View source for Coma (comet)

{{Short description|Cloud of gas or a trail around a comet or asteroid}}
{{Other uses|Coma (disambiguation)}}
[[File:Infrared Structure of Comet Holmes.jpg|thumb|400px|Structure of Comet Holmes in infrared, as seen by an infrared space telescope]]

The '''coma''' is the nebulous envelope around the [[comet nucleus|nucleus]] of a [[comet]], formed when the comet passes near the [[Sun]] in its [[highly elliptical orbit]]. As the comet warms, parts of it [[sublimation (chemistry)|sublimate]];<ref name=Combi2004>{{cite journal |last1=Combi|first1=Michael R. |last2=Harris|first2=W. M. |last3=Smyth|first3=W. H. |title=Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations |journal=Comets II |publisher=[[Lunar and Planetary Institute]] |volume=745 |pages=523–552 |date=2004 |bibcode=2004come.book..523C |url=https://lpi.usra.edu/books/CometsII/7023.pdf
}}</ref> this gives a comet a diffuse appearance when viewed through [[telescope]]s and distinguishes it from [[star]]s. The word ''coma'' comes from the Greek {{Wikt-lang|grc|κόμη}} ({{grc-transl|κόμη}}), which means "hair" and is the origin of the word ''comet'' itself.<ref name=chap14>{{cite web |url=http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT314/HTML/AT31402.HTM |publisher= |website=lifeng.lamost.org |title=Chapter 14, Section 2 {{pipe}} Comet appearance and structure |access-date=2017-01-08}}</ref><ref>{{cite Dictionary.com|comet |access-date=2016-01-02}}</ref>

The coma is generally made of [[volatile (astrogeology)|ice]] and [[comet dust]].<ref name=Combi2004/> Water composes up to 90% of the [[volatile (astrogeology)|volatiles]] that outflow from the nucleus when the comet is within {{convert|3–4|au|e6mi e6km|lk=on|abbr=unit}} from the Sun.<ref name=Combi2004/> The [[water|H<sub>2</sub>O]] 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 [[Comet tail|tail]] by [[radiation pressure|light pressure]].
<!--
[[Stardust (spacecraft)|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.-->

On 11 August 2014, astronomers released studies, using the [[Atacama Large Millimeter Array|Atacama Large Millimeter/Submillimeter Array (ALMA)]] for the first time, that detailed the distribution of [[Hydrogen cyanide|HCN]], [[Hydrogen isocyanide|HNC]], [[Formaldehyde|H<sub>2</sub>CO]], and [[dust]] inside the comae of [[comet]]s [[C/2012 F6 (Lemmon)]] and [[Comet ISON|C/2012 S1 (ISON)]].<ref name="NASA-20140811">{{cite web |last1=Zubritsky |first1=Elizabeth |last2=Neal-Jones |first2=Nancy |title=RELEASE 14-038 - NASA's 3-D Study of Comets Reveals Chemical Factory at Work |url=http://www.nasa.gov/press/2014/august/goddard/nasa-s-3-d-study-of-comets-reveals-chemical-factory-at-work |date=11 August 2014 |work=[[NASA]] |access-date=2014-08-12 }}</ref><ref name="AJL-20140811">{{cite journal |author=Cordiner, M.A. |title=Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array |date=11 August 2014 |journal=[[The Astrophysical Journal]] |volume=792 |pages=L2 |number=1  |doi=10.1088/2041-8205/792/1/L2 |display-authors=etal|arxiv = 1408.2458 |bibcode = 2014ApJ...792L...2C |s2cid=26277035 }}</ref> On 2 June 2015, NASA reported that the [[Rosetta (spacecraft)#Instruments|ALICE spectrograph]] on the [[Rosetta (spacecraft)|''Rosetta'' space probe]] studying comet [[67P/Churyumov–Gerasimenko]] determined that [[electron]]s (within {{convert|1|km|mi|abbr=on}} above the [[comet nucleus]]) produced from [[photoionization]] of [[water]] [[molecule]]s by [[Sunlight|solar radiation]], and not [[photon]]s 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">{{cite web |last1=Agle |first1=DC |last2=Brown |first2=Dwayne |last3=Fohn |first3=Joe |last4=Bauer |first4=Markus |title=NASA Instrument on Rosetta Makes Comet Atmosphere Discovery |url=http://www.jpl.nasa.gov/news/news.php?feature=4609 |date=2 June 2015 |work=[[NASA]] |access-date=2015-06-02 }}</ref><ref name="AA-20150602">{{cite journal |url=http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |title=Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta |journal=[[Astronomy and Astrophysics]] |volume=583 |pages=A8 |last1=Feldman |first1=Paul D. |last2=A'Hearn |first2=Michael F. |last3=Bertaux |first3=Jean-Loup |last4=Feaga |first4=Lori M. |last5=Parker |first5=Joel Wm. |last6=Schindhelm |first6=Eric |last7=Steiffl |first7=Andrew J. |last8=Stern |first8=S. Alan |last9=Weaver |first9=Harold A. |last10=Sierks |first10=Holger |last11=Vincent |first11=Jean-Baptiste |display-authors=5 |date=2 June 2015 |doi=10.1051/0004-6361/201525925|arxiv = 1506.01203 |bibcode = 2015A&A...583A...8F |s2cid=119104807 }}</ref>

