Editing Comet (section)

=== Nucleus ===
[[File:Comet Hartley 2.jpg|thumb|Nucleus of [[103P/Hartley]] as imaged during a [[Deep Impact (spacecraft)|spacecraft flyby]]. The nucleus is about 2&nbsp;km in length.]]
{{Main|Comet nucleus}}
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of [[rock (geology)|rock]], [[Comet dust|dust]], [[ice|water ice]], and frozen [[carbon dioxide]], [[carbon monoxide]], [[methane]], and [[ammonia]].<ref>{{cite journal |bibcode=1998A&A...330..375G |title=Making a comet nucleus |last1=Greenberg |first1=J. Mayo |volume=330 |date=1998 |pages=375 |journal=[[Astronomy & Astrophysics]]}}</ref> As such, they are popularly described as "dirty snowballs" after [[Fred Whipple]]'s model.<ref>{{cite web |url=http://starryskies.com/solar_system/Comet/dirty_snowballs.html |title=Dirty Snowballs in Space |publisher=Starryskies |access-date=15 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130129035627/http://starryskies.com/solar_system/Comet/dirty_snowballs.html |archive-date=29 January 2013}}</ref> Comets with a higher dust content have been called "icy dirtballs".<ref>{{cite news |url=http://www.timeshighereducation.co.uk/news/evidence-from-esas-rosetta-spacecraft-suggests-that-comets-are-more-icy-dirtball-than-dirty-snowball/199168.article |title=Evidence from ESA's Rosetta Spacecraft Suggests that Comets are more "Icy Dirtball" than "Dirty Snowball" |date=21 October 2005 |work=Times Higher Education}}</ref> The term "icy dirtballs" arose after observation of [[Tempel 1|Comet 9P/Tempel 1]] collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "[[Fried ice cream|deep fried ice cream]]", in that their surfaces are formed of dense crystalline ice mixed with [[organic compound]]s, while the interior ice is colder and less dense.<ref name="NASA-20150210" />

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. Nuclei contain a variety of organic compounds, which may include [[methanol]], [[hydrogen cyanide]], [[formaldehyde]], [[ethanol]], [[ethane]], and perhaps more complex molecules such as long-chain [[hydrocarbon]]s and [[amino acid]]s.<ref>{{cite web |last=Meech |first=M. |title=1997 Apparition of Comet Hale–Bopp: What We Can Learn from Bright Comets |url=http://www.psrd.hawaii.edu/Feb97/Bright.html |publisher=Planetary Science Research Discoveries |date=24 March 1997 |access-date=30 April 2013}}</ref><ref>{{cite web |title=Stardust Findings Suggest Comets More Complex Than Thought |url=http://stardust.jpl.nasa.gov/news/news110.html |publisher=NASA |date=14 December 2006 |access-date=31 July 2013}}</ref> In 2009, it was confirmed that the amino acid [[glycine]] had been found in the comet dust recovered by NASA's [[Stardust (spacecraft)|Stardust mission]].<ref>{{cite journal |doi=10.1111/j.1945-5100.2009.tb01224.x |title=Cometary glycine detected in samples returned by Stardust |date=2009 |last1=Elsila |first1=Jamie E. |last2=Glavin |first2=Daniel P. |last3=Dworkin |first3=Jason P. |display-authors=1 |journal=Meteoritics & Planetary Science |volume=44 |issue=9 |pages=1323 |bibcode=2009M&PS...44.1323E|doi-access=free }}</ref> In August 2011, a report, based on [[NASA]] studies of [[meteorite]]s found on Earth, was published suggesting [[DNA]] and [[RNA]] components ([[adenine]], [[guanine]], and related organic molecules) may have been formed on [[asteroid]]s and comets.<ref name="Callahan">{{cite journal |doi=10.1073/pnas.1106493108 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |date=2011 |last1=Callahan |first1=M. P. |last2=Smith |first2=K. E. |last3=Cleaves |first3=H. J. |last4=Ruzicka |first4=J. |last5=Stern |first5=J. C. |last6=Glavin |first6=D. P. |last7=House |first7=C. H. |last8=Dworkin |first8=J. P. |display-authors=1 |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=34 |pages=13995–8 |bibcode=2011PNAS..10813995C |pmid=21836052 |pmc=3161613|doi-access=free }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=NASA |date=8 August 2011 |access-date=31 July 2013 |archive-date=26 April 2020 |archive-url=https://web.archive.org/web/20200426055700/https://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |url-status=dead }}</ref>

