Asteroid bombardment and the core of Theia as possible sources for the Earth's late veneer component

Abstract

The silicate Earth contains Pt-group elements in roughly chondritic relative ratios, but with absolute concentrations <1% chondrite. This veneer implies addition of chondritelike material with 0.3-0.7% mass of the Earth's mantle or an equivalent planet-wide thickness of 5-20 km. The veneer thickness, 200-300 m, within the lunar crust and mantle is much less. One hypothesis is that the terrestrial veneer arrived after the moon-forming impact within a few large asteroids that happened to miss the smaller Moon.Alternatively, most of terrestrial veneer came from the core of the moon-forming impactor, Theia. The Moon then likely contains iron from Theia's core. Mass balances lend plausibility. The lunar core mass is ~1.6×10 21 kg and the excess FeO component in the lunar mantle is 1.3-3.5×10 21 kg as Fe, totaling 3-5×10 21 kg or a few percent of Theia's core. This mass is comparable to the excess Fe of 2.3-10×10 21 kg in the Earth's mantle inferred from the veneer component. Chemically in this hypothesis, Fe metal from Theia's core entered the Moon-forming disk. H 2 O and Fe 2 O 3 in the disk oxidized part of the Fe, leaving the lunar mantle near a Fe-FeO buffer. The remaining iron metal condensed, gathered Pt-group elements eventually into the lunar core. The silicate Moon is strongly depleted in Pt-group elements. In contrast, the Earth's mantle contained excess oxidants, H 2 O and Fe 2 O 3 , which quantitatively oxidized the admixed Fe from Theia's

Geological and Geochemical Constraints on the Origin and Evolution of Life

“…Their concentrations, thus, do not bear on the later asteroid flux (Ryder, 2002;Zahnle and Sleep, 2006;Brasser et al, 2016;Sleep, 2016). The predicted impact flux to the Earth is relatively benign.…”

“…The average flux was 2 · 10 14 mol/ year over 600 My. The accreted mass and the flux estimates need to be reduced by a factor of *10 if most of the veneer arrived within the core of Theia rather than accreted asteroids (Sleep, 2016). Further, almost all the extraterrestrial material arrived within large bodies that likely produced a few transient episodes of global reducing conditions.…”

“…In particular, numerous asteroid impacts on the Hadean Earth endangered life on an otherwise-habitable planet. This section utilizes the review articles by Zahnle and Sleep (2006) and Sleep (2016) and focuses on evolutionary bottlenecks from decimation of terrestrial life. I present basic conclusions but do not repeat the equations in these articles.…”

“…The Earth scientist and the biologist have two parallel tasks following previous essays (Sleep andBird, 2007, 2008; Sleep et al, 2011Sleep et al, , 2012: appraisal of whether the proposed venue could reasonably exist on the ancient Earth and conversely assuming that putative molecular biological hypotheses are correct to infer conditions on the early Earth. Given the collective ignorance of ancient environments, these tasks proceed in parallel.…”

Cubic zirconia in >2370 °C impact melt records Earth's hottest crust

“…The immediate, post-impact, thermal pulse far exceeds both ultra-high-temperature metamorphism in tectonic settings (Heisinger and Head, 2006;Korhonen et al, 2014) and the liquidus temperatures of target rocks, and is therefore important for the evolution of planet and asteroid surfaces. The heat associated with impacts had a profound significance on the early Earth, and has been shown to have affected processes in Earth's core (Arkani-Hamed and Ghods, 2011;Monteux et al, 2015;Sleep, 2016), mantle (Watters et al, 2009), crust and atmosphere (Marchi et al, 2016;Marchi et al, 2014;Marchi et al, 2013;Melosh, 2008).…”