Core of Theia as source of much of the Earth’s late veneer and implications of a relatively benign Late Heavy Bombardment

Abstract:

The mantle of the Earth contains Pt-group elements along with S, Se, and Te is roughly chondritic relative ratios, but with absolute concentrations of less than 1% chondrite. This property is expected for a mechanical mixture into the mantle which was previous depleted in Pt-group elemnts, but not for equilibrium partition between silicate and molten iron. The well-known late-veneer hypothesis states that this material arrived after the moon-forming impact within bodies with a total mass of 0.3-0.7% of the Earth’s mantle. The ratio of the mass of the lunar veneer component to the Moon’s mass is much less. For this situation to occur, most of the mass of the impactors was concentrated in the few largest bodies that crashed into the large Earth, rather than the small Moon. Several of these bodies were large enough to sterile the Earth by boiling the oceans. Alternatively, much of the terrestrial veneer came from the core of the moon-forming impactor Theia, rather than from later bodies. Recent lunar studies show that the Moon likely contains "excess" iron spewed from Theia’s core. The mass of the lunar core is ~1.6×10²¹ kg. There is also an excess FeO component in the lunar mantle of 1.3-3.5x10²¹ kg as Fe. The total excess (core + mantle) lunar Fe is 3-5x10²¹ kg or ~2% of Theia’s core. This mass is comparable to the excess Fe of 2.3-10x10²¹ kg in the Earth’s mantle inferred from the Pt-group veneer and chondritic Fe/Pt. Most likely Fe metal from Theia’s core entered the Moon-forming disk. Part of the Fe was oxidized by H₂O and Fe₂O₃ in the disk, leaving the lunar mantle near the Fe-FeO buffer. The remaining iron metal condensed, gathered lunar Pt-group elements from the disk, and became the lunar core. The lunar crust and mantle are hence strongly depleted in Pt-group elements, implying a thin late veneer. In contrast, the Earth’s mantle contained excess oxidants. The admixed Fe from Theia’s core was nearly quantitatively oxidized. H₂O and Fe₂O₃ components remained in the mantle with the Pt-group veneer. Solar system wide planetary accretion and late impact scenarios should not be calibrated to provide a thick veneer component to the Earth. For biology, it is conceivable that few or no ocean-boiling impacts frustrated life on the Earth. A nearly sterilzing impact may have left thermophile bottlenecks that root major clades of life.