Atmosphere
The gaseous envelope surrounding a planetary object, moon or star, (or possibly the habitable surfaces of an artificial structure. A typical atmosphere consists of a mixture of gases. Some of the most common are hydrogen, helium, methane, nitrogen, oxygen, argon, carbon dioxide, water vapour, and ammonia. Often an atmosphere is divided into zones of similar pressure, temperature, and composition.
 
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Development Notes
Text by M. Alan Kazlev

Initially published on 07 October 2001.