Atmosphere - Escape

Escape

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 lower 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. Interstellar planets, theoretically, may also retain thick atmospheres.

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 photo dissociated into hydrogen and oxygen by solar ultraviolet, the hydrogen escaped. Earth's magnetic field helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years the Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen.

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.

Read more about this topic:  Atmosphere

Famous quotes containing the word escape:

    We must understand how to hide in darkness in order to escape the gnat-swarms of utterly annoying admirers.
    Friedrich Nietzsche (1844–1900)

    Fine vapors escape from whatever is doing the living.
    The night is cold and delicate and full of angels
    Pounding down the living. The factories are all lit up,
    The chime goes unheard.
    We are together at last, though far apart.
    John Ashbery (b. 1927)

    When this immediate evil power has been defeated, we shall not yet have won the long battle with the elemental barbarities. Another Hitler, it may be an invisible adversary, will attempt, again, and yet again, to destroy our frail civilization. Is it true, I wonder, that the only way to escape a war is to be in it? When one is a part of an actuality does the imagination find a release?
    Ellen Glasgow (1873–1945)