Atmosphere is the general name for a layer of gases that may surround a material body of insufficient mass. The gases are attracted by the gravity well 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 (see gas giant).
Earth, Venus, Mars, and Pluto have atmospheres that envelop their surfaces, as do three of the satellites of the outer planets: Titan, Enceladus (moons of Saturn), and Triton (a moon of Neptune). In addition, the giant planets of the outer solar system - Jupiter, Saturn, Uranus, and Neptune - are composed predominantly of gases. Other bodies in the solar system possess extremely thin atmospheres. Such bodies are the Moon (sodium gas), Mercury (sodium gas), Europa (oxygen) and Io (sulfur).
Initial atmospheric makeup is generally related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. These original atmospheres underwent much evolution over time, with the varying properties of each planet resulting in very different outcomes.
Surface gravity, the force that holds down an atmosphere, differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter is able to retain light gases such as hydrogen and helium that escape from higher gravity objects. Second, the distance from the sun determines the energy available to heat atmospheric gas to the point where its molecules' thermal motion exceed the planet's escape velocity, the speed at which gas molecules overcome a planet's gravitational grasp. Thus, the distant and cold Titan, Triton, and Pluto are able to retain their atmospheres despite relatively low gravities.
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 hydrogen dioxide when, after being photodissociated into hydrogen and oxygen by solar ultraviolet, the hydrogen escaped. Earth's magnetic field helps to prevent this, as the solar wind greatly enhances the escape of hydrogen.
Other mechanisms that can cause atmosphere depletion are solar wind-induced sputtering, impact erosion, weathering, and sequestration—sometimes referred to as "freezing out"—into the regolith and polar caps.
Moreover, on Earth, atmospheric composition is largely governed by the by-products of the very life that it sustains.
From the perspective of the planetary geologist, atmospheres are important in the ways they shape planetary surfaces. Wind can transport particles, both eroding the surface and leaving deposits (eolian processes). Frost and precipitation can leave direct and indirect marks on a planetary surface. Climate changes can influence a planet's geological history. Conversely, studying surface geology leads to an understanding of the atmosphere and climate of a planet - both its present state and its past.
Interstellar planets, theoretically, may also retain thick atmospheres.