Planetary core: Difference between revisions
Three references, three different formats. From now on it uses Citation Style 1 horizontal-styled references, with last/first for authors. |
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{{For2|the Earth's core|[[Inner core]], [[Outer core]] and [[Structure of the Earth#Core]]}} |
{{For2|the Earth's core|[[Inner core]], [[Outer core]] and [[Structure of the Earth#Core]]}} |
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Different from a planetary core in the [[Core accretion theory]] where "planetary core" refers to a central accretionary body surrounded by a halo of dust and gas which serves to trap debris and increase the rate of accretion. |
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{{weasel|date=July 2012|article}} |
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[[Image:Terrestial Planets internal en.jpg|thumb|400px|The internal structure of the inner planets.]] |
[[Image:Terrestial Planets internal en.jpg|thumb|400px|The internal structure of the inner planets.]] |
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==Planetary Core== |
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The '''planetary core''' consists of the innermost layer(s) of a [[planet]]. |
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The '''planetary core''' consists of the innermost layer(s) of a [[planet]]. A planetary core may be composed of solid and liquid layers. <ref name="sci.1112328">{{cite journal |last=Solomon |first = S.C. |title = Hot News on Mercury's Core |journal=Science|pages=702-3 |year=2007 |volume=316 |issue = 5825 |doi=10.1126/science.1142328 |pmid=17478710 }} {{subscription required|date=May2012}}</ref> Cores of specific planets may be entirely solid or entirely liquid. <ref name="Williams and Nimmo 2004">{{cite journal |last=Williams |first=Jean-Pierre |last2=Nimmo |first2=Francis |title = Thermal evolution of the Martian core: Implications for an early dynamo |journal = Geology|pages=97-100 |year=2004 |volume=32 |issue=2 }}</ref> In our solar system, core size can range from about 20% (Moon) to 85% of a planet's radius (Mercury). |
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[[Jovian planets]] / [[Gas Giants]] also have cores, though the composition of these cores are still a matter of debate and range in possible composition from traditional stony/iron cors, to icey cores, or to fluid metallic hydrogen.<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref><ref name="Fortney and Hubbard 2003">{{cite journal |last=Fortney |first = Jonathan J. |last2=Hubbard |first2=William B. |title=Phase seperation in giant planets: inhomogeneous evolution of Saturn |journal = Icarus |publisher=Academic Press |volume=164 |year = 2003 |pages=228-243}}</ref><ref name = "Stevenson 1982">{{cite journal |last = Stevenson |first = D. J. |title = Formation of the Giant Planets |journal = Planet. Space Sci |publisher = Pergamon Press Ltd. |volume=30 |issue=8 |year = 1982 |pages =755-764}}</ref> Gas Giant cores are proportionally much smaller than those of terrestrial planets, though their cores can be considerably larger than the Earth nevertheless; Jupiter has a core 10-30 times heavier than Earth<ref name = "Stevenson 1982">{{cite journal |last = Stevenson |first = D. J. |title = Formation of the Giant Planets |journal = Planet. Space Sci |publisher = Pergamon Press Ltd. |volume=30 |issue=8 |year = 1982 |pages =755-764}}</ref>, and exoplanet [HD149026b] has a core 67 times the mass of the Earth.<ref name = "Sato, et al. 2005">{{cite journal |last = Sato |first = Bun'ei |last2 = al. |first2 = et |title = The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core |journal=The Astrophysical Journal |publisher = The American Astronomical Society |volume=633 |year = 2005 |month = November |pages = 465-473.}} </ref> |
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The core may be composed of solid and liquid layers.<ref name="sci.1112328">{{cite journal |last=Solomon |first=S.C. |title=Hot News on Mercury's Core |journal=Science |pages=702–3 |year=2007 |volume=316 |issue=5825 |doi=10.1126/science.1142328 |pmid=17478710 }} {{subscription required|date=May 2012}}</ref> The cores of [[Mars]] and [[Venus]] may be completely solid as they lack an internally generated [[magnetic field]].<ref name="magneto">{{cite encyclopedia |last=Luhmann |first=J. G |last2=Russell |first2=C. T |editor-last=Shirley |editor-first=J. H |editor2-last=Fainbridge |editor2-first=R. W |title=Mars: Magnetic Field and Magnetosphere |encyclopedia=Encyclopedia of Planetary Sciences |pages=454–456 |location=New York |publisher=Chapman and Hall |year=1997 |url=http://www-spc.igpp.ucla.edu/personnel/russell/papers/mars_mag/ }}</ref> In our solar system, core size can range from about 20% (the [[Moon]]) to 85% of a planet's radius ([[Mercury (planet)|Mercury]]). |
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{{TOC limit|limit=2}} |
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{{refimprove|date=November 2012|section}} |
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==Discovery== |
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[[Gas giant]]s also have iron-rich cores.<ref name="arxiv.6309">{{cite journal |url=http://arxiv.org/abs/1111.6309 |title=Rocky core solubility in Jupiter and giant exoplanets |first=Hugh F. |last=Wilson |first2=Burkhard |last2=Militzer |work= |publisher=arXiv |year=2011 }}</ref> Although these cores are proportionately much smaller than those of terrestrial planets, gas giants are so large that their cores can actually be larger than Earth. [[Jupiter]]'s core is thought to be approximately 12 times the mass of Earth (3% of Jupiter's total mass), and the [[extrasolar planet|exoplanet]] [[HD 149026 b]] is thought to have a core approximately 70 times the mass of Earth. |
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In 1798, [[Lord Cavendish]] calculated the average density of the earth to be 5.48 times the density of water (later refined to 5.53), this lead to the accepted belief that the Earth was much denser in its interior.<ref name = "Cavendish 1798">{{cite journal |last = Cavendish |first = H. |title = Experiments to determine the density of Earth |journal=Philosophical Transactions of the Royal Society of London |volume=88 |year = 1798 |pages=469-479}}</ref> Following the discovery of Iron-Meteorites, [[Wiechert]] in 1898 postulated that the Earth had a similar bulk composition to iron meteorites, but the iron had settled to the interior of the Earth, and later represented this by integrating the bulk density of the Earth with the missing Iron and Nickel as a core. <ref name = "Wiechert 1897">{{cite journal |last = Wiechert |first = E. |title = Uber die Massenverteilung im Inneren der Erde |journal = Nachr. K. Ges. Wiss. Goettingen, Math-K.L. |year = 1897 |pages=221-243}}</ref> The first detection of Earth's core occured in 1906 by Richard Dixon Oldham upon discovery of the P-wave shadow zone; the liquid outer core. <ref name = "Oldham 1906">{{cite journal | last = Oldham |first = Richard Dixon |title=The constitution of the interior of the Earth as revealed by Earthquakes |journal=G.T. Geological Society of London |volume = 62 |year = 1906 |pages=459-486}}</ref> By 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core.<ref name = "Transdyne Corporation">{{cite journal |last = Corporation |first = Transdyne |title=Richard D. Oldham's Discovery of the Earth's Core |editor = J. Marvin Hemdon. |publisher = Transdyne Corporation | year = 2009}} |http://nuclearplanet.com/Earth%20Core%20Discovery.html</ref> |
