Inverting Robert Oppenheimer’s memory of the line in the Bhagavad Gita, “I am become Death, the destroyers of worlds”, during his Road-to-Damascus moment when the first atomic weapon was tested, may seem an odd headline for an article on geochemistry. But geochemists sometimes do give the air of being on the verge of solving the “Big Question”. Alex Halliday of ETH in Zurich is one of them (Halliday, A.N. 2004, Mixing, volatile loss and compositional change during impact-driven accretion of the Earth. Nature, v. 427, p. 505-509). It is now well accepted that Earth’s early evolution was one of repeated big impacts during planetary accretion. It probably culminated in a collision with a Mars-sized planet that not only created the Moon from the debris splattered from both bodies, but set the Earth’s chemistry for all subsequent time; a sort of geochemists’ Year Zero. When that happened and what ensued has all manner of connotations (see Geoscience consensus challenged in EPN for January 2004). Halliday reviews evidence from several isotopic systems (Pb, Xe, Sr, W) that are reckoned to be appropriate “fingerprints” for the environments in which planets accreted. His treatment takes the data as a whole, rather than separated into one or another isotopic system. He begins with the assumption in most accretion models that metallic cores form continuously and in equilibrium with the silicate outer mantle of rocky planets. That is important in using W isotopes to model the “when”, since tungsten is likely to enter iron-rich metal rather than silicates (see Mantle and core do not mix in EPN February 2004). In fact estimates for the time taken for the Earth to gather 2/3 of its mass based on W isotopes (~11 Ma) are a lot faster than those based on other isotopes (between 15 to 40Ma). Halliday’s explanation is the seemingly sound one that when big things form from smaller ones (whatever contributed to core and mantle), the chances of them mixing and reaching equilibrium, before they definitively separate into the inner and outer Earth, are not good. Reviewing the somewhat bewildering permissiveness of isotopic data from Earth and Moon that bear on “Year Zero” he concludes that the massive loss of xenon (and other “volatile” elements) that characterises Earth, by comparison with what is known about the Solar System’s pre-planetary composition, was 50 to 80 Ma after the “start of the Solar System”. The Moon has provided insufficient data for its age of formation to be tied down isotopically. Although its Hf-W age might be >44 Ma relative to the Earth’s beginning, there again, perhaps >54 Ma, and it may have formed even later. Eventually we reach modelling (read “speculation”?) that takes us to the putative composition of the culprit for Year Zero, “Theia” (a Titan and the product of incestuous liaison between Uranus and his mother Gaia).
What seems odd to me is that some of the parent isotopes for those used in fingerprinting (e.g. 182Hf for 182W, and plutonium for a Xe isotope) can only form in supernovae events, and are so short-lived that the balance between their formation and their influence on partitioning of their daughters in planets is pretty delicate in terms of timing. Indeed all radioactive isotopes, and every element with greater atomic mass than iron, in the Solar System have this origin, because it is impossible for a star the size of the Sun to form them. Massive stars that become supernovas are common enough, and when they “go off” and what blend of heavy elements they produce depend on how big they were and when they formed. Interstellar material is surely a mix of debris from a number of such events of different ages, and new stars and planetary systems form from that. Maybe they are triggered by nearby supernovas, but that also contributes to the isotopic mix that has evolved since a galaxy formed. Just suppose that the mix for the Solar System was heterogeneous, with differently aged uranium, thorium, rubidium, hafnium and other elements heavier than can be formed inside small stars like the Sun, and must have formed in big ones that eventually blasted their products into interstellar space. If the Earth accreted as an open, non-equilibrated system, then what of the Solar System itself? Bit early to say, really….