In
planetary sciencePlanetary science is the scientific study of planets, moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation,...
,
planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, whereby the body evolves into compositionally distinct layers; the
denserThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
materials of a
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
sink to the center, while less dense materials rise to the surface. Such a process tends to create a
coreThe planetary core consists of the innermost layer of a planet.The core may be a solid or a liquid layer, as is Mercury's, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field...
and
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
. Sometimes a chemically distinct
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
forms on top of the mantle.
Heating
When the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
ignited in the
solar nebulaIn cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. It was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at...
,
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
,
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
and other volatile materials were evaporated in the area near the Sun. The
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
and
light pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
forced such low-density material away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres, but themselves remained, to accumulate in
protoplanetProtoplanets are moon-sized planets, or larger embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
s.
Protoplanets had higher concentrations of radioactive elements early in their history; the quantity of these has been reduced over time due to
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s. In melted zones, it was possible for denser materials to sink towards the center, while lighter phases rose to the surface. The compositions of some
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s show that differentiation took place in some asteroids.
When protoplanets
accreteAccretion may refer to:*Accretion , predictable changes in the price of certain securities-Accretion in science:In science, accretion is a process in which the size of something gradually increases by steady addition of smaller parts...
more material, the energy at impact causes local heating. In addition to this temporary heating, the gravitational force in a large enough body create pressures and temperatures which are sufficient to melt some of the materials. This allows
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
s and
densityThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
differences to mix and separate materials, and soft materials to spread out over the surface.
On
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, a large piece of molten
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
is sufficiently denser than
continental crustThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
material that it can force its way down through the crust to the
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
. In the outer solar system similar effects may take place but the materials are made of lighter elements: they may be hydrocarbons such as
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
,
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
as liquid or ice, or frozen
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
.
Chemical differentiation
Note that 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 that they're associated with. For instance, although the rare element
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
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.
Gravitational separation
Materials with a high
densityThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
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 commonest element that is likely to form a very dense molten metal phase, tends to congregate towards planetary interiors. With it, many
siderophileSiderophile means "iron-loving". This can refer to:* Siderophilic bacteria, bacteria that require or are facilitated by free iron* Siderophile elements, chemical elements such as iridium or gold that tend to bond with metallic iron, as described by the Goldschmidt classification* Siderophilia,...
elements (i.e. materials that like to 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
coreThe planetary core consists of the innermost layer of a planet.The core may be a solid or a liquid layer, as is Mercury's, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field...
, the less dense magnesium silicate-rich
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
and the relatively thin, light
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
composed mainly of silicates of aluminium, sodium, calcium and potassium. Even lighter still are the watery liquid
hydrosphereA hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
and the gaseous, nitrogen-rich
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
.
Lighter materials try to rise through material with a higher density. They may take on dome-shaped forms called
diapirA diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks...
s when doing so. On Earth,
salt domeA salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
s are salt diapirs in the crust which rise through surrounding rock. Diapirs of molten low-density silicate rocks such as
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
are well known and abundant in the Earth's upper crust. The hydrated, low-density
serpentiniteSerpentinite is a rock composed of one or more serpentine minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries...
formed by alteration of
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
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 volcanoThe term mud volcano or mud dome is used to refer to formations created by geo-excreted liquids and gases, although there are several different processes which may cause such activity. Temperatures are much cooler than igneous processes...
s.
Moon's KREEP
On the Moon, a distinctive
basaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most...
ic material has been found that is high in "incompatible elements" such as
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
(
periodic tableThe periodiс table of the chemical elements is a tabular display of the chemical elements...
symbol K),
rare earth elementAs defined by IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids...
s, and
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
and is often referred to by the abbreviation
KREEPKREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
. It also is high in
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
and
thoriumThorium is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal. Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust...
. 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.
Fractional melting and crystallization
MagmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
in the Earth is produced by
partial meltingPartial melting occurs when only a portion of a solid is melted. When that solid contains many different crystalline phases, some phases will melt while others will stay solid....
of a source rock, ultimately in the
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
. The melt extracts from its source a large portion of the "incompatible elements" 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 rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
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
The Soret effect is displayed when material is unevenly heated. Lighter material migrates toward hotter zones and heavier material migrates towards colder areas. Also known as Thermomigration and
ThermophoresisThermophoresis, also called thermomigration, thermodiffusion, or Sorét effect, or Ludwig-Soret effect, is a phenomenon observed when a mixture of two or more types of motile particles are subjected to the force of a temperature gradient. The phenomenon is observed at the scale of one millimeter or...
. This can affect differentiation in
magma chamberA magma chamber is a large underground pool of molten rock found beneath the surface of the Earth's crust. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s.
Differentiation through collision
The
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
seems to have been formed out of material splashed into orbit by the impact of a large body into the early Earth. Differentiation on Earth had probably separated many lighter materials toward the surface already, so the impact removed a disproportionate amount of silicate material from the 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.
Density differences on Earth
On
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, physical and chemical differentiation processes led to a crustal density of approximately 2700 kg/m
3 compared to the 3400 kg/m
3 density of the compositionally different mantle just below, and the average density of the planet as a whole is 5515 kg/m
3.