Changes

== Chemical composition ==

== Chemical composition ==

[[Image:Yerkeshalley2.jpg|right|thumb|300px|Illustration of how Halley's Comet might have looked to a naked-eye observer standing on the ground of Yerkes observatory in 1910. Moon at left, Halley's at right. Relative sizes are correct and this is the way an extremely bright comet actually looks. Although its appearance suggests motion, comets do not swoop through the sky; their position changes slowly over many hours.]]The ice portion of a comet is mostly water but might include other substances. These include [[ammonia]], [[carbon dioxide]], [[carbon monoxide]], [[methane]], and [[ethane]].<ref name=Brown/><ref name=Yeomans/>

[[Image:Yerkeshalley2.jpg|right|thumb|300px|Illustration of how Halley's Comet might have looked to a naked-eye observer standing on the ground of Yerkes observatory in 1910. Moon at left, Halley's at right. Relative sizes are correct and this is the way an extremely bright comet actually looks. Although its appearance suggests motion, comets do not swoop through the sky; their position changes slowly over many hours.]]The ice portion of a comet is mostly water but might include other substances. These include [[ammonia]], [[carbon dioxide]], [[carbon monoxide]], [[methane]], and [[ethane]].<ref name=Yeomans/><ref name=Brown/>

In 1998 and 1999, Meier ''et al.'' published at least three papers showing that comets are remarkably rich in [[deuterium]] or "heavy" [[hydrogen]]. This included deuterated water (HDO)<ref name=Meier1>Meier, Roland, Owen, Tobias C., Matthews, Henry E., ''et al.'' "[http://www.sciencemag.org/cgi/content/abstract/279/5352/842 A Determination of the HDO/H₂O Ratio in Comet C/1995 O1 (Hale-Bopp)]." ''Science'', 279(5352):842-844, February 6, 1998. {{doi|10.1126/science.279.5352.842}} Accessed July 20, 2008.</ref><ref name=Meier2>Meier, Roland, and Owens, Tobias C. "Cometary Deuterium". ''Space Science Review'', 90(1-2):33-43, 1999. Cited in Niemann HB, Atreya SK, Bauer SJ, ''et al.'' "[http://www-personal.umich.edu/~atreya/Articles/2005_GCMS_nature04122.pdf The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe]." ''Nature'', 438:779-784, 2005. {{doi|10.1038/nature04122}} Accessed July 20, 2008.</ref> and deuterated hydrogen cyanide (DCN)<ref name=Meier3>Meier, Roland, Owen, Tobias C., Jewitt, David C., ''et al.'' "[http://www.sciencemag.org/cgi/content/abstract/279/5357/1707 Deuterium in Comet C/1995 O1 (Hale-Bopp): Detection of DCN]." ''Science'' 279(5357):1707-1710, March 13, 1998. {{doi|10.1126/science.279.5357.1707}} Accessed July 20, 2008.</ref> In fact they have it in twice the concentration of deuterium in the seas of earth and 20 to 100 times the concentration in the rest of the solar system. In 1998 Meier stated flatly that{{cquote|Comets cannot be the only source for the oceans on Earth.}}

In 1998 and 1999, Meier ''et al.'' published at least three papers showing that comets are remarkably rich in [[deuterium]] or "heavy" [[hydrogen]]. This included deuterated water (HDO)<ref name=Meier1>Meier, Roland, Owen, Tobias C., Matthews, Henry E., ''et al.'' "[http://www.sciencemag.org/cgi/content/abstract/279/5352/842 A Determination of the HDO/H₂O Ratio in Comet C/1995 O1 (Hale-Bopp)]." ''Science'', 279(5352):842-844, February 6, 1998. {{doi|10.1126/science.279.5352.842}} Accessed July 20, 2008.</ref><ref name=Meier2>Meier, Roland, and Owens, Tobias C. "Cometary Deuterium". ''Space Science Review'', 90(1-2):33-43, 1999. Cited in Niemann HB, Atreya SK, Bauer SJ, ''et al.'' "[http://www-personal.umich.edu/~atreya/Articles/2005_GCMS_nature04122.pdf The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe]." ''Nature'', 438:779-784, 2005. {{doi|10.1038/nature04122}} Accessed July 20, 2008.</ref> and deuterated hydrogen cyanide (DCN)<ref name=Meier3>Meier, Roland, Owen, Tobias C., Jewitt, David C., ''et al.'' "[http://www.sciencemag.org/cgi/content/abstract/279/5357/1707 Deuterium in Comet C/1995 O1 (Hale-Bopp): Detection of DCN]." ''Science'' 279(5357):1707-1710, March 13, 1998. {{doi|10.1126/science.279.5357.1707}} Accessed July 20, 2008.</ref> In fact they have it in twice the concentration of deuterium in the seas of earth and 20 to 100 times the concentration in the rest of the solar system. In 1998 Meier stated flatly that{{cquote|Comets cannot be the only source for the oceans on Earth.}}

In July of 2004, the Stardust mission approached to within 150 miles of Comet Wild 2 and was able to sample its tail and return the samples to earth (January 2006). The returned dust was crystalline and included organic material, water ice, and many terrestrial minerals. These included [[aluminum]], [[magnesium]], [[calcium]], and [[titanium]].<ref name=Brown/>

In July 2004, the Stardust mission approached to within 150 miles of Comet Wild 2 and was able to sample its tail and return the samples to earth (January 2006). The returned dust was crystalline and included organic material, water ice, and many terrestrial minerals. These included [[aluminum]], [[magnesium]], [[calcium]], and [[titanium]].<ref name=Brown/>

One year later, on July 4, 2005, the Deep Impact mission conducted the first direct analysis of the rocky portion of a comet nucleus, by launching a projectile into Comet Tempel 1. The projectile liberated much material, including silicates, crystalline silicates, minerals that normally form in liquid water ([[calcium carbonate]] forms and clays), an organic material of still undetermined composition, sodium, and a very fine powder.<ref name=Brown/>

One year later, on July 4, 2005, the Deep Impact mission conducted the first direct analysis of the rocky portion of a comet nucleus, by launching a projectile into Comet Tempel 1. The projectile liberated much material, including silicates, crystalline silicates, minerals that normally form in liquid water ([[calcium carbonate]] forms and clays), an organic material of still undetermined composition, sodium, and a very fine powder.<ref name=Brown/>

=== Descriptive table ===

=== Descriptive table ===

The following table, adapted from Brown<ref name=Brown/>, gives orbital characteristics and composition of the 964 comets now known:

The following table, adapted from Brown,<ref name=Brown/> gives orbital characteristics and composition of the 964 comets now known:

{| class="wikitable"

{| class="wikitable"

{{reflist|2}}

{{reflist|2}}

== See Also ==

== See also ==

* [[Halley's Comet]]

* [[Halley's Comet]]

* [[Kuiper Belt]]

* [[Kuiper Belt]]