Venus' Atmosphere

The atmosphere of Venus is one of the thickest atmospheres in the Solar System, and is by far, the thickest of the terrestrial planets.  It is mainly carbon dioxide (CO2) with 96.5% of the atmosphere made up of CO2.  It has about 3.5% nitrogren (N2) and trace other elements with sulfur compounds beign a major portion.  Compare this to the Earth with 78% N2, 21% O2, and trace other gases (argon being the chief among those gases).

The atmosphere is so thick that the atmospheric pressure at the surface (what we would call sea level on Earth) is 92 times that of Earth.  A cubic meter of air on Earth has a mass of about 1.2 kilograms, or weighs about 10 pounds.  On Venus, the same volume of air has a mass of 67 kilograms, or weighs 600 pounds on Venus (on Earth, that volume of air would weigh 670 pounds).  This weigh is so heavy, that its atmosphere at the surface would squash you flat and kill you, if the oppressive heat didn't get you first.

Besides being oppressive, the heat is the most impressive thing about Venus' atmosphere.  Despite being farther from the Sun than Mercury, its surface temperature is hotter.  Because Mercury has such a thin, if lacking, atmosphere, it does not retain heat well.  With Venus thick atmosphere composed of mostly carbon dioxide, the atmosphere does a great job of retaining heat reflected and emitted by the surface of the planet.  Carbon dioxide a really good job of preventing infrared radiation from escaping into space which in turn heats up the atmosphere.  This lead to a runaway greenhouse effect which increases the heat on Venus' surface.  On Venus, surface temperatures can reach 462°C (864°F) where on Mercury, in sunlight, reaches "only" 420°C (788°F).  Mercury does drop below freezing on the side facing away from the Sun at -220°C (-364°F) because the lack of an atmosphere.

At the same time, it is nearly impossible to see the surface of Venus without some help.  The intense cloud cover does not allow visible light to escape.  On Earth, our clouds are made of water vapor and droplets.  Venus' clouds are hydrogen sulfide and sulfuric acid.  Not easy material for visible light to traverse.  These clouds allow 50% of the visible light to come through and heat the ground, leading to the reflection and emission of infrared light, while the other 50% is reflected into space.  What we see when we look at Venus is the cloud cover.

To see Venus itself, we use radio waves which have long enough wavelengths to travel through the clouds.  The reflected radio waves can then be detected and map the surface.  This was what the spacecraft Magellan did to show us the planetary features.  Any images of the surface of Venus are all false color.