Editing Uranus (section)

== Internal structure ==
[[File:Uranus, Earth size comparison 2.jpg|thumb|Size comparison of Earth and Uranus]]
Uranus's mass is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27&nbsp;g/cm<sup>3</sup> makes Uranus the second least dense planet, after Saturn.<ref name="Seidelmann Archinal A'hearn et al. 2007" /><ref name="Jacobson Campbell et al. 1992" /> This value indicates that it is made primarily of various ices, such as water, ammonia, and methane.<ref name="Podolak Weizman et al. 1995" /> The total mass of ice in Uranus's interior is not precisely known, because different figures emerge depending on the model chosen; it must be between 9.3&nbsp;and 13.5&nbsp;Earth masses.<ref name="Podolak Weizman et al. 1995" /><ref name="Podolak Podolak et al. 2000" /> [[Hydrogen]] and [[helium]] constitute only a small part of the total, with between 0.5 and 1.5&nbsp;Earth masses.<ref name="Podolak Weizman et al. 1995" /> The remainder of the non-ice mass (0.5 to 3.7&nbsp;Earth masses) is accounted for by [[rock (geology)|rocky material]].<ref name="Podolak Weizman et al. 1995" />

The standard model of Uranus's structure is that it consists of three layers: a rocky ([[silicate]]/[[iron–nickel alloy|iron–nickel]]) [[core (geology)|core]] in the centre, an icy [[mantle (geology)|mantle]] in the middle, and an outer gaseous hydrogen/helium envelope.<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> The core is relatively small, with a mass of only 0.55&nbsp;Earth masses and a radius less than 20% of the planet; the mantle comprises its bulk, with around 13.4&nbsp;Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5&nbsp;Earth masses and extending for the last 20% of Uranus's radius.<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> Uranus's core [[density]] is around 9&nbsp;g/cm<sup>3</sup>, with a [[pressure]] in the centre of 8&nbsp;million&nbsp;[[bar (unit)|bars]] (800 [[gigapascal|GPa]]) and a temperature of about 5000&nbsp;[[kelvin|K]].<ref name="Podolak Podolak et al. 2000" /><ref name="Faure2007" /> The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other [[Volatile (astrogeology)|volatiles]].<ref name="Podolak Weizman et al. 1995" /><ref name="Faure2007" /> This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.<ref name="Atreya2006" />

[[File:Uranus diagram.svg|thumb|Diagram of the interior of Uranus, listing each layer's composition|center|450x450px]]

The extreme pressure and temperature deep within Uranus may break up the methane molecules, with the carbon atoms condensing into crystals of [[diamond]] that rain down through the mantle like hailstones.<ref>{{cite web |url=http://www.spacedaily.com/news/carbon-99d.html |title=Is It Raining Diamonds on Uranus |website=Space Daily |date=1 October 1999 |access-date=17 May 2013 |archive-date=22 May 2013 |archive-url=https://web.archive.org/web/20130522005929/http://www.spacedaily.com/news/carbon-99d.html |url-status=live }}</ref><ref>{{Cite journal |title=Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions |journal=Nature Astronomy |last1=Kraus |first1=D. |last2=Vorberger |first2=J. |last3=Pak |first3=A. |last4=Hartley |first4=N. J. |last5=Fletcher |first5=L. B. |last6=Frydrych |first6=S. |last7=Galtier |first7=E. |last8=Gamboa |first8=E. J. |last9=Gericke |first9=D. O. |last10=Glenzer |first10=S. H. |last11=Granados |first11=E. |last12=MacDonald |first12=M. J. |last13=MacKinnon |first13=A. J. |last14=McBride |first14=E. E. |last15=Nam |first15=I. |last16=Neumayer |first16=P. |last17=Roth |first17=M. |last18=Saunders |first18=A. M. |last19=Schuster |first19=A. K. |last20=Sun |first20=P. |last21=van Driel |first21=T. |last22=Döppner |first22=T. |last23=Falcone |first23=R. W. |display-authors=1 |volume=1 |issue=9 |pages=606–611 |date=September 2017 |doi=10.1038/s41550-017-0219-9 |bibcode=2017NatAs...1..606K |s2cid=46945778 |url=https://cloudfront.escholarship.org/dist/prd/content/qt1k79p6pk/qt1k79p6pk.pdf?t=p5nsjk |access-date=23 October 2018 |archive-date=25 July 2018 |archive-url=https://web.archive.org/web/20180725002534/https://cloudfront.escholarship.org/dist/prd/content/qt1k79p6pk/qt1k79p6pk.pdf?t=p5nsjk |url-status=live }}</ref> This phenomenon is similar to diamond rains that are theorised by scientists to exist on [[Jupiter]], [[Saturn]], and [[Neptune]].<ref>{{cite web|url=https://www.businessinsider.com/diamond-rain-saturn-jupiter-2016-4|title=Lightning storms make it rain diamonds on Saturn and Jupiter|last1=Kane|first1=Sean|website=Business Insider|date=29 April 2016|access-date=22 May 2019|archive-date=26 June 2019|archive-url=https://web.archive.org/web/20190626211704/https://www.businessinsider.com/diamond-rain-saturn-jupiter-2016-4|url-status=live}}</ref><ref>{{cite news|url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/|title=It rains solid diamonds on Uranus and Neptune|last1=Kaplan|first1=Sarah|newspaper=The Washington Post|date=25 March 2017|access-date=22 May 2019|archive-date=27 August 2017|archive-url=https://web.archive.org/web/20170827011901/https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/|url-status=live}}</ref> Very-high-pressure experiments at the [[Lawrence Livermore National Laboratory]] suggest that an ocean of metallic liquid carbon, perhaps with floating solid 'diamond-bergs', may comprise the base of the mantle.<ref name="Eggert">{{Cite journal |last1=Eggert |first1=J. H. |last2=Hicks |first2=D. G. |last3=Celliers |first3=P. M. |last4=Bradley |first4=D. K. |last5=McWilliams |first5=R. S. |last6=Jeanloz |first6=R. |last7=Miller |first7=J. E. |last8=Boehly |first8=T. R. |last9=Collins |first9=G. W. |display-authors=etal |date=January 2010 |title=Melting temperature of diamond at ultrahigh pressure |journal=[[Nature Physics]] |language=en |volume=6 |issue=1 |pages=40–43 |bibcode=2010NatPh...6...40E |doi=10.1038/nphys1438 |issn=1745-2473 |doi-access=free}}</ref><ref name="Bland, Eric">{{cite news |last1=Bland |first1=Eric |title=Outer planets may have oceans of diamond |url=http://www.abc.net.au/science/articles/2010/01/18/2794635.htm |access-date=9 October 2017 |work=ABC Science |date=18 January 2010 |language=en-AU |archive-date=15 June 2020 |archive-url=https://web.archive.org/web/20200615182200/http://www.abc.net.au/science/articles/2010/01/18/2794635.htm |url-status=live }}</ref><ref>{{cite journal |url=http://www.astronomynow.com/news/n1001/21diamond/ |title=Oceans of diamond possible on Uranus and Neptune |last1=Baldwin |first1=Emily |journal=Astronomy Now |date=21 January 2010 |access-date=6 February 2014 |url-status=dead |archive-url=https://web.archive.org/web/20131203065217/http://www.astronomynow.com/news/n1001/21diamond/ |archive-date=3 December 2013 }}</ref>

