Colonization of Venus
Venus's atmosphere is made mostly out of carbon dioxide, which, after filtering from sulfuric acid, can be used to grow food. Because nitrogen and oxygen are lighter than the carbon-dioxide that makes the atmosphere, breathable-air-filled balloons will float at a height of about . At this height, the temperature is a manageable ; or if we could get higher (see ).
Secondary atmosphere
A secondary atmosphere is an atmosphere of a planet that did not form by accretion during the formation of the planet's star. A secondary atmosphere instead forms from internal volcanic activity, or by accumulation of material from comet impacts. It is characteristic of terrestrial planets, which includes the other terrestrial planets in the Solar System: Mercury, Venus, and Mars. Secondary atmospheres are relatively thin compared to primary atmospheres like Jupiter's. Further processing of a secondary atmosphere, for example by the processes of biological life, can produce a tertiary atmosphere, such as that of Earth.