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Coma (comet)
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<p>The coma is the nebulous envelope around the <a href="/facts/Comet_nucleus/dpJGJlqw">nucleus</a> of a <a href="/facts/Comet/w34utQCd">comet</a>, formed when the comet passes near the <a href="/facts/Sun/CadPCVHh">Sun</a> in its <a href="/facts/Highly_elliptical_orbit/93YbggYy">highly elliptical orbit</a>. As the comet warms, parts of it <a href="/facts/Sublimation_(chemistry)/KDoKokMn">sublimate</a>; this gives a comet a diffuse appearance when viewed through <a href="/facts/Telescope/BDwYahhR">telescopes</a> and distinguishes it from <a href="/facts/Star/PFbd6ICx">stars</a>. The word <i>coma</i> comes from the Greek κόμη (<i>kómē</i>), which means "hair" and is the origin of the word <i>comet</i> itself. </p><p>The coma is generally made of <a href="/facts/Volatile_(astrogeology)/yYCe49qX">ice</a> and <a href="/facts/Comet_dust/wE6apPVv">comet dust</a>. Water composes up to 90% of the <a href="/facts/Volatile_(astrogeology)/yYCe49qX">volatiles</a> that outflow from the nucleus when the comet is within 3–4 <a href="/facts/Astronomical_unit/gjsbu5xU">au</a> (280–370 million <a href="/facts/Mile/e22bY569">mi</a>; 450–600 million <a href="/facts/Kilometre/hdQp79QE">km</a>) from the Sun. The <a href="/facts/Water/0lkXnJPK">H2O</a> parent molecule is destroyed primarily through <a href="/facts/Photodissociation/nzRyzh8I">photodissociation</a> and to a much smaller extent <a href="/facts/Photoionization/HIJK2mHo">photoionization</a>. The <a href="/facts/Solar_wind/BvuB3dT8">solar wind</a> plays a minor role in the destruction of water compared to <a href="/facts/Photochemistry/GsXNBWon">photochemistry</a>. Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's <a href="/facts/Comet_tail/RbIDJMyC">tail</a> by <a href="/facts/Radiation_pressure/xA6japY0">light pressure</a>. </p><p>On 11 August 2014, astronomers released studies, using the <a href="/facts/Atacama_Large_Millimeter_Array/1hLdJxsw">Atacama Large Millimeter/Submillimeter Array (ALMA)</a> for the first time, that detailed the distribution of <a href="/facts/Hydrogen_cyanide/vNtA0Mxe">HCN</a>, <a href="/facts/Hydrogen_isocyanide/gfnqSpKo">HNC</a>, <a href="/facts/Formaldehyde/Ck8RLfqk">H2CO</a>, and <a href="/facts/Dust/vaNKP8wk">dust</a> inside the comae of <a href="/facts/Comet/w34utQCd">comets</a> <a href="/facts/C%2f2012_F6_(Lemmon)/f0YMmGEn">C/2012 F6 (Lemmon)</a> and <a href="/facts/Comet_ISON/uUgmKFIT">C/2012 S1 (ISON)</a>. On 2 June 2015, NASA reported that the <a href="/facts/Rosetta_(spacecraft)/ASd7ggs3">ALICE spectrograph</a> on the <a href="/facts/Rosetta_(spacecraft)/ASd7ggs3"><i>Rosetta</i> space probe</a> studying comet <a href="/facts/67P%2fChuryumov%25E2%2580%2593Gerasimenko/FBdA81uy">67P/Churyumov–Gerasimenko</a> determined that <a href="/facts/Electron/L0KBo5cW">electrons</a> (within 1 km (0.62 mi) above the <a href="/facts/Comet_nucleus/dpJGJlqw">comet nucleus</a>) produced from <a href="/facts/Photoionization/HIJK2mHo">photoionization</a> of <a href="/facts/Water/0lkXnJPK">water</a> <a href="/facts/Molecule/N9ljSfFq">molecules</a> by <a href="/facts/Sunlight/WPkJRC7s">solar radiation</a>, and not <a href="/facts/Photon/mFNhAEzA">photons</a> from the Sun as thought earlier, are responsible for the liberation of water and <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> molecules released from the comet nucleus into its coma. </p>