Planetary differentiation

Last updated November 28, 2023

The layers of the Earth, a differentiated planetary body Earth-crust-cutaway-english.svg — The layers of the Earth, a differentiated planetary body

In planetary science, 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 planets, the asteroid 4 Vesta, and natural satellites (such as the Moon).

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, 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 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, sodium, calcium and potassium. Even lighter still are the watery liquid hydrosphere and the gaseous, nitrogen-rich atmosphere.

Lighter materials tend to rise through material with a higher density. A light mineral such as plagioclase would rise. They may take on dome-shaped forms called diapirs when doing so. On Earth, salt domes are salt diapirs in the crust which rise through surrounding rock. Diapirs of molten low-density silicate rocks such as granite 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 is very dense as a pure element, it is chemically more compatible as a trace element in the Earth's light, silicate-rich crust than in the dense metallic core.^[1]

Heating

When the Sun ignited in the solar nebula, hydrogen, helium and other volatile materials were evaporated in the region around it. The solar wind and radiation pressure forced these low-density materials away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres,^[2] but themselves remained, to accumulate into protoplanets.

Protoplanets had higher concentrations of radioactive elements early in their history, the quantity of which has reduced over time due to 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 planets. 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 meteorites (achondrites) show that differentiation also took place in some asteroids (e.g. Vesta), that are parental bodies for meteoroids. The short-lived radioactive isotope ²⁶Al was probably the main source of heat.^[3]

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 reactions and density differences to mix and separate materials,^[4] and soft materials to spread out over the surface. Another external heat source is tidal heating.

On Earth, a large piece of molten iron is sufficiently denser than continental crust material to force its way down through the crust to the mantle.^[3]

In the outer Solar System, a similar process may take place but with lighter materials: they may be hydrocarbons such as methane, water as liquid or ice, or frozen carbon dioxide.^[5]

Fractional melting and crystallization

Magma 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 rocks 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 chambers. 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 basaltic material has been found that is high in "incompatible elements" such as potassium, rare earth elements, and phosphorus and is often referred to by the abbreviation KREEP.^[6] It is also high in uranium and thorium. 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's Moon probably formed out of material splashed into orbit by the impact of a large body into the early Earth.^[3] 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.^[3] On 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. ^[3] Examples include percolation, diking, diapirism, and the direct delivery of impacts are mechanisms involved in this process.^[3] 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.^[3] 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.^[3] The direct delivery of impacts occurs when an impactor of similar proportions strikes the target planetary body.^[3] During the impact, there is an exchange of pre-existing cores containing metallic material.^[3]

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.^[4] 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.^[4] Siderophile elements such as, sulfur, nickel, and cobalt can dissolve in molten iron; these elements help the differentiation of iron alloys.^[4]

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.^[3] Feeding zones and hit and run events are characteristics that can result after accretion.^[3]

Related Research Articles

Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon was formed from the ejecta of a collision between the early Earth and a Mars-sized planet, approximately 4.5 billion years ago in the Hadean eon. The colliding body is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies.

In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase.

KREEP, an acronym built from the letters K, REE and P, is a geochemical component of some lunar impact breccia and basaltic rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements and the heat-producing elements, namely radioactive uranium, thorium, and potassium.

A planetary core consists of the innermost layers of a planet. Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% to 85% of a planet's radius (Mercury).

Earth's crust is Earth's thick outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

The geology of the Moon is quite different from that of Earth. The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled faster. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.

The iron catastrophe is a postulated major geological event early in the history of Earth, where heavy metals such as iron and nickel congregated in the core during a geologically brief period.

<span class="mw-page-title-main">Fractional crystallization (geology)</span> Process of rock formation

Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within crust and mantle of a rocky planetary body, such as the Earth. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Fractional crystallization is also important in the formation of sedimentary evaporite rocks or simply fractional crystallization is the removal of early formed crystals from an Original homogeneous magma so that the crystals are prevented from further reaction with the residual melt.

The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The Lunar Magma Ocean was likely present on the Moon from the time of the Moon's formation to tens or hundreds of millions of years after that time. It is a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body. As the Moon accreted from the debris from the giant impact, gravitational potential energy was converted to thermal energy. Due to the rapid accretion of the Moon, thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the Lunar Magma Ocean explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material.

