Constraining the Theia and proto-Earth in the Moon-forming giant impact

The currently accepted model for the origin of the Moon is that of a giant impact between a proto-Earth and a Mars-sized body, Theia, that produced an initial protolunar disk that later accreted into the Earth and Moon couple [1-5].Here, we employ the smoothed particle hydrodynamic method to investigate how varying Theia’s mass, γ, and its core mass fraction (CMF) affect the giant-impact process and the post-impact compositions of the proto-Earth and the protolunar disk. We fix the total system mass at 1.02 Earth mass and explore impactor mass ratios γ of 0.13, 0.16, and 0.20 total mass, each at an oblique impact angle of ~45°, while varying Theia’s CMF from 10% to 70%.Our dynamical results show that, under the constraint of reproducing the present Earth-Moon angular momentum, Theia’s mass must not exceed ~0.15 Earth mass, as larger impactors produce too much angular momentum. Furthermore, Theia’s CMF strongly controls the post-impact material: an increased CMF yields higher iron concentrations in both the post-impact proto-Earth and the protolunar disk, while simultaneously diminishing the fraction of Theia-derived silicates in each.Using the results of our SPH simulations, we obtain the mass distribution in the resulting protolunar disk, from which we can try to derive some characteristics of Theia and proto-Earth. To translate these findings into geochemical features, we combine elemental (Fe, Si, Mg) and isotopic (Δ17O, ε50Ti, ε54Cr) data for Earth’s and Moon’s mantles with mass-balance modeling. Under both end-member scenarios—complete iron–silicate equilibration and zero equilibration—we find that Theia’s CMF must be