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In
planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), Astronomical object, celestial bodies (such as Natural satellite, moons, Asteroid, asteroids, Comets on Fire, comets) and planetary systems (in p ...
, planetary differentiation is the process of separating out the different components within a planetary body as a consequence of their physical or chemical behavior (e.g. density and chemical affinities). The process of planetary differentiation is mediated by
partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound wi ...
with heat from radioactive isotope decay and
planetary accretion In astrophysics, accretion is the accumulation of particles into a massive object by gravity, gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxy, galaxies, stars, and pl ...
. Planetary differentiation has occurred on planets,
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s, the asteroid
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
, and
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s (such as the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
).


Physical differentiation


Gravitational separation

High-
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
materials tend to sink through lighter materials. This tendency is affected by the relative structural strengths, but such strength is reduced at temperatures where both materials are plastic or molten.
Iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

Iron
, the most common element that is likely to form a very dense molten metal phase, tends to congregate towards planetary interiors. With it, many siderophile elements (i.e. materials that readily
alloy An alloy is an admixture of metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in ...
with iron) also travel downward. However, not all heavy elements make this transition as some chalcophilic heavy elements bind into low-density silicate and oxide compounds, which differentiate in the opposite direction. The main compositionally differentiated zones in the solid Earth are the very dense iron-rich metallic
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
, the less dense
magnesium-silicate
magnesium-silicate
-rich
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
and the relatively thin, light crust composed mainly of silicates of
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
,
sodium Sodium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

sodium
,
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
and
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
. Even lighter still are the watery liquid
hydrosphere The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
and the gaseous, nitrogen-rich
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
. Lighter materials tend to rise through material with a higher density. A light mineral such as
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical compositio ...
would rise. They may take on dome-shaped forms called
diapir A diapir (; , ) is a type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...
s when doing so. On Earth,
salt dome A salt dome is a type of structural dome formed when salt (or other evaporite An evaporite () is a water-soluble Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a s ...
s are
salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...

salt
diapirs in the crust which rise through surrounding rock. Diapirs of molten low-density silicate rocks such as
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cool ...

granite
are abundant in the Earth's upper crust. The hydrated, low-density
serpentinite Serpentinite is a rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemic ...

serpentinite
formed by alteration of
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
material at
subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate This is a list of tectonic plates on Earth's surface. Tec ...
s can also rise to the surface as diapirs. Other materials do likewise: a low-temperature, near-surface example is provided by mud volcanoes.


Chemical differentiation

Although bulk materials differentiate outward or inward according to their density, the elements that are chemically bound in them fractionate according to their chemical affinities, "carried along" by more abundant materials with which they are associated. For instance, although the rare element
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
is very dense as a pure element, it is chemically more
compatible Compatibility may refer to: Computing * Backward compatibility, in which newer devices can understand data generated by older devices * Compatibility card, an expansion card for hardware emulation of another device * Compatibility layer, compone ...
as a trace element in the Earth's light,
silicate In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
-rich crust than in the dense metallic core.


Heating

When the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
ignited in the
solar nebula The formation and evolution of the Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) ...
,
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
,
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

helium
and other volatile materials were evaporated in the region around it. The
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
and
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is Ab ...

radiation pressure
forced these low-density materials away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres, but themselves remained, to accumulate into
protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimal Planetesimals ...
s. Protoplanets had higher concentrations of
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nucleus is the small, dense region consisting of s and s ...

radioactive
elements early in their history, the quantity of which has reduced over time due to
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

radioactive decay
. For example, the hafnium-tungsten system demonstrates the decay of two unstable isotopes and possibly forms a timeline for accretion. Heating due to radioactivity, impacts, and gravitational pressure melted parts of protoplanets as they grew toward being
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s. In melted zones, it was possible for denser materials to sink towards the center, while lighter materials rose to the surface. The compositions of some
meteorite A meteorite is a solid piece of debris from an object, such as a comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varie ...
s (
achondrite An achondrite is a stony meteorite A meteorite is a solid piece of debris from an object, such as a comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that ...
s) show that differentiation also took place in some
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s (e.g. Vesta), that are parental bodies for meteoroids. The short-lived radioactive isotope
26Al
<sup>26</sup>Al
was probably the main source of heat. When protoplanets accrete more material, the energy of impact causes local heating. In addition to this temporary heating, the gravitational force in a sufficiently large body creates pressures and temperatures which are sufficient to melt some of the materials. This allows
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
s and
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
differences to mix and separate materials, and soft materials to spread out over the surface. Another external heat source is
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an object ...
. On
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, a large piece of molten
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
is sufficiently denser than
continental crust Continental crust is the layer of igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...
material to force its way down through the crust to the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
. In the outer Solar System, a similar process may take place but with lighter materials: they may be
hydrocarbons In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
such as
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
,
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
as liquid or ice, or frozen
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
.


