Atmosphere

This article is about atmospheres of celestial bodies. For Earth's atmosphere, see Atmosphere of Earth. For other uses, see Atmosphere (disambiguation).
"Atmospheric environment" redirects here. For the scientific journal, see Atmospheric Environment.
The atmospheric gases around Earth scatter blue light (shorter wavelengths) more than light toward the red end (longer wavelengths) of the visible spectrum; thus, a blue glow over the horizon is seen when observing Earth from outer space. The Moon is visible in the background.

An atmosphere (from Ancient Greek ἀτμός (atmós) 'vapour, steam' and σφαῖρα (sphaîra) 'sphere')[1] is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere; stars of low temperature might have outer atmospheres containing compound molecules.

The atmosphere of Earth is composed of nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.04%) and trace gases.[2] Most organisms use oxygen for respiration; lightning and bacteria perform nitrogen fixation which produces ammonia that is used to make nucleotides and amino acids; plants, algae, and cyanobacteria use carbon dioxide for photosynthesis. The layered composition of the atmosphere minimises the harmful effects of sunlight, ultraviolet radiation, solar wind, and cosmic rays and thus protects the organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.[3]

  1. ^ Cite error: The named reference etymology was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference earthcomposition was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference globalchange was invoked but never defined (see the help page).