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==Formation== |
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It is thought that some gas giants orbiting very close to their primaries may have their atmospheres stripped away, leaving only their core behind. This as-yet hypothetical class of planets are called "[[Chthonian planet]]s". |
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===Accretion=== |
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Planetary Systems form from a flattened disk of dust and gas which accrete rapidly (within thousands of years) into [[planetesimals]] around 10 km in diameter. From here gravity takes over to produce Moon to Mars sized [[Planetary embryo]]s (10^5 - 10^6 years) and these develop into planetary bodies over an additional 10-100 million years.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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Jupiter and Saturn most likely formed around previously existing rocky and/or icey bodies, rendering these previous primordial planets into gas-giant cores.<ref name = "Stevenson 1982">{{cite journal |last = Stevenson |first = D. J. |title = Formation of the Giant Planets |journal = Planet. Space Sci |publisher = Pergamon Press Ltd. |volume=30 |issue=8 |year = 1982 |pages =755-764}}</ref> This is the [[Planetary core accretion]] model of planet formation. |
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===Differentiation=== |
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Some [[natural satellite|moon]]s, [[asteroid]]s and other [[minor planet]]s may also have well-differentiated cores depending on their size and history. Jupiter's moons [[Io (moon)|Io]] and [[Europa (moon)|Europa]] are in many ways sisters of the terrestrial planets and have very substantial cores comprising about a third of their radii{{Citation needed|date=July 2012}}. The large asteroid [[4 Vesta]] is likewise believed{{Who|date=July 2012}} to have a differentiated structure with a distinct core. |
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[[Planetary differentiation]], is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components. <ref name = "Merriam Webster 2014">{{cite journal |last=Webster |first=Merriam |title=differentiation |year=2014}}http://www.merriam-webster.com/dictionary/differentiation</ref> |
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The [[Hafnium-182]]/[[Tungsten-182]] isotopic system has a half-life of 9 million years, and is approximated as an extinct system after 45 million years. Hafnium is a [[lithophile]] element and Tungsten is [[siderophile]]. Thus if metal segregation (between the Earth's core and mantle) occured in under 45 million years, [[silicate]] [[reservoirs]] develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated [[chondrite]] material.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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The observed Hf/W ratios in iron meteorites contrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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Several factors control segregation of a metal core including the crystallization of [[perovskite]]. Crystalization of perovskite in an early magma ocean is an oxidization process and may drive the production and extraction of iron metal from an original silicate melt. |
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:<math>Fe^{2+}+ O \rightleftharpoons 2*FeAlO_3 + Fe^0</math> |
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===Core Merging/Impacts=== |
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Impacts between planet-sized bodies in the early solar system are important aspects in the formation and growth of planets and planetary cores. |
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====Earth-Moon System==== |
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The [[Giant impact hypothesis]] states that an impact between a theoretical mars-sized planet [[Theia]] (greek goddess-mother of Selene) and the early Earth formed the modern Earth and moon. <ref name = "Halliday and N. 2000">{{cite journal |last = Halliday |last2=N. |first2=Alex |title = Terrrestrial accretion rates and the origin of the Moon |journal = Earth and Planetary Science Letters |publisher = Science |volume = 176 |issue=1|year = 2000|month=February | pages = 17-30}}</ref> During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core. <ref name = "Seti Institute 2012">{{cite journal | last = Institute |first = Seti | title = A new Model for the Origin of the Moon | publisher = Seti Institute | year = 2012}}http://www.seti.org/node/1458</ref> |
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====Mars==== |
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Core mergin between the proto-mars and another differentiated planetoid could have been as fast as a 1000 years or as slow as 300 000 years (depending on viscosity of both cores). <ref name = "Monteaux and Arkani-Hamed 2013">{{cite journal |last=Monteaux |first = Julien |last2 = Arkani-Hamed |first2 = Jafar |title = Consequences of giant impacts in early Mars: Core merging and Martian Dynamo evolution|journal=Journal of Geophysical Research: Planets |publisher = AGU Publications | year=2013 | month=November | pages = 84-87}}</ref> |
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==Chemistry== |
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===Determining Primary Composition=== |
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====Earth==== |
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Using the chondritic reference model and combining known compositions of the [[crust]] and [[mantle]], the unknown component, the composition of the inner and outer core, can be determined; 85% Fe, 5% Ni, 0.9% Cr, 0.25% Co, and all other [[Refractory elements]] at very low concentration.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> This leaves Earth's core with a 5-10% weight deficit for the outer core<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> and a 4-5% weight deficit for the inner core; <ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref>which is attributed to lighter elements that should be cosmically abundant and are iron-soluble; H, O, C, S, P, and Si.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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Earth's core contains half the Earth's [[vanadium]] and [[chromium]], and may contain considerably [[niobium]] and [[tantalum]].<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> Earth's core is depleted in [[germanium]] and [[gallium]].<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> |
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===Weight Deficit Components=== |
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====Earth==== |
