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Kadlag, Y. & Becker, H. Highly siderophile and chalcogen element constraints on the origin of components of the Allende and Murchison meteorites.

Fractionation of siderophile elements in the Earth’s upper mantle.

Because the Earth pulls siderophile, or iron-loving, elements such as iridium or gold inward to its core, the planet's crust contains relatively low concentrations of these molecules.

Meanwhile, space debris contains more siderophile elements than the Earth's crust, and space rocks such as asteroidstypically contain more of these elements than do comets, which are known as the "dirty snowballs" of the solar system.

However, it has not yet been possible to measure in a lab the solubility of all the highly siderophile elements over the full range of temperatures and pressures of the Earth's mantle, so for now this proposed explanation for the abundance of gold also remains no more than a hypothesis.