Editing Kuiper belt (section)

== Composition ==
[[File:2003 UB313 near-infrared spectrum.png|thumb|upright=1.35|The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines]]

Being distant from the Sun and major planets, Kuiper belt objects are thought to be relatively unaffected by the processes that have shaped and altered other Solar System objects; thus, determining their composition would provide substantial information on the makeup of the earliest Solar System.<ref name="Brown_2012a"/> Due to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine. The principal method by which astronomers determine the composition of a celestial object is [[spectroscopy]]. When an object's light is broken into its component colors, an image akin to a rainbow is formed. This image is called a [[spectrum]]. Different substances absorb light at different wavelengths, and when the spectrum for a specific object is unravelled, dark lines (called [[absorption line]]s) appear where the substances within it have absorbed that particular wavelength of light. Every [[element (chemistry)|element]] or [[compound (chemistry)|compound]] has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine its composition.

Analysis indicates that Kuiper belt objects are composed of a mixture of rock and a variety of ices such as water, [[methane]], and [[ammonia]]. The temperature of the belt is only about 50 [[Kelvin|K]],<ref name="Quaoar">{{cite journal |title=Crystalline water ice on the Kuiper belt object (50000) Quaoar |journal=Nature |volume=432 |issue=7018 |pages=731–3 |author=David C. Jewitt |author2=Jane Luu |name-list-style=amp |url=http://www2.ess.ucla.edu/~jewitt/papers/50000/Quaoar.pdf |date=2004 |access-date=21 June 2007 |archive-url=https://web.archive.org/web/20070621182808/http://www.ifa.hawaii.edu/~jewitt/papers/50000/Quaoar.pdf |archive-date=21 June 2007|bibcode=2004Natur.432..731J |doi=10.1038/nature03111 |pmid=15592406 |s2cid=4334385 }}</ref> so many compounds that would be gaseous closer to the Sun remain solid. The densities and rock–ice fractions are known for only a small number of objects for which the diameters and the masses have been determined. The diameter can be determined by imaging with a high-resolution telescope such as the [[Hubble Space Telescope]], by the timing of an [[occultation]] when an object passes in front of a star or, most commonly, by using the [[albedo]] of an object calculated from its infrared emissions. The masses are determined using the semi-major axes and periods of satellites, which are therefore known only for a few binary objects. The densities range from less than 0.4 to 2.6 g/cm<sup>3</sup>. The least dense objects are thought to be largely composed of ice and have significant porosity. The densest objects are likely composed of rock with a thin crust of ice. There is a trend of low densities for small objects and high densities for the largest objects. One possible explanation for this trend is that ice was lost from the surface layers when differentiated objects collided to form the largest objects.<ref name="Brown_2012a">{{cite journal |last1=Brown |first1=Michael E. |title=The Compositions of Kuiper Belt Objects |journal=Annual Review of Earth and Planetary Sciences |date=2012 |volume=40 |issue=1 |pages=467–494 |doi=10.1146/annurev-earth-042711-105352 |arxiv=1112.2764 |bibcode=2012AREPS..40..467B|s2cid=14936224 }}</ref>

[[File:Artist’s impression of exiled asteroid 2004 EW95.jpg|left|thumb|Artist's impression of plutino and possible former [[C-type asteroid]] {{mpl|120216|2004 EW|95}}<ref>{{cite web|title=Exiled Asteroid Discovered in Outer Reaches of Solar System – ESO telescopes find first confirmed carbon-rich asteroid in Kuiper Belt|url=https://www.eso.org/public/news/eso1814/|website=www.eso.org|access-date=May 12, 2018|archive-date=31 May 2019|archive-url=https://web.archive.org/web/20190531211838/https://www.eso.org/public/news/eso1814/|url-status=live}}</ref>]]

