Coma (comet)
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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;<ref name=Combi2004>Template:Cite journal</ref> this gives a comet a diffuse appearance when viewed through telescopes and distinguishes it from stars. The word coma comes from the Greek Template:Wikt-lang (Template:Grc-transl), which means "hair" and is the origin of the word comet itself.<ref name=chap14>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite Dictionary.com</ref>
The coma is generally made of ice and comet dust.<ref name=Combi2004/> Water composes up to 90% of the volatiles that outflow from the nucleus when the comet is within Template:Convert from the Sun.<ref name=Combi2004/> The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.<ref name=Combi2004/> The solar wind plays a minor role in the destruction of water compared to photochemistry.<ref name=Combi2004/> 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).<ref name="NASA-20140811">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="AJL-20140811">Template:Cite journal</ref> On 2 June 2015, NASA reported that the ALICE spectrograph on the Rosetta space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within Template:Convert 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.<ref name="NASA-20150602">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="AA-20150602">Template:Cite journal</ref>
SizeEdit
Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, even though the density is very low.<ref name=chap14/> About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.<ref name=atmosphere>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Great Comet of 1811 also had a coma roughly the diameter of the Sun.<ref name="primer">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="primer"/> At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.<ref name="primer"/>
X-raysEdit
Comets were found to emit X-rays in late-March 1996.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. Thomas E. Cravens was the first to propose an explanation in early 1997.<ref>Template:Cite journal</ref> The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping off" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
ObservationEdit
With a basic Earth-surface based telescope and some technique, the size of the coma can be calculated.<ref name=levy>Template:Cite book</ref> Called the drift method, one locks the telescope in position and measures the time for the visible disc to pass through the field of view.<ref name=levy/> That time multiplied by the cosine of the comet's declination, times .25, should equal the coma's diameter in arcminutes.<ref name=levy/> If the distance to the comet is known, then the apparent size of the coma can be determined.<ref name=levy/>
In 2015, it was noted that the ALICE instrument on the ESA Rosetta spacecraft to comet 67/P, detected hydrogen, oxygen, carbon and nitrogen in the coma, which they also called the comet's atmosphere.<ref name="esa.int">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Alice is an ultraviolet spectrograph, and it found that electrons created by UV light were colliding and breaking up molecules of water and carbon monoxide.<ref name="esa.int"/>
Hydrogen gas haloEdit
OAO-2 ('Stargazer') discovered large halos of hydrogen gas around comets.<ref name=o2>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did a close flyby of the comet in 1986.<ref name=Giotto>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A hydrogen gas halo was detected to be 15 times the diameter of Sun (12.5 million miles). This triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet was emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earth's surface because those wavelengths are blocked by the atmosphere.<ref name="usra">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft.<ref name="blogs.esa.int">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> One of the issues is where the hydrogen is coming from and how (e.g. Water splitting):
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A hydrogen gas halo three times the size of the Sun was detected by Skylab around Comet Kohoutek in the 1970s.<ref name="nasa">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> SOHO detected a hydrogen gas halo bigger than 1 AU in radius around Comet Hale–Bopp.<ref name="google">Template:Cite book</ref> Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light.<ref name="ase.tufts.edu">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The halos have been measured to be ten billion meters across, many times bigger than the Sun.<ref name="ase.tufts.edu"/> The hydrogen atom are very light so they can travel a long distance before they are themselves ionized by the Sun.<ref name="ase.tufts.edu"/> When the hydrogen atoms are ionized they are especially swept away by the solar wind.<ref name="ase.tufts.edu"/>
CompositionEdit
The Rosetta mission found carbon monoxide, carbon dioxide, ammonia, methane and methanol in the Coma of Comet 67P, as well as small amounts of formaldehyde, hydrogen sulfide, hydrogen cyanide, sulfur dioxide and carbon disulfide.<ref name="cnet">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The four top gases in 67P's halo were water, carbon dioxide, carbon monoxide, and oxygen.<ref name="phys.org">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The ratio of oxygen to water coming off the comet remained constant for several months.<ref name="phys.org"/>