The outer surfaces of cometary nuclei have a very low [[albedo]], making them among the least reflective objects found in the Solar System. The [[Giotto (spacecraft)|Giotto]] [[space probe]] found that the nucleus of [[Halley's Comet]] (1P/Halley) reflects about four percent of the light that falls on it,<ref name="dark">{{cite journal |title=The Activity and Size of the Nucleus of Comet Hale-Bopp (C/1995 O1) |journal=Science |last1=Weaver |first1=H. A. |last2=Feldman |first2=P. D. |last3=a'Hearn |first3=M. F. |last4=Arpigny |first4=C. |last5=Brandt |first5=J. C. |last6=Festou |first6=M. C. |last7=Haken |first7=M. |last8=McPhate |first8=J. B. |last9=Stern |first9=S. A. |last10=Tozzi |first10=G. P. |display-authors=1 |volume=275 |issue=5308 |pages=1900–1904 |date=1997 |pmid=9072959 |doi=10.1126/science.275.5308.1900 |bibcode=1997Sci...275.1900W|s2cid=25489175 }}</ref> and [[Deep Space 1]] discovered that [[19P/Borrelly|Comet Borrelly]]'s surface reflects less than 3.0%;<ref name="dark" /> by comparison, [[Bitumen|asphalt]] reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter [[Volatility (chemistry)|volatile]] [[Chemical compound|compounds]], leaving behind larger organic compounds that tend to be very dark, like [[tar]] or [[petroleum|crude oil]]. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their [[outgassing]] processes.<ref>{{cite book |url=https://books.google.com/books?id=PRqVqQKao9QC&pg=PA91 |page=91 |title=Habitability and Cosmic Catastrophes |isbn=978-3-540-76945-3 |last1=Hanslmeier |first1=Arnold |date=2008|publisher=Springer }}</ref>

Comet nuclei with radii of up to {{convert|30|km|mi|sp=us}} have been observed,<ref>{{cite journal |doi=10.1023/A:1021545031431 |title=The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity |date=2000 |last1=Fernández |first1=Yanga R. |journal=Earth, Moon, and Planets |volume=89 |issue=1 |pages=3–25 |bibcode=2002EM&P...89....3F|s2cid=189899565 }}</ref> but ascertaining their exact size is difficult.<ref>{{cite web |url=http://www2.ess.ucla.edu/~jewitt/nucleus.html | first=David | last=Jewitt |title=The Cometary Nucleus |publisher=Department of Earth and Space Sciences, UCLA |date=April 2003 |access-date=31 July 2013}}</ref> The nucleus of [[322P/SOHO]] is probably only {{convert|100|-|200|m|ft|sp=us}} in diameter.<ref name="soho1">{{cite web |title=SOHO's new catch: its first officially periodic comet |publisher=European Space Agency |url=http://www.esa.int/Our_Activities/Space_Science/SOHO_s_new_catch_its_first_officially_periodic_comet |access-date=16 August 2013}}</ref> A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than {{convert|100|m|ft|sp=us}} across.<ref>{{harvnb|Sagan|Druyan|1997|p=137}}</ref> Known comets have been estimated to have an average density of {{convert|0.6|g/cm3|oz/cuin|abbr=on}}.<ref name="Britt2006">{{cite journal |bibcode=2006LPI....37.2214B |title=Small Body Density and Porosity: New Data, New Insights |last1=Britt |first1=D. T. |last2=Consolmagno |first2=G. J. |last3=Merline |first3=W. J. |display-authors=1 |volume=37 |date=2006 |pages=2214 |journal=37th Annual Lunar and Planetary Science Conference |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |access-date=25 August 2013 |archive-url=https://web.archive.org/web/20081217064607/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |archive-date=17 December 2008 |url-status=dead}}</ref> Because of their low mass, comet nuclei do not [[gravitational collapse|become spherical]] under their own [[gravity]] and therefore have irregular shapes.<ref>{{cite web |url=https://history.nasa.gov/SP-467/ch7.htm |title=The Geology of Small Bodies |date=January 1984 |publisher=NASA |access-date=15 August 2013 |last1=Veverka |first1=J. }}</ref>