==Size==
[[File:17P-Holmes Auvergne 2007 11 02.jpg|left|Comet [[17P/Holmes]], 2007/11/02|thumb]]
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>{{cite web |author-link=David C. Jewitt |last=Jewitt |first=David |date=2007-11-09 |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=Comet Holmes Bigger Than The Sun |publisher=Institute for Astronomy at the University of Hawaii |access-date=2007-11-17}}</ref> The [[Great Comet of 1811]] also had a coma roughly the diameter of the Sun.<ref name="primer">{{cite web
  |title=The Comet Primer
  |publisher=Cometography.com
  |author=Gary W. Kronk
  |url=http://cometography.com/educate/comintro.html
  |access-date=2011-04-05|author-link=Gary W. Kronk
 }}</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&nbsp;[[Astronomical unit|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 [[Comet tail|tail]].<ref name="primer"/>

==X-rays==
[[File:PIA02118.jpg|thumb|150px|Tempel 1 in X-ray light by [[Chandra X-ray Observatory|Chandra]]]]
Comets were found to emit [[X-rays]] in late-March 1996.<ref>
{{cite web
 |title=First X-Rays from a Comet Discovered
 |url=http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html
 |publisher=[[Goddard Spaceflight Center]]
 |access-date=2006-03-05
}}</ref> This surprised researchers, because X-ray emission is usually associated with very [[black-body radiation|high-temperature bodies]]. [[Thomas E. Cravens]] was the first to propose an explanation in early 1997.<ref>{{cite journal
| last = Cravens
| first = T. E.
| date = 1997
| title = Comet Hyakutake x-ray source: Charge transfer of solar wind heavy ions
| journal = Geophysical Research Letters
| volume = 24
| issue = 1
}}</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]] [[photon]]s.<ref>
{{cite web
 |title=Interaction model – Probing space weather with comets
 |url=http://www.kvi.nl/~bodewits
 |archive-url=https://web.archive.org/web/20060213232726/http://www.kvi.nl/~bodewits
 |archive-date=2006-02-13
 |publisher=KVI atomics physics
 |access-date=2009-04-26
}}</ref>

==Observation==
With a basic Earth-surface based telescope and some technique, the size of the coma can be calculated.<ref name=levy>{{cite book|title=David Levy's Guide to Observing and Discovering Comets|author=Levy, D.H.|date=2003|publisher=[[Cambridge University Press]]|isbn=9780521520515|url=https://books.google.com/books?id=2AzBYCYV9ucC&pg=PA127|page=127|access-date=2017-01-08}}</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">{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Ultraviolet_study_reveals_surprises_in_comet_coma|publisher=esa.int|title=Ultraviolet study reveals surprises in comet coma / Rosetta / Space Science / Our Activities / ESA |access-date=2017-01-08}}</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 halo==
[[File:Kohoutek-uv.jpg|thumb|Artificially colored far-ultraviolet image (with film) of Comet Kohoutek (Skylab, 1973)]]
[[OAO-2]] ('Stargazer') discovered large halos of hydrogen gas around comets.<ref name=o2>{{cite web|url=http://www.sal.wisc.edu/~meade/OAO/|publisher=sal.wisc.edu|title=Orbiting Astronomical Observatory OAO-2|access-date=2017-01-08}}</ref> Space probe Giotto detected hydrogen ions at distance of 7.8&nbsp;million km away from Halley when it did a close flyby of the comet in 1986.<ref name=Giotto>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Giotto_overview|publisher=esa.int|title=Giotto overview / Space Science / Our Activities / ESA |access-date=2017-01-08}}</ref> A hydrogen gas halo was detected to be 15 times the diameter of Sun (12.5&nbsp;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">{{cite web|url=http://www.lpi.usra.edu/education/explore/comets/background/|publisher=lpi.usra.edu|title=About Comets|access-date=2017-01-08}}</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">{{cite web|url=http://blogs.esa.int/rosetta/2015/06/02/ultraviolet-study-reveals-surprises-in-comet-coma/|publisher=blogs.esa.int|title=Ultraviolet study reveals surprises in comet coma {{pipe}} Rosetta – ESA's comet chaser|access-date=2017-01-08}}</ref> One of the issues is where the hydrogen is coming from and how (e.g. [[Water splitting]]):

