The rain-out model is a

model A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure. Models c ...

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 their ...

that describes the first stage of

planetary differentiation 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 p ...

and

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

formation. According to this model, a

planetary body A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...

is assumed to be composed primarily of

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

s and NiFe (i.e. a mixture of

nickel Nickel is a chemical element with 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 slow t ...

and

iron Iron () is a chemical element with symbol Fe (from la, ferrum) 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, right in ...

). If temperatures within this body reach about 1500 K, the minerals and the metals will melt. This will produce an

emulsion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Alth ...

in which globules of liquid NiFe are dispersed in a

magma 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 natura ...

of liquid silicates, the two being

immiscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also appli ...

. 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. According to the rain-out model, core formation was a relatively rapid process, taking a few dozen millennia to reach completion. This occurred at the end of a lengthy process in which the planets were assembled from colliding planetary embryos. Only the collisions of such large embryos could generate enough heat to melt entire bodies. Furthermore, it was only after all of the iron and nickel delivered by impacting bodies had arrived that core formation could proceed to completion. However, this process of core formation was preceded by a long period of partial differentiation, in which some of the nickel and iron within the planetary embryos had begun to separate. The rain-out model can be invoked to explain core formation in all the

terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...

s, given that these consist primarily of silicates, nickel and iron. It can also be adapted to account for core formation in smaller bodies composed of ices and silicates. In such a case, it would be the denser silicates which would rain out to form a rocky core, while the volatile components would form an icy mantle.

References

* Planetary science Scientific models {{geology-stub

See also

References