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[[Sulphur]] is strongly siderophile and only moderately volatile and depleted in the silicate earth; thus may account for 1.9 weight% of Earth's core.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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By similar argument; [[phosphorous]] may be present up to 0.2 weight%. Hydrogen and carbon however are highly volatile and thus would have been lost during early accretion and therefore can only account for 0.1 to 0.2 weight % respectively.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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Silicon and Oxygen thus make up the remaining mass deficit of Earth's core; though the abundances of each are still a matter of controversy revolving largely around the pressure and oxidation state of Earth's core during its formation.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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No geochemical evidence exists to include any radioactive elements in Earth's core.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref>. Despite this, experimental evidence has found potassium to be strongly siderophile given the temperatures associated with core formation, thus there is potential for potassium in planetary cores of planets, and therefore [[potassium-40]] as well.<ref name = "Murthy, van Westrenen and Fei 2003">{{cite journal |last=Murthy|first=V. Rama|last2=van Westrenen |first2=Wim|last3=Fei|first3=Yingwei|title=Experimental evidence that potassium is a substantial radioactive heat source in planetary cores|journal = letters to nature | publisher= Nature| volume=423 |year = 2003| pages = 163-167}}</ref> |
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===Isotopic Composition=== |
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====Earth==== |
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Hafnium/Tungsten isotopic ratios, when compared with a chondritic reference frame, show a marked enrichment in the silicate earth indicating depletion in Earth's core. Iron meteorites, believed to be resultant from very early core fractionation processes, are also depleted.<ref name = "Wood, Walter and Jonathan 2006">{{cite journal |last=Wood |last2=Walter |last3=Jonathan |first = Bernard J. |first2=Michael J. |first3=Wade |title = Accretion of the Earth and segregation of its core | journal = Nature Reviews | publisher = Nature | volume = 441 | year = 2006 | month = June |pages = 825-833}}</ref> |
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Niobium/Tantalum isotopic ratios, when compared with a chondritic reference frame, show mild depletion in bulk silicate Earth and the moon.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> |
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===Palasite Meteorites=== |
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[[Pallasite meteorites]] formed at the core-mantle boundary of an early planetesimal. |
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==Dynamics== |
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===Dynamo=== |
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[[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. |
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A dynamo requires a source of thermal and/or compositional buoyancy as a driving force.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> |
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Thermal buoyancy from a cooling core alone cannot drive the necessary convection as indicated by modelling, thus compositional buoyancy (from 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">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> |
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Other celestial bodies which exhibit magnetic fields are Mercury, Earth, Jupiter, Ganymede, and Saturn.<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref> |
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===Stability and Instability=== |
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Small planetary cores may experience catastrophic energy release associated wtihp hase changes within their cores. Ramsey, 1950 found that the total energy released by such a phase change would be on the order of 10^29 joules; equivalent to the total energy release due to earthquakes through geologic time. Such an event could explain the asteroid belt. Such phase changes would only occur at specific mass to volume ratious, 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 = Royal Astronomical Society | volume=110 |month = April |year = 1950 |pages = 325-338}}</ref> |
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==Observed Types== |
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The following summarizes known information about the planetary cores of given non-stellar bodies. |
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===Within Our Solar System=== |
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====Mercury==== |
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Mercury has an observed magnetic field which is believed to be generated within its metallic core.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> Mercury's core occupies 85% of the planet's radius, making it the largest core relative to the size of the planet in our solar system; this indicates that much of mercury's surface may have been lost early in the solar system's history.<ref name = "NASA 2012">{{cite journal | last = NASA | year = 2012 |title = MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities | journal = News Releases | publisher = Press Conferences, NASA | city = The Woodlands, Texas | pages = 1-2}}</ref> Mercury has a solid silicate crust and mantle overlying a solid iron sulfide outer core layer, followed by a deeper liquid core layer, and then a possible solid inner core making a third layer.<ref name = "NASA 2012">{{cite journal | last = NASA | year = 2012 |title = MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities | journal = News Releases | publisher = Press Conferences, NASA | city = The Woodlands, Texas | pages = 1-2}}</ref> |
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====Venus==== |
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The composition of Venus' core varies significantly depending on the model used to calculate it, thus constraints are required.<ref name = "Fegley 2003">{{cite journal |last= Fegley |first = B. Jr. |title = Venus |journal = Treatise on Geochemistry |publisher = Elsevier |volume=1 |year = 2003 | pages=487-507}}</ref> |
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{|class="wikitable" |
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|- |
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!Element |
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!Chondritic Model |
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!Equilibrium Condensation Model |
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!Pyrolitic Model |
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|- |
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|Iron |
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|88.6% |
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|94.4% |
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|78.7% |
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|- |
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|Nickel |
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|5.5% |
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|5.6% |
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|6.6% |
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|- |
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|Cobalt |
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|0.26% |
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|Unknown |
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|Unknown |
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|- |