Having a mean density of 3,346.4 kg/m³, the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.

The rain-out model is a model of planetary science that describes the first stage of planetary differentiation and core formation. According to this model, a planetary body is assumed to be composed primarily of silicate minerals and NiFe. If temperatures within this body reach about 1500 K, the minerals and the metals will melt. This will produce an emulsion in which globules of liquid NiFe are dispersed in a magma of liquid silicates, the two being immiscible. Because the NiFe globules are denser than the silicates, they will sink under the influence of gravity to the centre of the planetary body—in effect, the globules of metal will "rain out" from the emulsion to the centre, forming a core.

In geochemistry, the primitive mantle is the chemical composition of the Earth's mantle during the developmental stage between core-mantle differentiation and the formation of early continental crust. The chemical composition of the primitive mantle contains characteristics of both the crust and the mantle.

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.

<span class="mw-page-title-main">Origin of the Moon</span> Theories explaining the formation of Earths Moon

The origin of the Moon is usually explained by a Mars-sized body striking the Earth, making a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be formed. Other proposed scenarios include captured body, fission, formed together, planetesimal collisions, and collision theories.

Magma oceans exist during periods of Earth's or any planet's or some natural satellite's accretion when the planet or the natural satellite is completely or partly molten.

The geochemistry of carbon is the study of the transformations involving the element carbon within the systems of the Earth. To a large extent this study is organic geochemistry, but it also includes the very important carbon dioxide. Carbon is transformed by life, and moves between the major phases of the Earth, including the water bodies, atmosphere, and the rocky parts. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respirated back into the atmosphere. However an important part of the carbon cycle involves the trapping of living matter into sediments. The carbon then becomes part of a sedimentary rock when lithification happens. Human technology or natural processes such as weathering, or underground life or water can return the carbon from sedimentary rocks to the atmosphere. From that point it can be transformed in the rock cycle into metamorphic rocks, or melted into igneous rocks. Carbon can return to the surface of the Earth by volcanoes or via uplift in tectonic processes. Carbon is returned to the atmosphere via volcanic gases. Carbon undergoes transformation in the mantle under pressure to diamond and other minerals, and also exists in the Earth's outer core in solution with iron, and may also be present in the inner core.

Core–mantle differentiation is the set of processes that took place during the accretion stage of Earth's evolution that results in the separation of iron-rich materials that eventually would conform a metal core, surrounded by a rocky mantle. According to the Safronov's model, protoplanets formed as the result of collisions of smaller bodies (planetesimals), which previously condensed from solid debris present in the original nebula. Planetesimals contained iron and silicates either already differentiated or mixed together. Either way, after impacting the Proto-Earth their materials very likely became homogenized. At this stage, the Proto-Earth was probably the size of Mars. Next followed the separation and stratification of the Proto-Earth's constituents, chiefly driven by their density contrasts. Factors such as pressure, temperature, and impact bodies in the primordial magma ocean were involved in the differentiation process.

Volcanic and igneous plumbing systems (VIPS) consist of interconnected magma channels and chambers through which magma flows and is stored within Earth's crust. Volcanic plumbing systems can be found in all active tectonic settings, such as mid-oceanic ridges, subduction zones, and mantle plumes, when magmas generated in continental lithosphere, oceanic lithosphere, and in the sub-lithospheric mantle are transported. Magma is first generated by partial melting, followed by segregation and extraction from the source rock to separate the melt from the solid. As magma propagates upwards, a self-organised network of magma channels develops, transporting the melt from lower crust to upper regions. Channelled ascent mechanisms include the formation of dykes and ductile fractures that transport the melt in conduits. For bulk transportation, diapirs carry a large volume of melt and ascent through the crust. When magma stops ascending, or when magma supply stops, magma emplacement occurs. Different mechanisms of emplacement result in different structures, including plutons, sills, laccoliths and lopoliths.

Over the years, our ability to detect, confirm, and characterize exoplanets and their atmospheres has improved, allowing researchers to begin constraining exoplanet interior composition and structure. While most exoplanet science is focused on exoplanetary atmospheric environments, the mass and radius of a planet can tell us about a planet's density, and hence, its internal processes. The internal processes of a planet are partly responsible for its atmosphere, and so they are also a determining factor in a planet's capacity to support life.