The bulk compositions of Uranus and Neptune are different from those of Jupiter and [[Saturn]], with ice dominating over gases, hence justifying their separate classification as [[ice giant]]s. There may be a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down [[superionic water]] in which the oxygen crystallises but the hydrogen ions move freely within the oxygen lattice.<ref>{{cite news |url=https://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets/ |title=Weird water lurking inside giant planets |work=New Scientist |first=David |last=Shiga |issue=2776 |date=1 September 2010 |access-date=11 February 2018 |archive-date=12 February 2018 |archive-url=https://web.archive.org/web/20180212143537/https://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets/ |url-status=live }}</ref>

Although the model considered above is reasonably standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow a scientific determination of which model is correct.<ref name="Podolak Podolak et al. 2000" /> The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.<ref name="Podolak Weizman et al. 1995" /> For the sake of convenience, a revolving [[oblate spheroid]] set at the point at which atmospheric pressure equals 1&nbsp;bar (100&nbsp;kPa) is conditionally designated as a "surface". It has equatorial and [[Geographical pole|polar]] radii of {{convert|25559|±|4|km|mi|abbr=on}} and {{convert|24973|±|20|km|mi|abbr=on}}, respectively.<ref name="Seidelmann Archinal A'hearn et al. 2007" /> This surface is used throughout this article as a zero point for altitudes.

=== Internal heat ===
Uranus's [[internal heat]] appears markedly lower than that of the other giant planets; in astronomical terms, it has a low [[thermal flux]].<ref name="Sromovsky & Fry 2005" /><ref name="Hanel Conrath et al. 1986" /> Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun,<ref name="Sromovsky & Fry 2005" /> but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the [[far infrared]] (i.e. heat) part of the spectrum is {{val|1.06|0.08}} times the solar energy absorbed in its [[atmosphere]].<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> Uranus's heat flux is only {{val|0.042|0.047|ul=W/m2}}, which is lower than the internal heat flux of Earth of about {{val|0.075|ul=W/m2}}.<ref name="Pearl Conrath et al. 1990" /> The lowest temperature recorded in Uranus's [[tropopause]] is {{convert|49|K|C F}}, making Uranus the coldest planet in the Solar System.<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" />

One of the hypotheses for this discrepancy suggests the Earth-sized impactor theorised to be behind Uranus's axial tilt left the planet with a depleted core temperature, as the impact caused Uranus to expel most of its primordial heat.<ref>{{Cite magazine |last=Hawksett |first=David |date=2005 |title=Ten Mysteries of the Solar System: Why is Uranus So Cold? |magazine=[[Astronomy Now]] |page=73}}</ref> Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface.<ref name="Podolak Weizman et al. 1995" /> For example, [[convection]] may take place in a set of compositionally different layers, which may inhibit upward [[Heat conduction|heat transport]];<ref name="Lunine 1993" /><ref name="Pearl Conrath et al. 1990" /> perhaps [[double diffusive convection]] is a limiting factor.<ref name="Podolak Weizman et al. 1995" />

In a 2021 study, the ice giants' interior conditions were mimicked by compressing water that contained minerals such as [[olivine]] and [[ferropericlase]], thus showing that large amounts of [[magnesium]] could be dissolved in the liquid interiors of Uranus and Neptune. If Uranus has more of this magnesium than Neptune, it could form a [[thermal insulation]] layer, thus potentially explaining the planet's low temperature.<ref>{{cite journal|last1=Taehyun|first1=Kim|display-authors=et al|title=Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets|url=https://www.researchsquare.com/article/rs-78494/v1.pdf?c=1631891793000|journal=[[Nature Astronomy]]|year=2021|volume=5|issue=8|pages=815–821|doi=10.1038/s41550-021-01368-2|bibcode=2021NatAs...5..815K|s2cid=238984160|access-date=20 May 2021|archive-date=26 February 2024|archive-url=https://web.archive.org/web/20240226144821/https://assets.researchsquare.com/files/rs-78494/v1/59e27b87-a2e1-4af2-8d68-877406ccb609.pdf?c=1637596106|url-status=live}}</ref>