Venus is sometimes referred to as Earthís sister planet due to the fact that it is the most like Earth in its density, size, and mass. Although it is much like our planet, it is one of the least-known of any of the terrestrial bodies. It is covered by a very dense atmosphere, filled with clouds that obscure Venusís surface. Venus has a very dense atmosphere a hundred times denser than that of Earth. Venusís atmosphere consists of 97 percent carbon dioxide. Venusís surface is very rocky, without water, and extremely hot- the temperature of Venus is more than 450 degrees C during both night and day. The pressure on the surface of Venus is 90 times that of the atmospheric pressure found on the Earth.
From the Earth one can observe the outer-most cloud layer of Venus which is 70 km above the surface of the planet. Above this outer-most layer up to the height of 90 km there is a layer of ice crystals that form a haze around the planet making observation in visible light extremely difficult. The upper haze is transparent when using ultraviolet radiation and one can observe the light and dark clouds in the highest layer. The clouds surrounding the planet primarily consist of sulfuric acid and other such materials.
The atmosphere of Venus is not a hazard to radar which has made the exploration of the planets surface possible. In the early 1980ís, radar on the spaceprobe Pioneer Venus Orbiter made the first global map of the planet.
Although the highest parts of Venus are an extreme difference compared to the lowest parts, Venus is smoother than Earthís surface. These great relative elevations, from 10.6 km to 1.5 km are located on only 8 percent of the planet. The greater part of Venusís surface, around 65 percent, consists of spreading, rolling plains. The other 27 percent contains lowlands, basins, and valleys, some reaching the depth of -2.5 km.
More detailed radar information from the probes Venera 15 and 16 have shown volcanoes and canyons. Igneous rocks-basalts-predominate the surface of Venus. In pictures taken by probes that have landed on the planet's surface one can find numerous sharp-edged rocks. This indicates that the planet is geologically active and is still forming surface features.