation, and then back towards the Sun. After passing behind, or in front of, the Sun it moves away from the Sun on the opposite side. The times when Venus is behind or in front of the Sun are called inferior and superior conjunctions. If the orbits of the Earth and Venus were in the same plane then at each conjunction Venus would either pass directly behind or in front of the Sun. The orbits are, however inclined to one another and so we see Venus pass in front of the Sun's disc, called a transit, only rarely. The last such transit was observed in 2004 at the Observatory.

If you know where to look, Venus can be seen even in the middle of the day, providing it is far enough away from the Sun. Through a small telescope Venus can be easily seen to have phases, like the Moon. It is very unusual for any details to be visible in the cloud structures, except for the most practiced observers.

Venus is best seen in the evening when it is to the east of the Sun and in the morning when it is to the west of the Sun. It is hard to mistake it for any other object as it is so bright. When near the horizon the effects of `twinkling' can give rise to amazing flashing colour effects which are often reported as peculiar objects, sometimes as UFOs.

Venusian volcanoes

no title A portion of Western Eistla Regio on Venus. The volcanoes Gula Mons (left) and Sif Mons (right) can be seen in the distance. The two mountains are approximately 730 km apart. Lava flows extend for hundreds of kilometres across the plains in the foreground. This image is provided courtesy of NASA/JPL from radar measurements made by the Magellan spacecraft.

The active volcano, Maat Mons, was discovered on Venus by the Magellan spacecraft. Active volcanoes are known to exist on only two planets in the solar system, Venus and the Earth. Extinct volcanoes are seen on Mars. Two satellites are known to have active volcanoes but these are very different from those on the Earth and on Venus.

On Io, moon of Jupiter, there are active sulphur volcanoes and on Triton, a moon of Neptune there are active volcanoes whose temperatures are far below zero centigrade. Opinions differ as to whether there is evidence for extinct volcanoes on the moon.

The planet rotates in retrograde in 243 days with respect to the stars (117 days with respect to the Sun, the Venusian day). Venus is named after the Roman Goddess of Love and Beauty (Greek Aphrodite).