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|Sulfur |
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|5.1% |
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|0% |
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|4.9% |
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|- |
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|Oxygen |
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|0% |
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|Unknown |
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|9.8% |
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|} |
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====Moon==== |
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The existence of a lunar core is still debated, however if it does have a core it would have formed synchronously with the Earth's own core at 45 million years post-start of the solar system based off of Hafnium-Tungsten evidence <ref name = "Munker, et al. 2003">{{cite journal | last = Munker |first = Carsten |last2 = Pfander |first2 = Jorg A |last3 = Weyer |first3 = Stefan | last4 = Buchl |first4 = Anette | last5 = Kleine |first5 = Thorsten |last6 = Mezger |first6 = Klaus |title = Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics | journal = Science Reports | publisher = Science | volume=301 | year = 2003 | month = July | pages = 84-87}}</ref> and the [[Giant impact hypothesis]]. Such a core may have hosted a geomagnetic dynamo early on in its history.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> |
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====Earth==== |
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The Earth has an observed [[magnetic field]] generated within its metallic core.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> The Earth has a 5-10% mass deficit for the entire core and a density deficit from 4-5% for the inner core.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> Fe/Ni value of the core is well constrained by chondritic meteorites.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> Sulfur, carbon, and phosphorous only account for ~2.5% of the light element component/mass deficit.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> No geochemical evidence for including any radioactive elements in the core<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref>. However experimental evidence has found that Potassium is strongly siderophile when dealing with temperatures associated with core-accretion, and thus potassium-40 could have provided an important source of heat contributing to the early Earth's dynamo, though in a lesser extent then on sulphur rich Mars.<ref name = "Murthy, van Westrenen and Fei 2003">{{cite journal |last=Murthy|first=V. Rama|last2=van Westrenen |first2=Wim|last3=Fei|first3=Yingwei|title=Experimental evidence that potassium is a substantial radioactive heat source in planetary cores|journal = letters to nature | publisher= Nature| volume=423 |year = 2003| pages = 163-167}}</ref> The core contains half the Earth's vanadium and chromium, and may contain considerably niobium and tantalum.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> The core is depleted in germanium and gallium.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> Core mantle differentiation occurred within the first 30 million years of Earth's history.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> Inner core crystallization timing is still largely unresolved.<ref name = "McDonough 2003">{{cite journal |last = McDonough |first = W. F. |title=Compositional Model for the Earth's Core |journal=Geochemistry of the Mantle and Core | city = Maryland | year = 2003 | publisher = University of Maryland Geology Department | pages = 547-568}}</ref> |
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====Mars==== |
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Mars possibly hosted a core-generated magnetic field in the past.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> The dynamo ceased within 0.5 billion years of the planet's formation.<ref name="Williams and Nimmo 2004">{{cite journal |last=Williams |first=Jean-Pierre |last2=Nimmo |first2=Francis |title = Thermal evolution of the Martian core: Implications for an early dynamo |journal = Geology|pages=97-100 |year=2004 |volume=32 |issue=2 }}</ref> Hf/W isotopes derived from the martian meteorite [[Zagami]], indicate rapid accretion and core differentiation of Mars; ie under 10 million years.<ref name = "Halliday and N. 2000">{{cite journal |last = Halliday |last2=N. |first2=Alex |title = Terrrestrial accretion rates and the origin of the Moon |journal = Earth and Planetary Science Letters |publisher = Science |volume = 176 |issue=1|year = 2000|month=February | pages = 17-30}}</ref> Potassium-40 could have been a major source of heat powering the early martian dynamo.<ref name = "Murthy, van Westrenen and Fei 2003">{{cite journal |last=Murthy|first=V. Rama|last2=van Westrenen |first2=Wim|last3=Fei|first3=Yingwei|title=Experimental evidence that potassium is a substantial radioactive heat source in planetary cores|journal = letters to nature | publisher= Nature| volume=423 |year = 2003| pages = 163-167}}</ref> |
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Core merging between proto-mars and another differentiated planetoid could have been as fast as a 1000 years or as slow as 300 000 years (depending on the viscosity of both cores and mantles).<ref name = "Monteaux and Arkani-Hamed 2013">{{cite journal |last=Monteaux |first = Julien |last2 = Arkani-Hamed |first2 = Jafar |title = Consequences of giant impacts in early Mars: Core merging and Martian Dynamo evolution |journal=Journal of Geophysical Research: Planets |publisher = AGU Publications | year=2013 | month=November | pages = 84-87}}</ref> |
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Impact-heating of the martian core would have resulted in stratification of the core and kill the martian dynamo for a duration between 150-200 million years.<ref name = "Monteaux and Arkani-Hamed 2013">{{cite journal |last=Monteaux |first = Julien |last2 = Arkani-Hamed |first2 = Jafar |title = Consequences of giant impacts in early Mars: Core merging and Martian Dynamo evolution |journal=Journal of Geophysical Research: Planets |publisher = AGU Publications | year=2013 | month=November | pages = 84-87}}</ref> |
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Modelling done by Williams, et al. 2004 suggests that in order for [[Mars]] to have had a functional dynamo, the Martian core was initially hotter by 150 kelvin than the mantle (agreeing with the differentiation history of the planet, as well as the impact hypothesis), and with a liquid core potassium-40 would have had opportunity to partition into the core providing an additinoal source of heat. The model further concludes that the core of mars is entirely liquid, as the latent heat of crystallization would have driven a longer lasting (greater than 1 billion years) dynamo.<ref name="Williams and Nimmo 2004">{{cite journal |last=Williams |first=Jean-Pierre |last2=Nimmo |first2=Francis |title = Thermal evolution of the Martian core: Implications for an early dynamo |journal = Geology|pages=97-100 |year=2004 |volume=32 |issue=2 }}</ref> |
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If the core of mars is liquid, the lower bound for sulfur would be 5 weight %.<ref name="Williams and Nimmo 2004">{{cite journal |last=Williams |first=Jean-Pierre |last2=Nimmo |first2=Francis |title = Thermal evolution of the Martian core: Implications for an early dynamo |journal = Geology|pages=97-100 |year=2004 |volume=32 |issue=2}}</ref> |
