What turns a comet on?
All of the comets in the Oort cloud, and most of those in the Kuiper belt, are invisible. They are the nuclei of comets that are seen only when they come near the Sun. Each nucleus is the solid, enduring part of a comet. It is just a gigantic ball of frozen water ice and other ices laced with darker dust and pieces of rock. Light from the distant Sun is much too feeble to warm the comet ices, which remain frozen solid at the low temperatures in the remote comet reservoirs. When a comet nucleus emerges from the deep freeze of outer space and moves toward the Sun, the increased solar heat causes the cometís surface material to sublimate, with gases escaping through fissures in the crust of the nucleus. At the low pressure conditions of space, the solid ice goes directly into gas without passing through a liquid state, in a process called sublimation, just as dry ice does on Earth and water ice in the right terrestrial circumstances. The escaping gases also carry along dust particles. The gas and dust make the comet grow in size, enabling it to be seen.
If a comet nucleus provides such huge quantities of gas and dust, and still survives for hundreds or thousands of trips near the Sun, then it ought to be mainly composed of water ice. The fact that the outer layers of periodic comets start releasing material near 3 AU from the Sun suggests that water ice dominates their nucleus, since the temperature of the Sunís radiation at 3 AU is approximately that required to vaporize water ice. Ices of other possible molecules begin to sublimate off the nucleus at much lower temperatures and greater distances from the Sun. The vaporization of these more volatile substances, such as carbon dioxide, methane, and ammonia, may initiate the production of gas and dust in the new comets.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University