Coma (cometary)

The comet Ikeya-Zhang exhibiting a bright, condensed coma (March 2002)

In astronomy, a coma (from the Greek κόμη, "hair") is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate.^[1] This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars.

The coma is generally made of ice and dust.^[1] Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun.^[1] The H₂O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.^[1] The solar wind plays a minor role in the destruction of water compared to photochemistry.^[1] Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure.

Comet 17P/Holmes, 2007/11/02

About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.^[2] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.^[3] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun.^[3] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.^[3]

Stardust was a NASA mission to recover samples of a comet's coma.

In some cases, such as the Great Comet of 1882, a comet develops a visible antitail or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail.

See also

• Coma (optics)
• Comet nucleus
• Extraterrestrial atmospheres

References

1. ^ ^{a b c d e} Combi, Michael R.; Harris, W. M.; Smyth, W. H. (2004). "Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations". Lunar and Planetary Institute (Comets II) 745: 523–552. Bibcode 2004come.book..523C. http://www.lpi.usra.edu/books/CometsII/7023.pdf. 
2. ^ Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. http://www2.ess.ucla.edu/~jewitt/holmes.html. Retrieved 2007-11-17. 
3. ^ ^{a b c} Gary W. Kronk. "The Comet Primer". Cometography.com. http://cometography.com/educate/comintro.html. Retrieved 2011-04-05.