The missing element
the planetesimal theory of the formation of the Earth, suggesting Earth's formation four billion years ago, is verified by analysing the elements present in the Earth's mantle. But till recently the unexpectedly small amount of niobium present in the mantel has led to doubts over the theory's veracity. Now, a group of scientists at the University of Bristol, uk, have an answer to why niobium is missing from the mantle: It is actually locked up in the core of the Earth (Nature , Vol 409, p75).
The Earth's mantle is the rocky, solid shell some 3000 kilometers deep, just beneath the outer crust. Beneath the mantle lie the liquid core and finally the solid inner core. The solid and liquid core is mostly iron and little amounts of other elements. Scientists believe that the Earth was formed when large rocky objects called planetesimals collided and joined together in the solar system some 4 billion years ago. These impacts released a lot of energy and as a result the planet melted. Iron, being heavier sank to the core while the silica compounds (making up the rocks) floated on top. There were other elements also in the early periods. Some of them are more soluble in the iron than in upper magma. These elements separated and were thus predicted to be present in higher concentrations in the core than in the mantle.
Scientists infer the composition of the early Earth by analysing the composition of particular kind meteorites called chondrites. These are as old as the Earth and are thought to be the residual planetesimals. They also have not segregated into iron and silica regions and thus one can determine the Earth's primitive composition from them. Among the other elements found in chondrites is niobium. It has been believed that niobium's chemical properties make it more soluble in molten silicate than in iron. Thus there should be abundant niobium in the mantle. But research has shown otherwise. Some scientists have speculated that maybe niobium instead of mixing through out the mantle is present only in small pockets that are yet to be found. But the model for such a scenario is contrived and not convincing.
Researchers investigated the properties of niobium in conditions that simulate those prevailing in the deepest regions of Earth. They found that at high temperatures and high pressures, niobium is more soluble in liquid iron than in silicate. Thus, niobium present in the Earth, they concluded, is locked up in the core and not in the mantle as previously believed. This study has solved the long-standing missing niobium puzzle and further strengthened the planetesimal theory of the formation of the Earth.
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