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====Ganymede==== |
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Ganymede has an observed magnetic field generated within its metallic core.<ref name = "Hauck and Van Orman 2011">{{cite journal |last = Hauck | first = S. A. |last2 = Van Orman |first2 = J. A. |title = Core petrology: Implications for the dynamics and evolution of planetary interiors |journal = The Smithosnian/NASA Astrophysics Data System |publisher = American Geophysical Union |year = 2011 |pages = 1-2}}</ref> |
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====Jupiter==== |
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Jupiter has an observed magnetic field generated within its core, indicating some metallic substance is present.<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref> Its magnetic field is the strongest in the solar system after the Sun's. |
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Jupiter has a rock and or ice core ten-thirty times the mass of the earth, and this core is likely soluble in the gas envelope above, and so primordial in composition. Since the core still exists, the outer envelope must have originally accreted onto a previously existing planetary core.<ref name = "Stevenson 1982">{{cite journal |last = Stevenson |first = D. J. |title = Formation of the Giant Planets |journal = Planet. Space Sci |publisher = Pergamon Press Ltd. |volume=30 |issue=8 |year = 1982 |pages =755-764}}</ref> |
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Thermal contraction/evolution models support the presence of [[metallic hydrogen]] within the core in large abundances (greater than Saturn).<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref> |
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====Saturn==== |
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[[Saturn]] has an observed magnetic field generated within its metallic core.<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref>. Metallic hydrogen is present within the core (in lower abundances than Jupiter).<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref> |
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Saturn has a rock and or ice core ten-thirty times the mass of the earth, and this core is likely soluble in the gas envelope above, and therefore it is primordial in composition. Since the core still exists, the envelope must have originally accreted onto previously existing planetary cores.<ref name = "Stevenson 1982">{{cite journal |last = Stevenson |first = D. J. |title = Formation of the Giant Planets |journal = Planet. Space Sci |publisher = Pergamon Press Ltd. |volume=30 |issue=8 |year = 1982 |pages =755-764}}</ref> |
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Thermal contraction/evolution models support the presence of [[metallic hydrogen]] within the core in large abundances (but still less than Jupiter).<ref name="Pollack, et al. 1977">{{cite journal |last = Pollack |first = James B. |last2 = Grossman |first2 = Allen S. |last3 = Moore |first3 = Ronald |last4 = Graboske |first4 = Harold C. Jr. |title = A Calculation of Saturn’s Gravitational Contraction History |journal = Icarus |publisher = Academic Press, Inc |year = 1977 |volume = 30 |pages = 111-128}}</ref> |
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===Extra-Solar=== |
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====Chthonian Planets==== |
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A [[Chthonian planet]] results when a gas giant has its outer atmosphere stripped away by its parent star, likely due to the planet's inward migration. All that's left from the encounter is the original core. See [[Chthonian planet]] for more information. |
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====Planets derived from Stellar cores and Diamond Planets==== |
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[[Diamond planet]]s; previously stars, are formed alongside the formation of a [[millisecond pulsar]]. The first such planet discovered was 18 times the density of water, and five times the size of Earth. Thus the planet cannot be gaseous, and must be composed of heavier elements which are also cosmically abundant like carbon and oxygen; making it likely crystalline like a diamond.<ref name = "National Geographic Society 2011">{{cite journal | last= Society |first= National Geographic | title = "Diamond" Planet Found; May be Stripped Star | journal = National Geographic | publisher = National Geographic | year = 2011 | month = August | day = 25}}http://news.nationalgeographic.com/news/2011/08/110825-new-planet-diamond-pulsar-dwarf-star-space-science/</ref> |
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[[PSR J1719-1438 is a 5.7 millisecond pulsar found to have a companion with a mass similar to Jupiter but a density of 23 grams / cubic centimeter, suggesting that the companion is an ultralow mass carbon white dwarf, likely the core of an ancient star.<ref name = "Bailes, et al. 2011">{{cite journal | last = Bailes | first = M. |last2 = al. |first2 = et |title = Transformation of a Star into a Planet in a Millsecond Pulsar Binary | journal = Science Reports | publisher = Science |volume = 333 | month = September | year = 2011 | pages = 1717-1720}}</ref> |
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====Hot Ice Planets==== |
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Extra Solar Planets with moderate densities (more dense than Jovian planets, but less dense than terrestrial planets) suggests that such planets like [[GJ1214b]] and [[GJ436]] are composed of primarily water. Internal pressures of such water-worlds would result in exotic phases of water forming on the surface and within their cores.<ref name = "MessageToEagle.com 2012">{{cite journal |last=MessageToEagle.com | title = Hot Ice Planets | year = 2012 | month = April | day = 9}}http://www.messagetoeagle.com/hoticeplanets.php</ref> |
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==References== |
==References== |
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{{reflist|2}} |
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[[Category:Planets]] |
[[Category:Planets]] |
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[[Category:Planetary science]] |
[[Category:Planetary science]] |
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[[Category:Chemistry]] |
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[[Category:Geology]] |
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[[Category:Astrophysics]] |
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[[Category:Isotopes]] |
Revision as of 21:35, 13 April 2014
Different from a planetary core in the Core accretion theory where "planetary core" refers to a central accretionary body surrounded by a halo of dust and gas which serves to trap debris and increase the rate of accretion.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Terrestial_Planets_internal_en.jpg/400px-Terrestial_Planets_internal_en.jpg)
Planetary Core
The planetary core consists of the innermost layer(s) of a planet. A planetary core may be composed of solid and liquid layers. [1] Cores of specific planets may be entirely solid or entirely liquid. [2] In our solar system, core size can range from about 20% (Moon) to 85% of a planet's radius (Mercury).