[[File:Comet wild 2.jpg|thumb|220px|Comet [[81P/Wild]] exhibits jets on light side and dark side, stark relief, and is dry.]]
Roughly six percent of the [[near-Earth asteroid]]s are thought to be the [[Extinct comet|extinct nuclei of comets]] that no longer experience outgassing,<ref name="dormant">{{cite journal |doi=10.1016/j.icarus.2006.02.016 |arxiv=astro-ph/0603106v2 |date=2006 |title=The size–frequency distribution of dormant Jupiter family comets |last1=Whitman |first1=K. |last2=Morbidelli |first2=A. |last3=Jedicke |first3=R. |display-authors=1 |journal=Icarus |volume=183 |issue=1 |pages=101–114 |bibcode=2006Icar..183..101W|s2cid=14026673 }}</ref> including [[14827 Hypnos]] and [[3552 Don Quixote]].

Results from the [[Rosetta (spacecraft)|''Rosetta'']] and [[Philae (spacecraft)|''Philae'']] spacecraft show that the nucleus of [[67P/Churyumov–Gerasimenko]] has no magnetic field, which suggests that magnetism may not have played a role in the early formation of [[planetesimal]]s.<ref name="esa20150414">{{cite news |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_find_comet_not_magnetised |title=Rosetta and Philae Find Comet Not Magnetised |publisher=European Space Agency |first=Markus |last=Bauer |date=14 April 2015 |access-date=14 April 2015}}</ref><ref name="nature20150414">{{cite journal |title=Rosetta's comet has no magnetic field |journal=[[Nature (journal)|Nature]] |first=Quirin |last=Schiermeier |date=14 April 2015 |doi=10.1038/nature.2015.17327|s2cid=123964604 }}</ref> Further, the [[Rosetta (spacecraft)#Instruments|ALICE spectrograph]] on ''Rosetta'' determined that [[electron]]s (within {{convert|1|km|mi|abbr=on}} above the [[comet nucleus]]) produced from [[photoionization]] of water molecules by [[Sunlight|solar radiation]], and not [[photon]]s from the Sun as thought earlier, are responsible for the degradation of water and [[carbon dioxide]] [[molecule]]s released from the comet nucleus into its coma.<ref name="NASA-20150602">{{cite web |last1=Agle |first1=D. C. |last2=Brown |first2=Dwayne |last3=Fohn |first3=Joe |last4=Bauer |first4=Markus |display-authors=1 |title=NASA Instrument on Rosetta Makes Comet Atmosphere Discovery |url=http://www.jpl.nasa.gov/news/news.php?feature=4609 |date=2 June 2015 |publisher=[[NASA]] |access-date=2 June 2015}}</ref><ref name="AA-20150602">{{cite journal |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=1 |title=Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta |url=http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-url=https://web.archive.org/web/20150608071334/http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-date=2015-06-08 |url-status=live |date=2 June 2015 |journal=[[Astronomy & Astrophysics]] |doi=10.1051/0004-6361/201525925 |access-date=3 June 2015 |arxiv=1506.01203 |bibcode=2015A&A...583A...8F |volume=583 |pages=A8|s2cid=119104807 }}</ref> Instruments on the ''Philae'' lander found at least sixteen organic compounds at the comet's surface, four of which ([[acetamide]], [[acetone]], [[methyl isocyanate]] and [[propionaldehyde]]) have been detected for the first time on a comet.<ref name="wapo20150730">{{cite news |url=https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |archive-url=https://web.archive.org/web/20181223235109/https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |url-status=dead |archive-date=23 December 2018 |title=Philae probe finds evidence that comets can be cosmic labs |newspaper=The Washington Post |agency=Associated Press |first=Frank |last=Jordans |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="esa20150730">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Science_on_the_surface_of_a_comet |title=Science on the Surface of a Comet |publisher=European Space Agency |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="SCI-20150731">{{cite journal |last1=Bibring |first1=J.-P. |last2=Taylor |first2=M.G.G.T. |last3=Alexander |first3=C. |last4=Auster |first4=U. |last5=Biele |first5=J. |last6=Finzi |first6=A. Ercoli |last7=Goesmann |first7=F. |last8=Klingehoefer |first8=G. |last9=Kofman |first9=W. |last10=Mottola |first10=S. |last11=Seidenstiker |first11=K.J. |last12=Spohn |first12=T. |last13=Wright |first13=I. |display-authors=1 |title=Philae's First Days on the Comet – Introduction to Special Issue |date=31 July 2015 |journal=[[Science (journal)|Science]] |volume=349 |number=6247 |page=493 |doi=10.1126/science.aac5116 |bibcode=2015Sci...349..493B |pmid=26228139|doi-access=free }}</ref>