Fractional melting and crystallization

Magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

Magma
in the Earth is produced by
partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound wi ...
of a source rock, ultimately in the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
. The melt extracts a large portion of the "incompatible elements" from its source that are not stable in the major minerals. When magma rises above a certain depth the dissolved minerals start to crystallize at particular pressures and temperatures. The resulting solids remove various elements from the melt, and melt is thus depleted of those elements. Study of trace elements in
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
s thus gives us information about what source melted by how much to produce a magma, and which minerals have been lost from the melt.


Thermal diffusion

When material is unevenly heated, lighter material migrates toward hotter zones and heavier material migrates towards colder areas, which is known as
thermophoresisThermophoresis (also thermomigration, thermodiffusion, the Soret effect, or the Ludwig–Soret effect) is a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperatu ...
, thermomigration, or the Soret effect. This process can affect differentiation in
magma chamber A magma chamber is a large pool of liquid rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemica ...

magma chamber
s. A deeper understanding of this process can be drawn back to a study done on the Hawaiian lava lakes. The drilling of these lakes led to the discovery of crystals formed within magma fronts. The magma containing concentrations of these large crystals or
phenocrysts 300px, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
demonstrated differentiation through the chemical melt of crystals.


Moon's KREEP

On the Moon, a distinctive
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic material has been found that is high in "incompatible elements" such as
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
,
rare earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides, or the lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical element Image:Simple ...
s, and
phosphorus Phosphorus is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
and is often referred to by the abbreviation
KREEP KREEP, an acronym An acronym is a word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed ...

KREEP
. It is also high in
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
and
thorium Thorium is a weakly radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nucleus is the sma ...

thorium
. These elements are excluded from the major minerals of the lunar crust which crystallized out from its primeval
magma oceanMagma oceans exist during periods of Earth's or any planet's accretion when the planet is completely or partly molten. In the early Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the autho ...
, and the KREEP basalt may have been trapped as a chemical differentiate between the crust and the mantle, with occasional eruptions to the surface.


Differentiation through collision

Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
's
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
probably formed out of material splashed into orbit by the impact of a large body into the early Earth.{{Citation, last=Nimmo, first=Francis, title=Early Differentiation and Core Formation, date=2015, url=http://dx.doi.org/10.1002/9781118860359.ch5, work=The Early Earth: Accretion and Differentiation, pages=83–102, place=Hoboken, NJ, publisher=John Wiley & Sons, Inc, access-date=, last2=Kleine, first2=Thorsten Differentiation on Earth had probably already separated many lighter materials toward the surface, so that the impact removed a disproportionate amount of silicate material from Earth, and left the majority of the dense metal behind. The Moon's density is substantially less than that of Earth, due to its lack of a large iron core. On
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, physical and chemical differentiation processes led to a crustal density of approximately 2700 kg/m3 compared to the 3400 kg/m3 density of the compositionally different mantle just below, and the average density of the planet as a whole is 5515 kg/m3.


Core formation mechanisms

Core formation utilizes several mechanisms in order to control the movement of metals into the interior of a planetary body. Examples include
percolation Fig. 2: Percolation in a square lattice (Click to animate) In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ...

percolation
, diking, diapirism, and the direct delivery of impacts are mechanisms involved in this process. The metal to silicate density difference causes percolation or the movement of a metal downward. Diking is a process in which a new rock formation forms within a fracture of a pre-existing rock body. For example, if minerals are cold and brittle, transport can occur through fluid cracks. A sufficient amount of pressure must be met for a metal to successfully travel through the fracture toughness of the surrounding material. The size of the metal intruding and the viscosity of the surrounding material determines the rate of the sinking process. The direct delivery of impacts occurs when an impactor of similar proportions strikes the target planetary body. During the impact, there is an exchange of pre-existing cores containing metallic material. The planetary differentiation event is said to have most likely happened after the accretion process of either the asteroid or a planetary body. Terrestrial bodies and iron meteorites consist of Fe-Ni alloys. The Earth's core is primarily composed Fe-Ni alloys. Based on the studies of short lived
radionuclides A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of proton A proton is a subatomic par ...
, the results suggest that core formation process occurred during an early stage of the solar system. Siderophile elements such as,
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
,
nickel Nickel is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

nickel
, and
cobalt Cobalt is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical element ...

cobalt
can dissolve in molten iron; these elements help the differentiation of iron alloys. The first stages of accretion set up the groundwork for core formation. First, terrestrial planetary bodies enter a neighboring planet's orbit. Next, a collision would take place and the terrestrial body could either grow or shrink. However, in most cases, accretion requires multiple collisions of similar sized objects to have a major difference in the planet's growth. Feeding zones and hit and run events are characteristics that can result after accretion.


See also

* Core-mantle differentiation * Iron catastrophe * Rain-out model


References

Planetary science