A comet is a small icy body that orbits the Sun and has a visible atmosphere (coma) and sometimes one or more tails. The word comet comes from the Greek word kometes, which means “long haired.” It is the appearance of the glowing coma and tail that identifies a newly discovered object as a comet rather than an asteroid. Here is a closer look at the parts of a comet, its composition, and appearance.
Parts of a Comet
The four visible parts of a comet are its nucleus, coma, ion tail, and dust tail. However, there are invisible regions, too.
- Nucleus: The nucleus is the solid “dirty snowball” core of a comet. It consists mainly of water ice, other volatile ices (carbon dioxide, methane, ammonia, carbon monoxide), silicate dust, and organic particles (methanol, hydrogen cyanide, ethanol, formaldehyde, ethane, amino acids, hydrocarbons). A typical comet is a few kilometers in diameter. The albedo or reflection of a comet’s surface tends to be a bit redder than the color of the Sun. But, cometary nuclei range in color from very red to slightly blue.
- Coma: The coma is the atmosphere that escapes from the nucleus. As the comet nears the Sun, the solar wind sublimates the volatile ice into vapor, carrying some dust particles along. Coma color changes according to “seasons” on a comet. As a comet nears the Sun, its coma sometimes glows green. When a coma is green, it is because ultraviolet light excites electrons in cyanide/cyanogen (CN) and diatomic carbon (C2), which emit green light as the electrons return to lower energy states. A comet has a coma, while an asteroid lacks this feature.
- Hydrogen envelope: An invisible cloud of hydrogen surrounds the coma. The hydrogen cloud that surrounds a comet can be millions of kilometers in diameter, but the neutral hydrogen gas only appears to instruments and not human eyes.
- Dust tail: Solar radiation blows the dusty vapor of the coma back, forming the dust tail. The comet’s orbit also affects the tail, so it usually curves back behind the comet’s path. Usually, the tail is yellow or white in color. The dust tail extends up to 10 million kilometers behind the nucleus and coma.
- Ion tail: Unlike the dust tail, the ion tail points almost exactly away from the Sun. Solar radiation ionizes volatile gases in the coma and pushes this plasma away from the comet. The ion tail often has a blue glow from CO+ ions. This tail is narrow and extends back 100 million kilometers behind the nucleus. The ion tail often has rays and streamers from particles interacting with the solar wind.
Other Features of Comets
Close inspection of a comet shows that its surface is unstable. The uneven heating from the Sun produces jets of gas, snow, and dust. Sublimating dry ice powers these jets. The force of expulsion can tear a comet apart.
Some comets have a third tail, located between the dust and ion tails. This salt tail ionizes, to it is subject to the solar wind, but contains particles intermediate in size between the dust and plasma of the other two tails.
Where Do Comets Come From?
Comets form from material left over from the solar system formation 4.6 billion years ago. Most nuclei appear to originate from two different regions in the solar system. Most are fragments of Kuiper belt objects. The Kuiper belt, in turn, is a region beyond the orbit of Neptune that contains asteroids, comets, and other small icy bodies. However, long-period and Halley-type seem to come from around the gas giant planets, but got ejected into the Oort cloud, where they remained until gravity drew them toward the Sun. The Oort cloud is a region of icy debris that ranges from 2000 to 200,000 AU (0.03 to 3.2 light years) outward from the Sun. It include an inner disc region and outer spherical volume.
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Are you interested in learning more? Make your own model comet and explore the parts of comets and how they work.