Iron is a ubiquitous element with a rich (i.e., complex) chemical behavior. It possesses three oxidation states, metallic iron (Fe0), ferrous iron (Fe2+) and ferric iron (Fe3+). The distribution of these oxidation states is markedly stratified in the Earth.

Planetary objects other than Earth show similar stratification, including Mars, which has a metallic iron-rich core, a ferrous iron-rich mantle, and a ferric iron-rich surface, giving Mars its red color through the presence of nano-crystalline hematite. Many questions pertaining to the establishment and implications of such stratification remain unanswered, such as what were the P...

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