Venus has a much more dense atmosphere than the Earth, as a result of a ‘runaway greenhouse effect’ that evaporated all the surface water and prevented carbon dioxide from becoming locked up within limestone rock as happened on Earth.

The average surface pressure on Venus is about 90 times that on Earth (almost as much pressure as you would experience at a depth of a kilometre in the Earth’s oceans), and the atmosphere is 96% carbon dioxide, 3.5% nitrogen, 0.015% sulphur dioxide, 0.01% water vapour and 0.007% argon. The clouds that hide the surface from view (except by using radar) lie between 45 and 65 km from the surface, and consist of water droplets with dissolved sulphur dioxide (making them essentially sulphuric acid!).

The dense atmosphere, in particular the carbon dioxide, traps the Sun’s energy and makes the surface and lower atmosphere much hotter than they would otherwise be, even though the overlying cloud layer actually reflects a very high proportion of the sunlight.

Separating the effects of the highly reflective cloud cover tending to make Venus colder, from the ‘greenhouse effect’ of the dense carbon dioxide atmosphere (mostly below the clouds) that traps heat and tends to make it warmer would give the following results:

1. With no atmosphere, Venus would have a surface temperature of 130^oC
2. With global clouds (which are present) but no greenhouse effect, Venus would have a surface temperature of minus 50^oC
3. With global clouds and the actual greenhouse effect, Venus's real surface temperature averages at 470^oC

The main goal of the Venus Express mission is to find out more about the composition, evolution and circulation of the atmosphere. The images which follow, from previous missions, illustrate our present state of knowledge.

 

From Earth, Venus is a very bright object in the sky, but it appears virtually featureless through conventional telescopes. This montage shows Venus imaged between 27 February and 8 June 2004 through the same telescope. Notice how the apparent size of Venus increases as its faster orbit carries it between the Earth and the Sun, while at the same time we see a progressively smaller illuminated portion of the globe. The 8 June image was recorded as Venus passed across the Sun’s disk, which is a yellow rim (most of which has been removed) around the unilluminated planet. [Image: Statis Kalyvas and ESO VT-2004 programme]

 

Venus passing across the Sun’s disk on 8 June 2004. Observation of the fuzzy edge of the planet during rare ‘transits’ such as this provided some of the first evidence that Venus has an atmosphere. [Image: Statis Kalyvas and ESO VT-2004 programme]

There are many more images of the transit in our Transit of Venus gallery.

 

Venus as a crescent, seen from the Hubble Space Telescope in orbit about the Earth. [Image: NASA]

 

Ultraviolet image revealing structure in the top of Venus’s sulphuric acid cloud layer, recorded by the Pioneer Venus orbiter in 1979. [Image: NASA]

A series of views of the sulphuric acid cloud tops of Venus obtained over a two-day period by the Galileo probe as it was receding after a fly-by in 1996. The lower two images were taken only two hours apart; if you look carefully at these you should make out some cloud features migrating from right to left. This is actually in the opposite direction to the slow rotation of the planet beneath the clouds! [Image: NASA-JPL Photojournal]

Two views of Venus in different wavelengths. The top image was recorded light, and shows the top of the uppermost sulphuric acid cloud layer. The lower image was recorded in the near infrared, and reveals atmospheric structure about 10 km below the top of the cloud layer. [Image: NASA-JPL Photojournal]

The probable structure of Venus’s atmosphere showing the main cloud layer and also how temperature (blue line) varies with height. The temperature scale is in K (kelvins) where 273 K = 0^oC. [Graphic: NASA]