What Are Comets Made Of?

Comet Structure

• A comet consists of three parts: the nucleus, coma and tail. Comets lurk in the dark outer regions of space and are invisible until they approach the sun. Every time a comet makes a pass by the sun, some of the frozen ice and organic matter in the nucleus evaporates under the effects of solar radiation and solar wind, causing a glowing aurora to appear as the coma and the tail. With repeated fly-bys around the sun, the comet inevitably decreases in size until it completely disintegrates.

The Nucleus

• The chemical composition of a comet's nucleus is mainly ice, which includes water ice and carbon dioxide ice. Comets also carry organic compounds such as formaldehyde, ammonia, hydrogen cyanide, hydrogen sulfide and methanol, as well as silicates, dust and rocks. Although astronomers often refer to a comet as a dirty snowball, a more accurate description of a comet's nucleus is a dirty ice ball.

  The size ranges from a few hundred yards to 25 miles or possibly more in diameter. Comet Hale-Bopp, the comet that flashed across the sky in 1997, had a nucleus estimated to be 24.8 miles in diameter.

The Coma

• As the comet approaches the sun, a nebulous envelope called the coma forms around its nucleus and some of its matter sublimates as it warms, giving it a fuzzy appearance. The coma is composed of ice and dust, with water making up 90 percent of the volatile compounds that evaporate during its passage around the sun.

The Tail

• Comet tails form from the gas and dust given off by the nucleus as it approaches the sun. There are two types of comet tails; both form from the gas and dust spinning off the nucleus. Dust tails form from dust particles blowing off of the nucleus. They can be hundreds of miles in length, and the scattered dust resulting from the comet's passage shows up in the Earth's atmosphere as meteor showers whenever this planet passes through the remains of one of their tails.

  Plasma tails can form from gases sublimating from the nucleus and ionizing by the solar wind, a stream of charged particles burning away from the sun at a speed of hundreds of miles per second. Gas particles evaporating from the nucleus of the comet become ionized or charged and glow with a dramatic ion tail. The ions with the longest lifetimes are carbon monoxide ions, which emit a blue color.