The Late Veneer and Earth's habitability


Tuesday, January 10 2017 - 12:00 pm, PST
Norm Sleep
Stanford University

Asteroid impacts were a hazard to any life on the Hadean Earth. A traditional approach to geochemical models of the asteroid impactors uses the concentration of highly siderophile elements including the Pt-group in the silicate Earth. These elements occur in roughly chondritic relative ratios, but with absolute concentrations <1% chondrite. This veneer component implies addition of chondrite-like 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. The accretion of a large number of small bodies would provide comparable thicknesses to both bodies, as the effect of gravity is modest.

There are two traditional ways to explain the different veneers on the earth and the Moon. 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. Several impacted, then likely boiled the ocean and exterminated any life. These impacts provided the water and oxidant inventory to the silicate Earth.

Alternatively, most of the terrestrial veneer came from the core of the moon-forming impactor, Theia. The second hypothesis has the implication that that the Moon contains iron from Theia’s core. Mass balances lend plausibility to this theory. The lunar core mass is ~1.6x1021 kg and the excess FeO component in the lunar mantle is 1.3-3.5x1021 kg as Fe, totaling 3-5x1021 kg or a few percent 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 veneer component.

Chemically in this hypothesis, Fe metal from Theia’s core entered the Moon-forming disk. H2O and Fe2O3 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, H2O and Fe2O3, which quantitatively oxidized the admixed Fe from Theia’s core, retaining Pt-group elements. In this hypothesis, asteroid impacts was relatively benign with ~1 terrestrial event that left only thermophile survivors. Tungsten isotopes, sulfur group elements, and halogens may help select the applicable mechanism.

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