{{blockquote|First, an ultraviolet photon from the Sun hits a water molecule in the comet's coma and ionises it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into two hydrogen atoms and one oxygen, and energising them in the process. These atoms then emit ultraviolet light that is detected at characteristic wavelengths by Alice.<ref name="blogs.esa.int">{{cite web|url=http://blogs.esa.int/rosetta/2015/06/02/ultraviolet-study-reveals-surprises-in-comet-coma/|publisher=blogs.esa.int|title=Ultraviolet study reveals surprises in comet coma {{pipe}} Rosetta – ESA's comet chaser|access-date=2017-01-08}}</ref>}}

A hydrogen gas halo three times the size of the Sun was detected by [[Skylab]] around [[Comet Kohoutek]] in the 1970s.<ref name="nasa">{{cite web|url=https://history.nasa.gov/SP-404/ch4.htm|publisher=history.nasa.gov|title= SP-404 Skylab's Astronomy and Space Sciences Chapter 4 Observations of Comet Kohoutek |date=January 1979 |access-date=2017-01-08 |last1=Lundquist |first1=C. A. }}</ref> SOHO detected a hydrogen gas halo bigger than 1 AU in radius around [[Comet Hale–Bopp]].<ref name="google">{{cite book|title=Great Comets|author=Burnham, R.|date=2000|publisher=Cambridge University Press|isbn=9780521646000|url=https://books.google.com/books?id=HkFKMksEwPcC&pg=PA127|page=127|access-date=2017-01-08}}</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">{{cite web|url=https://ase.tufts.edu/cosmos/view_chapter.asp?id=12&page=5|publisher=ase.tufts.edu|title=NASA's Cosmos|access-date=2017-01-08}}</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"/>

==Composition==
[[File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|right|C/2006 W3 (Christensen) – emitting carbon gas (infrared image)]]
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">{{cite web|url=http://www.cnet.com/news/the-scent-of-a-comet-rotten-eggs-and-pee/|publisher=cnet.com|title=The scent of a comet: Rotten eggs and pee – CNET|access-date=2017-01-08}}</ref>

The four top gases in 67P's halo were water, carbon dioxide, carbon monoxide, and oxygen.<ref name="phys.org">{{cite web|url=http://phys.org/news/2015-10-rosetta-molecular-oxygen-comet-67p.html|publisher=phys.org|title=Rosetta finds molecular oxygen on comet 67P (Update)|access-date=2017-01-08}}</ref> The ratio of oxygen to water coming off the comet remained constant for several months.<ref name="phys.org"/>

==Coma spectrum==
[[File:Ssc2005-18a.jpg|thumb|left|500px|Three coma spectra compared]]
{{Clear}}

==See also==
*[[Coma (optics)]]
*[[Comet nucleus]]
*[[Extraterrestrial atmospheres]]

==References==
{{reflist}}

== External links ==
{{Scholia}}
* [http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT314/HTML/AT31402.HTM  Comet appearance and structure]
* [http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/comets.html NASA - Comets]
* [https://ase.tufts.edu/cosmos/view_chapter.asp?id=12&page=5 NASA - Cosmos - Comets (Chapter 14)]

{{Comets|nonobject=yes}}
{{Atmospheres}}

{{DEFAULTSORT:Coma (Cometary)}}
[[Category:Comets]]