Coma (comet)

File:17P-Holmes Auvergne 2007 11 02.jpg, 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. About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.{{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}} The Great Comet of 1811 also had a coma roughly the diameter of the Sun.{{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

}} 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. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.

File:PIA02118.jpg]]

Comets were found to emit X-rays in late-March 1996.

{{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

}} 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.{{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

}} 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.

{{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

}}

With a basic Earth-surface based telescope and some technique, the size of the coma can be calculated.{{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}} 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. That time multiplied by the cosine of the comet's declination, times .25, should equal the coma's diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined.

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.{{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}} 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.

File:Kohoutek-uv.jpg

OAO-2 ('Stargazer') discovered large halos of hydrogen gas around comets.{{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}} 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.{{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}} 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.{{cite web|url=http://www.lpi.usra.edu/education/explore/comets/background/|publisher=lpi.usra.edu|title=About Comets|access-date=2017-01-08}} The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft.{{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}} 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.{{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}}}}

A hydrogen gas halo three times the size of the Sun was detected by Skylab around Comet Kohoutek in the 1970s.{{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. }} SOHO detected a hydrogen gas halo bigger than 1 AU in radius around Comet Hale–Bopp.{{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}} Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light.{{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}} The halos have been measured to be ten billion meters across, many times bigger than the Sun. The hydrogen atom are very light so they can travel a long distance before they are themselves ionized by the Sun. When the hydrogen atoms are ionized they are especially swept away by the solar wind.

File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg

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.{{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}}

The four top gases in 67P's halo were water, carbon dioxide, carbon monoxide, and oxygen.{{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}} The ratio of oxygen to water coming off the comet remained constant for several months.

File:Ssc2005-18a.jpg

{{Clear}}

• Coma (optics)
• Comet nucleus
• Extraterrestrial atmospheres

{{reflist}}

{{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)]

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{{Atmospheres}}

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Category:Comets