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Atmospheric gases are lost to space when the individual molecules' thermal motion exceeds the escape velocity of the body. Since a gas at any particular temperature will have molecules moving at a wide range of velocities, there will almost always be some slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermal kinetic energy, and so gases of low molecular weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have both lost much of their water when, after being photodissociated into hydrogen and oxygen by solar ultraviolet, the hydrogen escaped. Earth's ozone layer helps to prevent this.

Other mechanisms that can cause atmosphere depletion are solar wind-induced sputtering, impact erosion, weatheringWeathering is the process of decomposition and/or disintegration of rocks in situ, that is, in place. It is not to be confused with erosion, which is the movement of rocks and/or weathering products by water, wind, ice or gravity. The breakdown products,, and sequestration—sometimes referred to as "freezing out"—into the regolithRegolith is a layer of loose, heterogeneous material covering solid rock. Regolith is present on Earth, the Moon, some asteroids, and other planets. The causes of regolith on Earth are erosion, weathering and biological processes. On Earth, regolith is ma and polar cap s.

See also: Earth's atmosphereEarth's atmosphere is the layer of gases surrounding the planet Earth and retained by the Earth's gravity. It consists of nitrogen (78. 1% by volume) and oxygen (20. 9%), with small amounts of argon (0. 9%), carbon dioxide (variable, but around 0. 035%),, stellar atmosphereDifferent stars have different atmospheres. All stars possess a so-called photosphere, which is the lowest and coolest part of the star's atmosphere and as the photosphere is the part which we see with our eyes (hence the name) you can think of it as the, Interstellar planetExtrasolar planets An interstellar planet is a hypothetical type of planet that has been ejected from its solar system by a proto- gas giant to become an outcast, drifting in interstellar space. Theoretical ideas on the atmospheres of interstellar exoplan

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Celestial globeThroughout Chinese history, astronomers have created celestial globes to assist the observation of the stars. The earliest celestial globe was dated back to 52 BC in the West Han dynasty by the astronomers Geng Shou-chang and Luo-xia Hong . The first wate Celestial sphereIn astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected, from their corresponding terran equivalents, a Celestial poleThe two celestial poles are the imaginary points where the Earth's spin axis intersects the celestial sphere. The sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 ( sidereal) hours. This phenomenon is due to Celestial coordinate systemIn astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. There are different celestial coordinate systems each using a coordinate grid projected on the celestial sphere, in analogy to the geographic coordinate s Celestial mechanicsCelestial Mechanics Astrodynamics Celestial mechanics is a term for the application of physics, historically Newtonian mechanics, to astronomical objects such as stars and planets. After Einstein explained the anomalous precession of Mercury's perihelion, Celestial navigationCelestial navigation is a position fixing technique that was the first system devised to help sailors locate themselves on a featureless ocean. Celestial navigation uses angular measurements (sights) between the horizon and a common celestial object. Celestial equator Celestial body Celestial body atmosphere Celestial observation Celestial Intervention Agency Celestial Chorus Celestial Kingdom Celestial spheres Celestial event