Jovian planets / Gas Giants also have cores, though the composition of these cores are still a matter of debate and range in possible composition from traditional stony/iron cors, to icey cores, or to fluid metallic hydrogen.[3][4][5] Gas Giant cores are proportionally much smaller than those of terrestrial planets, though their cores can be considerably larger than the Earth nevertheless; Jupiter has a core 10-30 times heavier than Earth[5], and exoplanet [HD149026b] has a core 67 times the mass of the Earth.[6]
Discovery
In 1798, Lord Cavendish calculated the average density of the earth to be 5.48 times the density of water (later refined to 5.53), this lead to the accepted belief that the Earth was much denser in its interior.[7] Following the discovery of Iron-Meteorites, Wiechert in 1898 postulated that the Earth had a similar bulk composition to iron meteorites, but the iron had settled to the interior of the Earth, and later represented this by integrating the bulk density of the Earth with the missing Iron and Nickel as a core. [8] The first detection of Earth's core occured in 1906 by Richard Dixon Oldham upon discovery of the P-wave shadow zone; the liquid outer core. [9] By 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core.[10]
Formation
Accretion
Planetary Systems form from a flattened disk of dust and gas which accrete rapidly (within thousands of years) into planetesimals around 10 km in diameter. From here gravity takes over to produce Moon to Mars sized Planetary embryos (10^5 - 10^6 years) and these develop into planetary bodies over an additional 10-100 million years.[11] Jupiter and Saturn most likely formed around previously existing rocky and/or icey bodies, rendering these previous primordial planets into gas-giant cores.[5] This is the Planetary core accretion model of planet formation.
Differentiation
Planetary differentiation, is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components. [12] The Hafnium-182/Tungsten-182 isotopic system has a half-life of 9 million years, and is approximated as an extinct system after 45 million years. Hafnium is a lithophile element and Tungsten is siderophile. Thus if metal segregation (between the Earth's core and mantle) occured in under 45 million years, silicate reservoirs develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated chondrite material.[11] The observed Hf/W ratios in iron meteorites contrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years.[11] Several factors control segregation of a metal core including the crystallization of perovskite. Crystalization of perovskite in an early magma ocean is an oxidization process and may drive the production and extraction of iron metal from an original silicate melt.
Core Merging/Impacts
Impacts between planet-sized bodies in the early solar system are important aspects in the formation and growth of planets and planetary cores.
Earth-Moon System
The Giant impact hypothesis states that an impact between a theoretical mars-sized planet Theia (greek goddess-mother of Selene) and the early Earth formed the modern Earth and moon. [13] During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core. [14]
Mars
Core mergin between the proto-mars and another differentiated planetoid could have been as fast as a 1000 years or as slow as 300 000 years (depending on viscosity of both cores). [15]
Chemistry
Determining Primary Composition
Earth
Using the chondritic reference model and combining known compositions of the crust and mantle, the unknown component, the composition of the inner and outer core, can be determined; 85% Fe, 5% Ni, 0.9% Cr, 0.25% Co, and all other Refractory elements at very low concentration.[11] This leaves Earth's core with a 5-10% weight deficit for the outer core[16] and a 4-5% weight deficit for the inner core; [16]which is attributed to lighter elements that should be cosmically abundant and are iron-soluble; H, O, C, S, P, and Si.[11] Earth's core contains half the Earth's vanadium and chromium, and may contain considerably niobium and tantalum.[16] Earth's core is depleted in germanium and gallium.[16]
Weight Deficit Components
Earth
Sulphur is strongly siderophile and only moderately volatile and depleted in the silicate earth; thus may account for 1.9 weight% of Earth's core.[11] By similar argument; phosphorous may be present up to 0.2 weight%. Hydrogen and carbon however are highly volatile and thus would have been lost during early accretion and therefore can only account for 0.1 to 0.2 weight % respectively.[11] Silicon and Oxygen thus make up the remaining mass deficit of Earth's core; though the abundances of each are still a matter of controversy revolving largely around the pressure and oxidation state of Earth's core during its formation.[11] No geochemical evidence exists to include any radioactive elements in Earth's core.[16]. Despite this, experimental evidence has found potassium to be strongly siderophile given the temperatures associated with core formation, thus there is potential for potassium in planetary cores of planets, and therefore potassium-40 as well.[17]
Isotopic Composition
Earth
Hafnium/Tungsten isotopic ratios, when compared with a chondritic reference frame, show a marked enrichment in the silicate earth indicating depletion in Earth's core. Iron meteorites, believed to be resultant from very early core fractionation processes, are also depleted.[11] Niobium/Tantalum isotopic ratios, when compared with a chondritic reference frame, show mild depletion in bulk silicate Earth and the moon.[18]
Palasite Meteorites
Pallasite meteorites formed at the core-mantle boundary of an early planetesimal.
