Celestial body atmosphere

Atmosphere is the general name for a layer of gases that may surround a
material body of sufficient mass. The gases are attracted by the gravity of
the body, and held fast if gravity is sufficient and the atmosphere's
temperature is low. Some planets consist mainly of various gases, and thus
have very deep atmospheres.

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, weathering, and sequestration into the regolith
and polar caps.

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