Rain-out model
Encyclopedia
The rain-out model is a model
of planetary science
which 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
(i.e. a mixture of nickel
and iron
). 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.
According to the rain-out model, core formation was a relatively rapid process, taking a few tens of 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
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.
Scientific modelling
Scientific modelling is the process of generating abstract, conceptual, graphical and/or mathematical models. Science offers a growing collection of methods, techniques and theory about all kinds of specialized scientific modelling...
of planetary science
Planetary science
Planetary 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,...
which describes the first stage of planetary differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...
and core formation. According to this model, a planetary body
Planet
A 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,...
is assumed to be composed primarily of silicate minerals and NiFe
NiFe
NiFe or Nife is a general shorthand expression for a mixture of nickel and iron . NiFe is used to describe nickel-iron batteries, various chemical reactions that involve a nickel-iron catalyst or component, and in geology to describe the general composition of the Earth's core.The affinity of...
(i.e. a mixture of nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
). If temperatures within this body reach about 1500 K, the minerals and the metals will melt. This will produce an emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
in which globules of liquid NiFe are dispersed in a magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
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.
According to the rain-out model, core formation was a relatively rapid process, taking a few tens of 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
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 are the inner planets closest to the Sun...
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
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
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.