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.[18] Thermal buoyancy from a cooling core alone cannot drive the necessary convection as indicated by modelling, thus compositional buoyancy (from 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.[18] Other celestial bodies which exhibit magnetic fields are Mercury, Earth, Jupiter, Ganymede, and Saturn.[3]
Stability and Instability
Small planetary cores may experience catastrophic energy release associated wtihp hase changes within their cores. Ramsey, 1950 found that the total energy released by such a phase change would be on the order of 10^29 joules; equivalent to the total energy release due to earthquakes through geologic time. Such an event could explain the asteroid belt. Such phase changes would only occur at specific mass to volume ratious, and an example of such a phase change would be the rapid formation or dissolution of a solid core component.[19]
Observed Types
The following summarizes known information about the planetary cores of given non-stellar bodies.
Within Our Solar System
Mercury
Mercury has an observed magnetic field which is believed to be generated within its metallic core.[18] Mercury's core occupies 85% of the planet's radius, making it the largest core relative to the size of the planet in our solar system; this indicates that much of mercury's surface may have been lost early in the solar system's history.[20] Mercury has a solid silicate crust and mantle overlying a solid iron sulfide outer core layer, followed by a deeper liquid core layer, and then a possible solid inner core making a third layer.[20]
Venus
The composition of Venus' core varies significantly depending on the model used to calculate it, thus constraints are required.[21]
Element | Chondritic Model | Equilibrium Condensation Model | Pyrolitic Model |
---|---|---|---|
Iron | 88.6% | 94.4% | 78.7% |
Nickel | 5.5% | 5.6% | 6.6% |
Cobalt | 0.26% | Unknown | Unknown |
Sulfur | 5.1% | 0% | 4.9% |
Oxygen | 0% | Unknown | 9.8% |
Moon
The existence of a lunar core is still debated, however if it does have a core it would have formed synchronously with the Earth's own core at 45 million years post-start of the solar system based off of Hafnium-Tungsten evidence [22] and the Giant impact hypothesis. Such a core may have hosted a geomagnetic dynamo early on in its history.[18]
Earth
The Earth has an observed magnetic field generated within its metallic core.[18] The Earth has a 5-10% mass deficit for the entire core and a density deficit from 4-5% for the inner core.[16] Fe/Ni value of the core is well constrained by chondritic meteorites.[16] Sulfur, carbon, and phosphorous only account for ~2.5% of the light element component/mass deficit.[16] No geochemical evidence for including any radioactive elements in the core[16]. However experimental evidence has found that Potassium is strongly siderophile when dealing with temperatures associated with core-accretion, and thus potassium-40 could have provided an important source of heat contributing to the early Earth's dynamo, though in a lesser extent then on sulphur rich Mars.[17] The core contains half the Earth's vanadium and chromium, and may contain considerably niobium and tantalum.[16] The core is depleted in germanium and gallium.[16] Core mantle differentiation occurred within the first 30 million years of Earth's history.[16] Inner core crystallization timing is still largely unresolved.[16]
Mars
Mars possibly hosted a core-generated magnetic field in the past.[18] The dynamo ceased within 0.5 billion years of the planet's formation.[2] Hf/W isotopes derived from the martian meteorite Zagami, indicate rapid accretion and core differentiation of Mars; ie under 10 million years.[13] Potassium-40 could have been a major source of heat powering the early martian dynamo.[17] Core merging between proto-mars and another differentiated planetoid could have been as fast as a 1000 years or as slow as 300 000 years (depending on the viscosity of both cores and mantles).[15] Impact-heating of the martian core would have resulted in stratification of the core and kill the martian dynamo for a duration between 150-200 million years.[15] Modelling done by Williams, et al. 2004 suggests that in order for Mars to have had a functional dynamo, the Martian core was initially hotter by 150 kelvin than the mantle (agreeing with the differentiation history of the planet, as well as the impact hypothesis), and with a liquid core potassium-40 would have had opportunity to partition into the core providing an additinoal source of heat. The model further concludes that the core of mars is entirely liquid, as the latent heat of crystallization would have driven a longer lasting (greater than 1 billion years) dynamo.[2] If the core of mars is liquid, the lower bound for sulfur would be 5 weight %.[2]
Ganymede
Ganymede has an observed magnetic field generated within its metallic core.[18]
Jupiter
Jupiter has an observed magnetic field generated within its core, indicating some metallic substance is present.[3] Its magnetic field is the strongest in the solar system after the Sun's. Jupiter has a rock and or ice core ten-thirty times the mass of the earth, and this core is likely soluble in the gas envelope above, and so primordial in composition. Since the core still exists, the outer envelope must have originally accreted onto a previously existing planetary core.[5] Thermal contraction/evolution models support the presence of metallic hydrogen within the core in large abundances (greater than Saturn).[3]
Saturn
Saturn has an observed magnetic field generated within its metallic core.[3]. Metallic hydrogen is present within the core (in lower abundances than Jupiter).[3] Saturn has a rock and or ice core ten-thirty times the mass of the earth, and this core is likely soluble in the gas envelope above, and therefore it is primordial in composition. Since the core still exists, the envelope must have originally accreted onto previously existing planetary cores.[5] Thermal contraction/evolution models support the presence of metallic hydrogen within the core in large abundances (but still less than Jupiter).[3]
Extra-Solar
Chthonian Planets
A Chthonian planet results when a gas giant has its outer atmosphere stripped away by its parent star, likely due to the planet's inward migration. All that's left from the encounter is the original core. See Chthonian planet for more information.
