NATS 1750 Lecture Notes - Lecture 20: Late Heavy Bombardment, Cyanobacteria, Barometer

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An atmosphere = the thin layer of gases held by a planet"s gravity. The layer of gases generally extends up at most a few hundred kilometres. Since gases are always in motion, this means that they can escape a planet"s gravity. Lighter gases tend to move faster, so it"s easier to keep an atmosphere of heavier gases. A smaller planet like earth cannot hold on to significant amounts of things like h (hydrogen) and helium since they are very light gases. Even the heavier gases that are in an atmosphere, however, can escape with time. This is why any planet with a significant atmosphere must have some source to renew it. Atmosphere was primarily nitrogen, with about 20% of carbon dioxide. Photosynthetic cyanobacteria extracted carbon dioxide and released oxygen. Earth"s atmosphere is predominantly nitrogen and oxygen today. 78% nitrogen, 21% oxygen, and 1% of other gases [mostly water vapour, but also carbon dioxide, methane, argon and helium]

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