Editing Planetary core (section)

==Dynamics==

===Dynamo===
[[Dynamo theory]] is a proposed mechanism to explain how celestial bodies like the Earth generate magnetic fields. The presence or lack of a magnetic field can help constrain the dynamics of a planetary core. Refer to [[Earth's magnetic field]] for further details. A dynamo requires a source of thermal and/or compositional buoyancy as a driving force.<ref name="Hauck and Van Orman 2011" /> Thermal buoyancy from a cooling core alone cannot drive the necessary convection as indicated by modelling, thus compositional buoyancy (from [[Phase transition|changes of phase]]) is required. On Earth the buoyancy is derived from [[crystallization]] of the inner core (which can occur as a result of temperature). Examples of compositional buoyancy include precipitation of iron alloys onto the inner core and liquid immiscibility both, which could influence convection both positively and negatively depending on ambient temperatures and pressures associated with the host-body.<ref name="Hauck and Van Orman 2011" /> Other celestial bodies that exhibit magnetic fields are Mercury, Jupiter, Ganymede, and Saturn.<ref name="Pollack, et al. 1977" />

=== Core heat source ===
A planetary core acts as a heat source for the outer layers of a planet. In the Earth, the heat flux over the core mantle boundary is 12 terawatts.<ref name="nimmo 2015">{{Cite book |last=Nimmo |first=F. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780444538024001391 |title=Treatise on geophysics |date=2015 |publisher=[[Elsevier]] |isbn=978-0-444-53803-1 |location=Amsterdam |pages=27–55 |chapter=Energetics of the Core |doi=10.1016/b978-0-444-53802-4.00139-1}}</ref> This value is calculated from a variety of factors: secular cooling, differentiation of light elements, [[Coriolis force]]s, [[radioactive decay]], and [[latent heat]] of crystallization.<ref name="nimmo 2015" /> All planetary bodies have a primordial heat value, or the amount of energy from accretion. Cooling from this initial temperature is called secular cooling, and in the Earth the secular cooling of the core transfers heat into an insulating [[silicate]] mantle.<ref name="nimmo 2015" /> As the inner core grows, the latent heat of crystallization adds to the heat flux into the mantle.<ref name="nimmo 2015" />

===Stability and instability===
Small planetary cores may experience catastrophic energy release associated with phase changes within their cores. Ramsey (1950) found that the total energy released by such a phase change would be on the order of 10<sup>29</sup> joules; equivalent to the total energy release due to [[earthquake]]s through [[geologic time]]. Such an event could explain the [[asteroid belt]]. Such phase changes would only occur at specific mass to volume ratios, and an example of such a phase change would be the rapid formation or dissolution of a solid core component.<ref name="Ramsey 1950">{{cite journal |last=Ramsey |first=W.H. |title=On the Instability of Small Planetary Cores |journal=  Monthly Notices of the Royal Astronomical Society|volume=110 |issue=4 |date=April 1950 |pages=325–338 |doi=10.1093/mnras/110.4.325|bibcode = 1950MNRAS.110..325R |doi-access= free}}</ref>

=== Trends in the Solar System ===

==== Inner rocky planets ====
All of the rocky inner planets, as well as the moon, have an iron-dominant core. Venus and Mars have an additional major element in the core. Venus’ core is believed to be iron-nickel, similarly to Earth. Mars, on the other hand, is believed to have an iron-sulfur core and is separated into an outer liquid layer around an inner solid core.<ref name="stevenson 2001" /> As the orbital radius of a rocky planet increases, the size of the core relative to the total radius of the planet decreases.<ref name="solomon 1979" /> This is believed to be because differentiation of the core is directly related to a body's initial heat, so Mercury's core is relatively large and active.<ref name="solomon 1979" /> Venus and Mars, as well as the moon, do not have magnetic fields. This could be due to a lack of a convecting liquid layer interacting with a solid inner core, as Venus’ core is not layered.<ref name="de pater 2015" /> Although Mars does have a liquid and solid layer, they do not appear to be interacting in the same way that Earth's liquid and solid components interact to produce a dynamo.<ref name="stevenson 2001" />

==== Outer gas and ice giants ====
Current understanding of the outer planets in the solar system, the ice and gas giants, theorizes small cores of rock surrounded by a layer of ice, and in Jupiter and Saturn models suggest a large region of liquid metallic hydrogen and helium.<ref name="de pater 2015" /> The properties of these metallic hydrogen layers is a major area of contention because it is difficult to produce in laboratory settings, due to the high pressures needed.<ref>{{Cite journal|last=Castelvecchi|first=Davide|date=2017-01-26|title=Physicists doubt bold report of metallic hydrogen|journal=Nature|volume=542|issue=7639|pages=17|doi=10.1038/nature.2017.21379|pmid=28150796|issn=0028-0836|bibcode=2017Natur.542...17C|doi-access=free}}</ref> Jupiter and Saturn appear to release a lot more energy than they should be radiating just from the sun, which is attributed to heat released by the hydrogen and helium layer. Uranus does not appear to have a significant heat source, but Neptune has a heat source that is attributed to a “hot” formation.<ref name="de pater 2015" />