atmospheres

An atmosphere is a layer of gas held around a planet, moon, or other body by gravity. It forms a gaseous envelope that interacts with radiation and solar wind and can influence surface conditions and climate. The composition and thickness of an atmosphere vary greatly among worlds and determine many physical and chemical processes at and near the surface.

The atmospheric composition is dominated by a few gases with trace amounts of others. Earth’s atmosphere consists mainly of nitrogen and oxygen, with argon, carbon dioxide, neon, and water vapor in smaller amounts. Other planets have very different makeups: Venus is thick and CO2-rich; Mars is thin and CO2-dominated; gas giants have hydrogen and helium as the main components.

Atmospheres are structured in layers defined by temperature and composition: troposphere, stratosphere, mesosphere, thermosphere, and exosphere on Earth; other bodies may have different or fewer layers. Temperature generally changes with altitude, creating stable and unstable layers that drive weather and climate. In the upper atmosphere, solar radiation and chemical reactions create ionized regions called ionospheres.

Formation and evolution depend on a body's gravity and energy input. A strong gravity is needed to retain light gases; volcanic outgassing and delivery by comets add materials; atmospheric loss processes such as thermal escape, sputtering, and solar wind erode atmospheres. Magnetic fields and planetary winds can influence retention.

Study of atmospheres provides insights into habitability, geology, and planetary history. Observations use spectroscopy from Earth and spacecraft, and in situ measurements by landers and probes.