{| class="wikitable center" style="text-align: center; width: 530px; margin: 0.1em auto;"
|+Properties of some comets
|-
! width="120" | Name
! width="120" | Dimensions<br />(km)
! width="120" | Density<br />([[Gram|g]]/cm<sup>3</sup>)
! width="120" | Mass<br />([[Kilogram|kg]])<ref name="mass">
Halley: Using the [[Volume#Formulas|volume of an ellipsoid]] of 15×8×8&nbsp;km * a [[rubble pile]] density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass (m=d*v) of 3.02E+14&nbsp;kg.<br />
Tempel 1: Using a spherical diameter of 6.25&nbsp;km; [[Volume#Formulas|volume of a sphere]] * a density of 0.62&nbsp;g/cm<sup>3</sup> yields a mass of 7.9E+13&nbsp;kg.<br />
19P/Borrelly: Using the [[Volume#Formulas|volume of an ellipsoid]] of 8x4x4km * a density of 0.3&nbsp;g/cm<sup>3</sup> yields a mass of 2.0E+13&nbsp;kg.<br />
81P/Wild: Using the [[Volume#Formulas|volume of an ellipsoid]] of 5.5x4.0x3.3&nbsp;km * a density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass of 2.28E+13&nbsp;kg.</ref>
!Refs
|-
| align="left" | [[Halley's Comet]]
| 15 × 8 × 8
| 0.6
| 3{{e|14}}
|<ref>{{cite web |url=https://astrosociety.org/file_download/inline/6f0b9235-b5eb-40e6-88a3-6dd45e3b6f92 |title=What Have We Learned About Halley's Comet? |date=1986 |publisher=Astronomical Society of the Pacific |access-date=4 October 2013}}</ref><ref>{{cite journal |title=Is the nucleus of Comet Halley a low density body? |journal=Nature |last1=Sagdeev |first1=R. Z. |last2=Elyasberg |first2=P. E. |last3=Moroz |first3=V. I. |display-authors=1 |volume=331 |issue=6153 |pages=240 |date=1988 |issn=0028-0836 |doi=10.1038/331240a0 |bibcode=1988Natur.331..240S|s2cid=4335780 }}</ref>
|-
| align="left" | [[Tempel 1]]
| 7.6 × 4.9
| 0.62
| 7.9{{e|13}}
|<ref name="Britt2006" /><ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9P |title=9P/Tempel 1 |publisher=JPL |access-date=16 August 2013}}</ref>
|-
| align="left" | [[19P/Borrelly]]
| 8 × 4 × 4
| 0.3
| 2.0{{e|13}}
|<ref name="Britt2006" />
|-
| align="left" | [[81P/Wild]]
| 5.5 × 4.0 × 3.3
| 0.6
| 2.3{{e|13}}
|<ref name="Britt2006" /><ref name="wild2">{{cite web |title=Comet 81P/Wild 2 |publisher=The Planetary Society |url=http://www.planetary.org/explore/topics/asteroids_and_comets/wild2.html |access-date=20 November 2007 |url-status=dead |archive-url=https://web.archive.org/web/20090106004009/http://planetary.org./explore/topics/asteroids_and_comets/wild2.html |archive-date=6 January 2009 }}</ref>
|-
| align="left" | [[67P/Churyumov–Gerasimenko]]
| 4.1 × 3.3 × 1.8
| 0.47
| 1.0{{e|13}}
|<ref>{{cite web |url=http://www.esa.int/spaceinimages/Images/2015/01/Comet_vital_statistics |title=Comet vital statistics |publisher=European Space Agency |date=22 January 2015 |access-date=24 January 2015}}</ref><ref>{{cite web |url=http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/ |title=Determining the mass of comet 67P/C-G |publisher=European Space Agency |first=Emily |last=Baldwin |date=21 August 2014 |access-date=21 August 2014}}</ref>
|}