Initially, detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their color.<ref name="KBOKBO">{{cite web |title=Surfaces of Kuiper Belt Objects |author=Dave Jewitt |work=University of Hawaii |url=http://www2.ess.ucla.edu/~jewitt/kb/kb-colors.html |date=2004 |access-date=21 June 2007 |archive-url=https://web.archive.org/web/20070609094911/http://www.ifa.hawaii.edu/~jewitt/kb/kb-colors.html |archive-date=9 June 2007}}</ref> These first data showed a broad range of colors among KBOs, ranging from neutral grey to deep red.<ref name="color">{{cite journal |doi=10.1086/300299 |title=Optical-Infrared Spectral Diversity in the Kuiper Belt |date=1998 |last1=Jewitt |first1=David |last2=Luu |first2=Jane |journal=The Astronomical Journal |volume=115 |issue=4 |pages=1667–1670 |bibcode=1998AJ....115.1667J|s2cid=122564418 |url=http://pdfs.semanticscholar.org/1749/a1869f99bc927872bebce8fac8682feaf3e6.pdf |archive-url=https://web.archive.org/web/20200412143631/http://pdfs.semanticscholar.org/1749/a1869f99bc927872bebce8fac8682feaf3e6.pdf |url-status=dead |archive-date=2020-04-12 }}</ref> This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to [[hydrocarbon]]s.<ref name=color/> This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of the volatile ices from their surfaces to the effects of [[cosmic ray]]s.<ref name=Davies_2001/>{{rp|page=118}} Various solutions were suggested for this discrepancy, including resurfacing by impacts or [[outgassing]].<ref name=KBOKBO/> Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in color was too extreme to be easily explained by random impacts.<ref>{{cite journal |doi=10.1086/323304 |title=Colors and Spectra of Kuiper Belt Objects |date=2001 |last1=Jewitt |first1=David C. |last2=Luu |first2=Jane X. |journal=The Astronomical Journal |volume=122 |issue=4 |pages=2099–2114 |arxiv=astro-ph/0107277 |bibcode=2001AJ....122.2099J|s2cid=35561353 }}</ref> The radiation from the Sun is thought to have chemically altered methane on the surface of KBOs, producing products such as [[tholin]]s. [[Makemake]] has been shown to possess a number of hydrocarbons derived from the radiation-processing of methane, including [[ethane]], [[ethylene]] and [[acetylene]].<ref name="Brown_2012a"/>

Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition.<ref name=Quaoar/> In 1996, Robert H. Brown et al. acquired spectroscopic data on the KBO 1993 SC, which revealed that its surface composition is markedly similar to that of [[Pluto]], as well as Neptune's moon [[Triton (moon)|Triton]], with large amounts of methane ice.<ref name="rbrown">{{cite journal |doi=10.1126/science.276.5314.937 |title=Surface Composition of Kuiper Belt Object 1993SC |date=1997 |last1=Brown |first1=R. H. |journal=Science |volume=276 |issue=5314 |pages=937–9 |pmid=9163038 |last2=Cruikshank |first2=DP |last3=Pendleton |first3=Y |last4=Veeder |first4=GJ |bibcode=1997Sci...276..937B |s2cid=45185392 }}</ref> For the smaller objects, only colors and in some cases the albedos have been determined. These objects largely fall into two classes: gray with low albedos, or very red with higher albedos. The difference in colors and albedos is hypothesized to be due to the retention or the loss of [[hydrogen sulfide]] (H<sub>2</sub>S) on the surface of these objects, with the surfaces of those that formed far enough from the Sun to retain H<sub>2</sub>S being reddened due to irradiation.<ref name="Wong_Brown_2016">{{cite journal|last1=Wong|first1=Ian|last2=Brown|first2=Michael E.|date=2017|title=The bimodal color distribution of small Kuiper Belt objects|arxiv=1702.02615|doi=10.3847/1538-3881/aa60c3|volume=153|issue=4|journal=The Astronomical Journal|page=145|bibcode = 2017AJ....153..145W |s2cid=30811674 |doi-access=free }}</ref>

The largest KBOs, such as Pluto and [[Quaoar]], have surfaces rich in volatile compounds such as methane, [[solid nitrogen|nitrogen]] and [[carbon monoxide]]; the presence of these molecules is likely due to their moderate vapor pressure in the 30–50&nbsp;K temperature range of the Kuiper belt. This allows them to occasionally boil off their surfaces and then fall again as snow, whereas compounds with higher boiling points would remain solid. The relative abundances of these three compounds in the largest KBOs is directly related to their [[surface gravity]] and ambient temperature, which determines which they can retain.<ref name="Brown_2012a"/> Water ice has been detected in several KBOs, including members of the Haumea family such as {{mpl-|19308|1996 TO|66}},<ref>{{cite journal |doi=10.1086/317277 |title=Near-Infrared Spectroscopy of the Bright Kuiper Belt Object 2000 EB173 |date=2000 |last1=Brown |first1=Michael E. |last2=Blake |first2=Geoffrey A. |last3=Kessler |first3=Jacqueline E. |journal=The Astrophysical Journal |volume=543 |issue=2 |pages=L163 |bibcode=2000ApJ...543L.163B|citeseerx=10.1.1.491.4308 |s2cid=122764754 }}</ref> mid-sized objects such as [[38628 Huya]] and [[20000 Varuna]],<ref>{{cite journal |date=2001 |title=NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106 |author1=Licandro |author2=Oliva |author3=Di MArtino |doi=10.1051/0004-6361:20010758 |journal=Astronomy and Astrophysics |volume=373 |issue=3 |pages=L29 |arxiv=astro-ph/0105434 |bibcode=2001A&A...373L..29L|s2cid=15690206 }}</ref> and also on some small objects.<ref name="Brown_2012a"/> The presence of crystalline ice on large and mid-sized objects, including [[50000 Quaoar]] where [[ammonia]] [[hydrate]] has also been detected,<ref name=Quaoar/> may indicate past tectonic activity aided by melting point lowering due to the presence of ammonia.<ref name="Brown_2012a"/>