Atmosphere

Atmosphere, the gas and aerosol envelope that extends from the ocean, land, and ice-covered surface of a planet outward into space. The density of the atmosphere decreases outward, because the gravitational attraction of the planet, which pulls the gases and aerosols inward, is greatest close to the surface. Atmospheres of some planetary bodies, such as Mercury, are almost nonexistent, as the primordial atmosphere has escaped the relatively low gravitational attraction of the planet and has been released into space. Other planets, such as Venus, Earth, Mars, and the giant outer planets of the solar system, have retained an atmosphere. In addition, Earth’s atmosphere has been able to contain water in each of its three phases, which has been essential for the development of life on the planet. The evolution of Earth’s current atmosphere is not completely understood. It is thought that the current atmosphere resulted from a gradual release of gases both from the planet’s interior and from the metabolic activities of life-forms-as opposed to the primordial atmosphere, which developed by outgassing (venting) during the original formation of the planet. Current volcanic gaseous emissions include water vapour (H₂O), carbon dioxide (CO₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), carbon monoxide (CO), chlorine (Cl), fluorine (F), and diatomic nitrogen (N₂; consisting of two atoms in a single molecule), as well as traces of other substances. Approximately 85 percent of volcanic emissions are in the form of water vapour. In contrast, carbon dioxide is about 10 percent of the effluent.