Evidence for Collisional Erosion of the Earth

Iron excess of the Earth. The Fe/Mg ratio of the whole Earth is at least 10% higher than the Fe/Mg ratio of the average solar system. Improved estimates of the chemical composition of the primitive upper mantle yield an MgO content of 36.33 ± 0.4 wt% [1], which transforms into a whole Earth Mg content of 14.90 wt%, assuming 32% for the mass of the core and that the core is Mg-free. The FeO content of the Earth’s mantle is well established at 8.10 ± 0.05 %. Adding to this the Earth’s core (85 wt% Fe) gives a whole Earth Fe content of 31.52 wt%. The resulting Fe/Mg ratio is 2.12. whereas the CI-ratio, representative of that of the solar system, is 1.92 ± 0.08 [2]. Thus the Earth has about 12 wt% more Fe than CI-meteorites and the average solar system. (The solar Fe/Mg ratio measured directly in the solar photosphere is 1.87±0.4; and is thus the same as the CI ratio, but its uncertainty is too large for this comparison to be useful). The Fe/Mg ratio of the whole Earth (Fe/Mg)WE can be written as: (Fe/Mg)WE = 2.3(1-Mg#)/Mg#+(Fec/MgPM)*Mc/(1-Mc)