This is a picture of dry ice (frozen CO2) sublimating (turning to a gas).
Click on image for full size
Why Pluto has an Atmosphere
Because the orbit of Pluto is so strange, Pluto is sometimes close enough to the sun for the ices on its surface to sublimate. Sublimation describes what happens when a frozen material changes to gaseous form. (Evaporation describes what happens when a liquid changes to gaseous form).
The most common example of sublimation is that of dry ice, which is the common name of frozen CO2. When dry ice is exposed to the air it begins to sublimate, or change to vapor, before your very eyes. This happens to dry ice because at room temperature the frozen gas would rather be a gas than frozen solid.
When Pluto comes close enough to the sun, the surface of solid Nitrogen sublimates to produce a substantial atmosphere with winds and clouds. Because the planet is so small, however, it does not have enough gravity to bind an atmosphere for very long. Thus Pluto's atmosphere is being rapidly produced and rapidly lost at the same time. This means that the atmosphere is not in equilibrium.
By 1999, Pluto will leave the neighborhood of Neptune and drift further away from the sun in its orbit. As it gets further from the sun it will no longer produce an atmosphere. Thus it will soon be too late for humankind to study Pluto's unique environment. These are good reasons to explore Pluto now.
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