Wednesday 23 April 2014

A model of planetary formation that explains the Earth’s lithophile element depletion.

CI chondrites (Carbonaceous Ivuna Chondrites; Ivuna from the Ivuna Salt Flats in Tanzania where the first meteorite of this class was discovered) have a chemistry that closely matches that of the Solar photosphere, and are therefore widely taken as a model for the abundances of elements in the nebulae from which the planets condensed. The Earth is considerably depleted in lithophile elements ('rock loving' elements; elements that remain close to the Earths surface - lithosphere - as they bond easily with oxygen; mostly f and s block metals, plus the lanthanides) compared to these meteorites, something which planetary scientists have struggled to explain.

If the Earth had formed in a part of the Solar System’s protoplanetary disk to hot for these elements to condense, then they would be more-or-less absent from the Earth’s crust. Had the Earth been reheated by some cataclysmic event after its formation, leading to the loss of volatile lithophile elements, then there should be an isotopic signature to this depletion (i.e. lighter isotopes would preferentially be lost), but the Earth shows no isotopic variation compared to CI chondrites (this is not a minor point, the Earth is, for example, 80% depleted in potassium compared to CI chondrites, so a heating episode that resulted in this level of loss would undoubtedly be sufficient for isotopic fractionation to occur).

An artists impression of planetary formation within a protoplanetary disk. Universe Today.

In a paper published on the arXiv database at Cornell University Library on 16 April 2014, Alexander Hubbard of the Department of Astrophysics at the American Museum of Natural History and Denton Ebel, of the Department of Earth and Planetary Sciences also at the American Museum of Natural History, describe a new theory of planetary formation, which seeks to explain the depletion in lithophile elements seen on the Earth.

Hubbard and Ebel consider that at the time of planetary accretion in the Solar System then the Sun may have been prone to FU Orionis type outbusts (a type of stellar activity first documented in the young star FU Orionis, but subsequently observed in other young stars). Such outbursts are thought to occur when large volumes of material from a protoplanetary disk are periodically dumped onto the surface of a young star, causing it to flare spectacularly for a brief period (less than a century), and causing the star’s luminosity to rise by a factor of four to six.

Hubbard and Ebel envisage a young Solar System in which matter in the protoplanetary disk slowly forms fluffy, snow-like particles, which precipitate onto the growing planets. Periodically this system is bathed in extra energy from FU Orionis type outbursts from the young Sun. This raises the temperature across the developing system, potentially by several hundred degrees in the Earth’s vicinity, although never for any great length of time.

These bursts of heat can have two effects on the mineral snowflakes raining down on the forming planets. The volatile lithophile elements (which have low melting and evaporation points) completely disassociate from the dust, returning to a gaseous state and being blown away. The heavier elements are also heated but not sufficiently to evaporate, instead being melted into more solid, hail-like particles which then rain down on the growing planets at a higher rate. This would interrupt the accumulation of lithophile elements on the developing Earth, but not completely remove them, which fits with the observed data.

Schematic of the model. Left panel: initial state, with fluffy, vertically mixed dust in equilibrium with low amounts of volatiles in the gas. Middle panel: immediately after heating, the dust contracts, and the volatiles evaporate, entering the gas phase. Right panel: after settling and cooling, only the low altitude volatiles recondense into the settled grains, leaving them depleted compared to their initial state. Hubbard & Ebel (2014).

Hubbard & Ebel believe that this model is capable of explaining the Earth’s deficit of volatile lithophile elements, and that furthermore it should be amenable to testing, since the heating caused would have an effect throughout the Solar System, but not the same effect, thus planets moving outwards in the Solar System should be depleted steadily in less elements, with the elements missing from the outer planets being the most volatile.

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