References

↑ Hazen, Robert M.; Ewing, Rodney C.; Sverjensky, Dimitri A. (2009). "Evolution of uranium and thorium minerals". American Mineralogist. 94 (10): 1293–1311. Bibcode:2009AmMin..94.1293H. doi:10.2138/am.2009.3208. ISSN 1945-3027. S2CID 4603681.
↑ Ahrens, T J (1993). "Impact Erosion of Terrestrial Planetary Atmospheres". Annual Review of Earth and Planetary Sciences. 21 (1): 525–555. Bibcode:1993AREPS..21..525A. doi:10.1146/annurev.ea.21.050193.002521. hdl: 2060/19920021677 . ISSN 0084-6597.
1 2 3 4 5 6 7 8 9 10 11 12 Nimmo, Francis; Kleine, Thorsten (2015), "Early Differentiation and Core Formation", The Early Earth: Accretion and Differentiation, Geophysical Monograph Series, Hoboken, NJ: John Wiley & Sons, Inc, pp. 83–102, doi:10.1002/9781118860359.ch5, ISBN 9781118860359
1 2 3 4 Sohl, Frank; Breuer, Doris (2014), "Differentiation, Planetary", in Amils, Ricardo; Gargaud, Muriel; Cernicharo Quintanilla, José; Cleaves, Henderson James (eds.), Encyclopedia of Astrobiology, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 1–5, doi:10.1007/978-3-642-27833-4_430-2, ISBN 978-3-642-27833-4 , retrieved 2021-11-08
↑ Prialnik, Dina; Merk, Rainer (2008). "Growth and evolution of small porous icy bodies with an adaptive-grid thermal evolution code: I. Application to Kuiper belt objects and Enceladus". Icarus. 197 (1): 211–220. Bibcode:2008Icar..197..211P. doi:10.1016/j.icarus.2008.03.024. ISSN 0019-1035.
↑ Warren, Paul H.; Wasson, John T. (1979). "The origin of KREEP". Reviews of Geophysics. 17 (1): 73–88. Bibcode:1979RvGSP..17...73W. doi:10.1029/RG017i001p00073. ISSN 1944-9208.

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[1] Hazen, Robert M.; Ewing, Rodney C.; Sverjensky, Dimitri A. (2009). "Evolution of uranium and thorium minerals". American Mineralogist. 94 (10): 1293–1311. Bibcode:2009AmMin..94.1293H. doi:10.2138/am.2009.3208. ISSN 1945-3027. S2CID 4603681.

[2] Ahrens, T J (1993). "Impact Erosion of Terrestrial Planetary Atmospheres". Annual Review of Earth and Planetary Sciences. 21 (1): 525–555. Bibcode:1993AREPS..21..525A. doi:10.1146/annurev.ea.21.050193.002521. hdl: 2060/19920021677 . ISSN 0084-6597.

[:0-3] 1 2 3 4 5 6 7 8 9 10 11 12 Nimmo, Francis; Kleine, Thorsten (2015), "Early Differentiation and Core Formation", The Early Earth: Accretion and Differentiation, Geophysical Monograph Series, Hoboken, NJ: John Wiley & Sons, Inc, pp. 83–102, doi:10.1002/9781118860359.ch5, ISBN 9781118860359

[:1-4] 1 2 3 4 Sohl, Frank; Breuer, Doris (2014), "Differentiation, Planetary", in Amils, Ricardo; Gargaud, Muriel; Cernicharo Quintanilla, José; Cleaves, Henderson James (eds.), Encyclopedia of Astrobiology, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 1–5, doi:10.1007/978-3-642-27833-4_430-2, ISBN 978-3-642-27833-4 , retrieved 2021-11-08

[5] Prialnik, Dina; Merk, Rainer (2008). "Growth and evolution of small porous icy bodies with an adaptive-grid thermal evolution code: I. Application to Kuiper belt objects and Enceladus". Icarus. 197 (1): 211–220. Bibcode:2008Icar..197..211P. doi:10.1016/j.icarus.2008.03.024. ISSN 0019-1035.

[6] Warren, Paul H.; Wasson, John T. (1979). "The origin of KREEP". Reviews of Geophysics. 17 (1): 73–88. Bibcode:1979RvGSP..17...73W. doi:10.1029/RG017i001p00073. ISSN 1944-9208.

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