Gold is arguably the most enigmatic element on Earth. Its value as a form of currency, an industrial metal, and its cultural significance have made it the most sought-after metal all over the world. It wouldn’t be unfair to say that gold is the stuff of legend. A tad dramatic perhaps? But then the story of gold is dramatic! Its presence on Earth tells of stellar cataclysms and extraterrestrial travel.
The gold in your ring, wristwatch, or the odd lowly dental filling, was born out of the collisions of super-dense neutron stars, much before the birth of the Solar System. The young, molten, and still-accreting Earth grew out of the debris of the stellar nebula, gradually accumulating the elements that form today’s world. Gold, with its high density higher, sank to the primordial core with compatriots like iron and nickel, where it remains today, locked deep beyond our reach. And yet, despite it inaccessibility, gold is everywhere from our jewellery to our computers.
The presence of gold in the shallow depths of the Earth’s crust is not due to the serendipitous stranding of some of the metal while the rest journeyed to the centre of the Earth. Gold also came to us through the meteorites from outer space. A 3.9 billion year (Gyr) old event known as the Terminal Bombardment, which was also responsible for the Moon’s spectacular cratering, brought the noble metal to Earth in a flurry of meteorite impacts, say geologists.
Although the origin of ‘minable gold’ was a matter of speculation for years, the most conclusive evidence for gold’s extraterrestrial origin came quite recently. In 2011, a team of geochemists from the University of Bristol measured isotopes of tungsten, a metal with geochemical behaviour similar to gold, in 3.8 billion year old rocks from Greenland. Derived from a primordial mantle reservoir with composition similar to the early Earth, the rocks were ideal to ascertain the effect Terminal Bombardment had on Earth’s composition. The analysis demonstrated that the isotopic composition of tungsten in the Greenland rocks was significantly different than that of more modern samples, suggesting that large scale meteorite impacts (which could change the isotopic ratios in the rocks) had altered the Earth’s tungsten composition. The geochemically similar behaviours of gold and tungsten make the meteorite hypothesis a likely candidate to explain the presence of gold in what should have otherwise been a distinctly non-shiny Earthen crust.