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Venus, the second planet from the Sun, has an atmosphere about 96 times denser at surface level than the Earth's. Venus' atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen, thought to be similar to the Earth's atmosphere approximately 4.4 billion years ago. In the Earth's case, most of the carbon dioxide was absorbed by the seas, precipitating out as carbonates, but Venus lacks surface water or biomass to sequester the carbon dioxide, so it remains airborne.
Venus may be considered as an extreme example of global warming, with an average surface temperature of 461.85 °C (863 °F). This is not just due to Venus' proximity to the Sun, but because of the "greenhouse effect" — the Sun can deliver heat to Venus' atmosphere, but it retains that heat due to the large amount of greenhouse gases — carbon dioxide, sulfuric acid — present. On Earth, which has an atmosphere 100 times less dense, more of the energy radiates away.
Although the surface of Venus may be considered one of the most uninhabitable areas of the inner solar system, at approximately 50-65 km (31-40 mi) above the surface, the temperature and pressure of Venus' atmosphere is similar to that of Earth's. Because the pressure is similar, balloons filled with breathable air (21% oxygen, 78% nitrogen) would float at this level as long as they remain structurally intact. Not only that, but the extremely slow rotation of the planet itself could be avoided. The equatorial clouds at this level rotate around the planet about once every 20 hours. A colony suspended here would be carried along by the wind, experiencing a regular night and day, much like people living on Earth. These factors have caused some space scientists to call this region the most habitable in the solar system outside Earth, trumping Mars.
Because the planet lacks its own magnetic field, Venus' atmosphere is constantly assailed by the solar wind. The charged solar wind strips away hydrogen, helium, and oxygen atoms, producing a long magnetotail composed of ions, extending many planetary diameters behind Venus.
Venus' atmosphere is filled with clouds of sulfuric acid, which reflect 75% of incoming light. Their many layers have historically served to obscure the surface of Venus, leaving humanity to speculate about the world beneath. Nothing was known of Venus' surface until the 1970s, when radar pulses were beamed at the planet by the 300 m radio telescope at Arecibo Observatory. This revealed surface features as small as 5 km (3 mi) in width.