Fig. 1.

The iron content of terrestrial bodies of approximate Mars size reflects the mean redox conditions of core formation (oxygen fugacity, fO₂, shown relative to the iron-wüstite [FeO] buffer). A Mars-like planet forming under conditions around 10× more oxidizing than the Earth possesses a mantle in excess of double the FeO content. In contrast, a Mercury-like planet forming under conditions ∼100× more reducing leaves its mantle denuded in FeO (<0.5 Earth) and also, a host of other transition elements (Cr < 0.5 Earth, Ni ∼ 0.1 Earth, Mo ∼ 0.01 Earth). The arrow represents the upper limit of Mercury’s mantle FeO content (14). The model follows ref. 75 and assumes all planets are of Mars mass, with metallic cores fully equilibrating with the mantle at depths of ∼1/3 of the growing mantle; further details are given in SI Appendix.