Earth’s mantle contains far more gold, platinum, and other metal-loving elements than it should, and a decades-old theory says a rain of extraterrestrial debris delivered them after the core formed—but new computer simulations may show that theory is wrong. The problem is that the “Late Veneer” hypothesis rests on lab experiments conducted at pressures and temperatures far lower than those that actually existed during Earth’s core formation. Those experiments suggested the core should have stripped the mantle of these elements, but the new calculations will model how these elements actually behave under the extreme conditions of planetary differentiation. If the core was never as efficient at removing them as assumed, the entire Late Veneer concept may be unnecessary. This is fundamental science with no immediate practical application. If successful, the work would force a fundamental reset in how scientists understand Earth’s accretion and early evolution—and could reshape interpretations of the chemical makeup of other rocky bodies such as Mars, the Moon, and the asteroid Vesta. Such paradigm shifts in planetary formation have historically fed into broader models of how habitable worlds emerge, though any such connections remain speculative at this stage.
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The Late Veneer (LV) is one of those fundamental paradigms in the formation and subsequent evolution of Earth that endured since the 1970s. In simple terms, the LV refers to a small amount of currently unknown material added to the Earth after its initial formation and the effective closure of the core. The evidence for the occurrence of a LV rests almost entirely on the interpretation of HSE (highly siderophile element) abundances in the mantle. Based on expected partitioning behaviours, these elements should have been almost entirely sequestered into the metallic core during planetary differentiation, leaving the Earth’s mantle devoid of them. In reality though, the abundance of HSEs in the upper mantle is at least three-orders of magnitude higher than expected. This requires a source of HSEs to the Earth after core-closure. Hence, the “Late Veneer” hypothesis. However, the partitioning data which underpins the LV model is from experiments at much lower pressures (P) and temperatures (T) than those extant at core-formation. Moreover, available data show such a large effect of P and T on this partitioning that the core may not be so effective at sweeping up these metal-loving elements as previously thought. It is, therefore, entirely possible that a LV is not needed at all to explain observations. Resolving this important issue would have far-reaching consequences for Earth accretion and force a re-set in our understanding of planetary formation. The aim of this project is to use state of the art ab initio calculations to retrieve partition coefficients of all the HSEs (which include Os, Ir, Ru, Rh, Pt, Pd, Au, and Re) between the core and mantle under the extreme P/T conditions appropriate for planetary differentiation. We do this because such partition coefficients are difficult or impossible to achieve experimentally. The ultimate vision is to use these new results to assess whether there is a need for a Late Veneer at all, or whether something more evolutionarily complex is required. This new work will provide robust constraints on the processes by which the Earth accreted and evolved to its current state. We foresee its future use in the interpretation of HSE inventories for other sampled silicate bodies (i.e. Mars, Moon, Vesta).
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