Editing Uranus (section)

=== 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>