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Comets and Sublimation - Windows to the Universe

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Arches National Park Geology Tour provides an extensive, visually rich description of the geology of Arches, by Deborah Ragland, Ph.D. See our DVD collection.
Sublimation of frozen carbon dioxide ("dry ice") is common at the surfaces of comets when they warm up upon approaching the Sun. This is an artist's depiction of sublimation on a comet's surface.
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JPL

Sublimation

Evaporation is not quite the correct term to describe what happens to a comet as it approaches the Sun. The correct term is sublimation. The term describes what happens when a frozen material changes to gaseous form. Evaporation describes what happens when a liquid changes to a vapor.

The most common example of sublimation is that of dry ice, which is the common name of frozen carbon dioxide. 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 molecules in the ice begin to move so rapidly that they escape the solid and become gases.

When a comet approaches the sun, the comet comes to a region of space where it is warm enough that the ices inside the nucleus become gases. That is when the tails and coma of the comet form.

Last modified November 13, 2009 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF