There the similarities end. Venus has no surface water, a toxic, heavy atmosphere made up almost entirely of carbon dioxide with clouds of sulphuric acid and at the surface the atmospheric pressure is over 90 times that of the Earth at sea-level.

The surface of Venus is the hottest in the solar system at a searing 750 K (477 °C). This high temperature has been caused by a catastrophic greenhouse effect due to the carbon dioxide rich atmosphere. Incident sunlight is trapped by the atmosphere and cannot radiate out into space with a resulting boost to the surface temperature by over 475 K.

The final anomaly between the two worlds is the rotation of Venus. Firstly its axis of rotation is inclined at 177.36 degrees (compared to 23.5 degrees on Earth). This means that Venus rotates in a retrograde direction from east to west, making the Sun rise in the west and set in the east. Further to this the rotation is very slow: a sidereal day on Venus lasts 243 Earth days. This is even longer than a Venusian year which is only 224.7 Earth days.

Despite several missions to Venus there is still much that is unknown about our celestial neighbour. The Russian Venera landers touched down in the 1980s and survived the hostile environment for about 60 minutes during which time images were taken revealing a barren world. Other missions to Venus, both Russian Venera orbiters, and NASA's Magellan spacecraft have concentrated on radar mapping the surface.

From the data returned by these missions scientist believe that the surface of Venus is relatively young - it appears to have undergone resurfacing around 400 million years ago. The topography consists of vast plains (covered by lava flows) and mountainous regions.

Impact craters cover the surface. Unlike other worlds, however, there are virtually no craters less than 2 km in size. The impactors that would cause them simply burn up in the thick atmosphere. Those that do exist are believed to be caused by the fragmentation of a large meteorite just before impact with the surface.

Table comparing various parameters for the two planets.