planetary differentiation
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planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of ...
, planetary differentiation is the process by which the chemical elements of a planetary body accumulate in different areas of that body, due to their physical or chemical behavior (e.g. density and chemical affinities). The process of planetary differentiation is mediated by partial melting with heat from radioactive isotope decay and planetary accretion. Planetary differentiation has occurred on planets,
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s, the asteroid
4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...
, and natural satellites (such as the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
).


Physical differentiation


Gravitational separation

High- 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; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
, 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 a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
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, the less dense magnesium-silicate-rich mantle and the relatively thin, light crust composed mainly of silicates of
aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
,
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
,
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
and
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
. Even lighter still are the watery liquid
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the Planetary surface, surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to ch ...
and the gaseous, nitrogen-rich
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
. Lighter materials tend to rise through material with a higher density. A light mineral such as
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
would rise. They may take on dome-shaped forms called diapirs when doing so. On Earth,
salt dome A salt dome is a type of structural dome formed when salt (or other evaporite minerals) intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered us ...
s are
salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
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 coo ...
are abundant in the Earth's upper crust. The hydrated, low-density serpentinite formed by alteration of mantle material at subduction zones 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; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
is very dense as a pure element, it is chemically more compatible as a trace element in the Earth's light,
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
-rich crust than in the dense metallic core.


Heating

When the Sun ignited in the solar nebula,
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
,
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
and other volatile materials were evaporated in the region around it. The
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
and 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 disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitatio ...
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 loses energy by radiation. A material containing unstable nuclei is conside ...
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 conside ...
. 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 a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
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 rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...
s ( achondrites) show that differentiation also took place in some
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s (e.g. Vesta), that are parental bodies for meteoroids. The short-lived radioactive isotope 26Al 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 chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
s and density differences to mix and separate materials, and soft materials to spread out over the surface. Another external heat source is tidal heating. On
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, a large piece of molten
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
is sufficiently denser than
continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...
material to force its way down through the crust to the mantle. In the outer Solar System, a similar process may take place but with lighter materials: they may be hydrocarbons such as
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
,
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
as liquid or ice, or frozen
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
.


Fractional melting and crystallization

Magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
in the Earth is produced by partial melting of a source rock, ultimately in the mantle. 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 ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
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 thermophoresis, thermomigration, or the Soret effect. This process can affect differentiation in
magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...
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 demonstrated differentiation through the chemical melt of crystals.


Lunar KREEP

On the Moon, a distinctive
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic material has been found that is high in "incompatible elements" such as
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
,
rare earth element The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...
s, and
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
and is often referred to by the abbreviation KREEP. It is also high in
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
and
thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...
. These elements are excluded from the major minerals of the lunar crust which crystallized out from its primeval magma ocean, 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
probably formed out of material splashed into orbit by the impact of a large body into the early Earth.{{Citation, last1=Nimmo, first1=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, series=Geophysical Monograph Series , doi=10.1002/9781118860359.ch5 , isbn=9781118860359 , url-access=subscription 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, 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 In physics, chemistry, and materials science, percolation () refers to the movement and filtration, filtering of fluids through porous materials. It is described by Darcy's law. Broader applications have since been developed that cover connecti ...
, 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, the results suggest that core formation process occurred during an early stage of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
. Siderophile elements such as
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
and
cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
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