Coma (comet)

The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes near the Sun in its highly elliptical orbit. As the comet warms, parts of it sublimate;^[1] this gives a comet a diffuse appearance when viewed through telescopes and distinguishes it from stars. The word coma comes from the Greek κόμη (kómē), which means "hair" and is the origin of the word comet itself.^[2][3]

The coma is generally made of ice and comet dust.^[1] Water composes up to 90% of the volatiles that outflow from the nucleus when the comet is within 3–4 au (280–370 million mi; 450–600 million km) from 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.

On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H₂CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).^[4][5] On 2 June 2015, NASA reported that the ALICE spectrograph on the Rosetta space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the liberation of water and carbon dioxide molecules released from the comet nucleus into its coma.^[6][7]