Comets A comet is generally considered to consist of a small, sharp nucleus embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple proposed in 1949 that the nucleus, containing practically all the mass of the comet, is a “dirty snowball” conglomerate of ices and dust. Major proofs of the snowball theory rest on various data. For one, of the observed gases and meteoric particles that are ejected to provide the coma and tails of comets, most of the gases are fragmentary molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH4 (methane), NH3 (ammonia), and H2O (water), which may exist as ices or more complex, very cold compounds in the nucleus. A....

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