Extraterrestrial atmosphere star undergoing runaway greenhouse effect. For planets in the habitable zone, it was thought that water photolysis would be strongly limited by cold-trapping of water vapour in the lower atmosphere. However, the extent of HO cold-trapping depends strongly on the amount of non-condensible gases in the atmosphere such as nitrogen N and argon. In the absence of such gases, the likelihood of build-up of oxygen also depends in complex ways on the planet's accretion history, internal chemistry, atmospheric dynamics, and orbital state. Therefore, oxygen, on its own, cannot be considered a robust biosignature. The ratio of nitrogen and argon to
Extraterrestrial atmosphere greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics. Due to its small size (and thus its small gravity), Mercury has no substantial atmosphere. Its extremely thin atmosphere mostly consists of a small amount of helium and traces of sodium, potassium, and oxygen. These gases derive from the solar wind, radioactive decay, meteor impacts, and breakdown of Mercury's crust. Mercury's atmosphere is not stable and is constantly being refreshed because of its atoms escaping into space as a result of the planet's heat. Venus' atmosphere is mostly composed of carbon dioxide. It contains minor amounts of nitrogen and