Planetary differentiation
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 travel downward. However, not all heavy elements make this transition as some chalcophilic heavy elements bind into low-density silicate and oxide compounds. These compounds differentiate in the opposite direction.
The main compositionally differentiated zones in the solid Earth include the very dense iron-rich metallic core. The less dense magnesium-silicate-rich mantle sits above the core. A relatively thin, light crust composed mainly of silicates of aluminium, sodium, calcium and potassium forms the outermost layer. 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.
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. Elements are 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. This behavior allows specific elements to migrate against gravity based on their bonding preferences.
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 show that differentiation also took place in some asteroids. Vesta serves as an example of a parental body for meteoroids where this process occurred. The short-lived radioactive isotope 26Al was probably the main source of heat driving these early changes.
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 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. 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 sufficient to melt some of the materials. This allows chemical reactions and density differences to mix and separate materials. 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.
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. Melt is thus depleted of those elements.
Study of trace elements in igneous rocks gives us information about what source melted by how much to produce a magma. It also reveals which minerals have been lost from the melt. 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.
On the Moon, a distinctive basaltic material has been found that is high in incompatible elements such as potassium, rare earth elements, and phosphorus. This material is often referred to by the abbreviation KREEP. 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. The KREEP basalt may have been trapped as a chemical differentiate between the crust and the mantle. Occasional eruptions brought it to the surface.
This unique composition serves as a record of early solar system differentiation. The presence of these specific elements helps scientists understand the thermal history of the Moon. The concentration of radioactive isotopes like uranium and thorium provided heat for later geological activity. Such findings allow researchers to reconstruct the conditions present during the formation of planetary bodies in our solar system.
Earth's Moon probably formed out of material splashed into orbit by the impact of a large body into the early Earth. Differentiation on Earth had probably already separated many lighter materials toward the surface. The impact removed a disproportionate amount of silicate material from Earth. It 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, 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. The average density of the planet as a whole is 5515 kg/m3. This stark contrast highlights how collision events can strip away layers and alter the final structure of celestial bodies. The process explains why moons often have lower densities than their parent planets.
Core formation utilizes several mechanisms in order to control the movement of metals into the interior of a planetary body. Examples of mechanisms involved in this process include percolation, diking, diapirism, and the direct delivery of impacts. 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.
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Common questions
What is planetary differentiation and how does it work?
Planetary differentiation is the process where high-density materials sink through lighter materials while low-density materials rise to form distinct layers. This tendency occurs when temperatures are high enough for both materials to become plastic or molten, allowing iron and siderophile elements to congregate toward planetary interiors.
How did heating sources cause differentiation in early solar system bodies?
Heating from radioactivity, impacts, and gravitational pressure melted parts of protoplanets as they grew into planets. The short-lived radioactive isotope 26Al was probably the main source of heat driving these early changes in asteroids like Vesta.
Why does Earth have a higher density than its Moon?
Earth has an average density of 5515 kg/m3 because it retains a large iron core, whereas the Moon formed from material splashed into orbit by a collision that removed most silicate material but left dense metal behind. The Moon's density is substantially less than Earth due to its lack of a large iron core.
What mechanisms allow metals to move into planetary cores?
Core formation utilizes several mechanisms including percolation, diking, diapirism, and direct delivery of impacts. Percolation involves the movement of metal downward driven by the metal-to-silicate density difference, while diking forms new rock within fractures when sufficient pressure overcomes fracture toughness.
How do trace elements help scientists understand magma history?
Study of trace elements in igneous rocks reveals which minerals were lost from the melt and how much source rock melted to produce magma. Research on Hawaiian lava lakes showed that crystals formed within magma fronts demonstrated differentiation through chemical melt of phenocrysts.