Planets derived from Stellar cores and Diamond Planets
Diamond planets; previously stars, are formed alongside the formation of a millisecond pulsar. The first such planet discovered was 18 times the density of water, and five times the size of Earth. Thus the planet cannot be gaseous, and must be composed of heavier elements which are also cosmically abundant like carbon and oxygen; making it likely crystalline like a diamond.[23] [[PSR J1719-1438 is a 5.7 millisecond pulsar found to have a companion with a mass similar to Jupiter but a density of 23 grams / cubic centimeter, suggesting that the companion is an ultralow mass carbon white dwarf, likely the core of an ancient star.[24]
Hot Ice Planets
Extra Solar Planets with moderate densities (more dense than Jovian planets, but less dense than terrestrial planets) suggests that such planets like GJ1214b and GJ436 are composed of primarily water. Internal pressures of such water-worlds would result in exotic phases of water forming on the surface and within their cores.[25]
References
- ^ Solomon, S.C. (2007). "Hot News on Mercury's Core". Science. 316 (5825): 702–3. doi:10.1126/science.1142328. PMID 17478710. (subscription required)
- ^ a b c d Williams, Jean-Pierre; Nimmo, Francis (2004). "Thermal evolution of the Martian core: Implications for an early dynamo". Geology. 32 (2): 97–100. Cite error: The named reference "Williams and Nimmo 2004" was defined multiple times with different content (see the help page).
- ^ a b c d e f g Pollack, James B.; Grossman, Allen S.; Moore, Ronald; Graboske, Harold C. Jr. (1977). "A Calculation of Saturn's Gravitational Contraction History". Icarus. 30. Academic Press, Inc: 111–128.
- ^ Fortney, Jonathan J.; Hubbard, William B. (2003). "Phase seperation in giant planets: inhomogeneous evolution of Saturn". Icarus. 164. Academic Press: 228–243.
- ^ a b c d e Stevenson, D. J. (1982). "Formation of the Giant Planets". Planet. Space Sci. 30 (8). Pergamon Press Ltd.: 755–764.
- ^ Sato, Bun'ei; al., et (2005). "The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core". The Astrophysical Journal. 633. The American Astronomical Society: 465-473.
{{cite journal}}
: Unknown parameter|month=
ignored (help) - ^ Cavendish, H. (1798). "Experiments to determine the density of Earth". Philosophical Transactions of the Royal Society of London. 88: 469–479.
- ^ Wiechert, E. (1897). "Uber die Massenverteilung im Inneren der Erde". Nachr. K. Ges. Wiss. Goettingen, Math-K.L.: 221–243.
- ^ Oldham, Richard Dixon (1906). "The constitution of the interior of the Earth as revealed by Earthquakes". G.T. Geological Society of London. 62: 459–486.
- ^ Corporation, Transdyne (2009). J. Marvin Hemdon. (ed.). "Richard D. Oldham's Discovery of the Earth's Core". Transdyne Corporation.
{{cite journal}}
: Cite journal requires|journal=
(help) |http://nuclearplanet.com/Earth%20Core%20Discovery.html - ^ a b c d e f g h i Wood, Bernard J.; Walter, Michael J.; Jonathan, Wade (2006). "Accretion of the Earth and segregation of its core". Nature Reviews. 441. Nature: 825–833.
{{cite journal}}
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ignored (help) - ^ Webster, Merriam (2014). "differentiation".
{{cite journal}}
: Cite journal requires|journal=
(help)http://www.merriam-webster.com/dictionary/differentiation - ^ a b Halliday; N., Alex (2000). "Terrrestrial accretion rates and the origin of the Moon". Earth and Planetary Science Letters. 176 (1). Science: 17–30.
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- ^ Munker, Carsten; Pfander, Jorg A; Weyer, Stefan; Buchl, Anette; Kleine, Thorsten; Mezger, Klaus (2003). "Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics". Science Reports. 301. Science: 84–87.
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ignored (help) - ^ Society, National Geographic (2011). ""Diamond" Planet Found; May be Stripped Star". National Geographic. National Geographic.
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ignored (help)http://news.nationalgeographic.com/news/2011/08/110825-new-planet-diamond-pulsar-dwarf-star-space-science/ - ^ Bailes, M.; al., et (2011). "Transformation of a Star into a Planet in a Millsecond Pulsar Binary". Science Reports. 333. Science: 1717–1720.
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ignored (help) - ^ MessageToEagle.com (2012). "Hot Ice Planets".
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ignored (help)http://www.messagetoeagle.com/hoticeplanets.php