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= Rain-out model =

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The ”’rain-out model”’ is a [[Scientific modelling|model]] of [[planetary science]] that describes the first stage of [[planetary differentiation]] and [[Core (geology)|core]] formation. According to this model, a [[Planet|planetary body]] is assumed to be composed primarily of [[Silicate mineral|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 dozen millennia to reach completion. This occurred at the end of a lengthy process in which the planets were assembled from colliding [[Planetary embryo|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 planets]], 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 [[Volatile (astrogeology)|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.

== History ==

The rain-out model was first presented in 1978 and then again in 1980 in a series of articles by Wayne L. Slattery of the [[Harvard–Smithsonian Center for Astrophysics|Harvard-Smithsonian Center for Astrophysics]] published in ”[[Earth, Moon, and Planets]]” (then titled ”The Moon and the Planets”). In this article, Slattery presented his theoretical reasoning about how protoplanetary conditions paralleled those of Earth’s atmosphere. This claim did not gain much traction at the time and was grouped with other little-known hypotheses of the day.

To this day, the rain-out model has few supporters with Slattery’s original series of articles gaining only 19 citations since publication (as of late 2025).

== Physical mechanism ==

== See also ==

* [[Planetary differentiation]]

* [[Internal structure of Earth]]

* [[Iron catastrophe]]

* [[Core–mantle differentiation|Core-mantle differentiation]]

* [[Earth’s outer core]]