Accretion models for the Earth and terrestrial planets are based on the distribution of siderophile (iron-loving) elements between metal and silicate.
* Homogeneous accretion argued that in the primitive solar system, particles were mixed and contained all of the elements of the sun's corona (except for Hydrogen and Helium which, because of their volatility escaped). These particles collected into planets which were homogeneous, i.e. the same in the core as they were on the surface. Later, the planets heated up, melting solid particles, leading to the differentiated structure we see today, with the heaviest material in the core.
* Heterogeneous accretion argued that in the primitive solar system, particles were already differentiated, and that when planets formed, the first particles to join together were the heaviest ones (iron and nickel), with lighter elements join the planet later. Thus, according to this theory, the planets had their layered structures from the start.
http://www.msstate.edu/dept/GeoSciences/CT/TIG/WEBSITES/LOCA...
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