Evidence for late meteorite bombardment on Earth

The Earth (and the other terrestrial planets) formed via the accretional build-up of planetesimals and larger Moon-sizes protoplanets. The heat from this process melted the proto-Earth, resulting in a magma ocean that allowed the heavy elements to sink towards the centre and the lighter elements to float towards the surface. As a results of this planet-wide differentiation, the interior of the Earth contains an iron-rich core and a silicate-rich mantle.  This differentiation should also have caused the siderophiles (which are “iron-loving” metals, like gold, copper, platinum and osmium) to be depleted from the crust and mantle and be locked up in the Earth’s iron-rich core. However, the mantle is surprisingly abundant in siderophiles.  One hypothesis to explain this is the “late veneer” theory, whereby primitive meteorites were accreted by the Earth and added to the mantle after the core had formed.  This period of smaller impacts is thought to have lasted about 500 million years, ending 4-3.8 billion years ago.

To test the late veneer hypothesis, researcher need to determine the composition of the Earth’s mantle before and after the bombardment period.  Using some of the oldest rocks in Greenland, Willbold and collaborators have made extremely sensitive measurements of the tungsten (W) isotope ratio.  182-W results from the radioactive decay of the now-extinct hafnium isotope 182-Hf.  During the formation of the Earth’s core, the siderophile tungsten would have sunk to the core, while the non-siderophile hafnium would have stayed in the mantle. After core formation, the 182-Hf in the mantle would have continued to decay, resulting in an enrichment of 182-W in the mantle.  The half-life of 182-Hf is 8.9 million years, so after about 50 millions years there would be no more 182-Hf left in the Earth’s mantle and hence from that point onwards the amount of 182-W in the Earth’s mantle would remain fixed.  Any changes in the tungsten isotope ratio after this time must have come from other material added to the mantle.  In this week’s edition of the journal Nature, Willbold et al. present results from ultra high-precision analysis of tungsten isotopes in a sample of 3.8 billion year old rocks from  Greenland which support the late veneer hypothesis. If the late veneer added just 0.5% of the Earth’s mass in the form of primate chondrite meteorites, which are low in 182-W and enriched with other precious metals, this would lower the mantle’s overall 182-W abundance.  Willbold et al. find that the old Greenland rocks are enriched in 182-W relative to the Earth’s average, which they argue supports the late veneer theory.

Earth's accretion (a, b, c) and effects of the late veneer (d, e). From Kleine 2011 (Nature, 477, p168)

Willbold et al. also speculate that this late period of meteorite impacts might actually have  initiated the large-scale convection in the Earth’s mantle which drives plate tectonics.

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