Sunday, September 22, 2013

did gold come from outer space?

bbc | For the chieftains of pre-Columbian America, the dazzling yellow stuff they found glinting at the bottom of streams or buried in the rocky ground captured the power of the sun god. They dressed themselves in battle armour wrought from the enchanted metal, believing it would protect them.

They were sadly deceived.

Gold, an unusually soft metal, wasn't any match for the steel of the Spanish. But the Native Americans may well have been right in believing the element was otherworldly.

"Why do you find nuggets of gold on the surface of the Earth?" asks science writer John Emsley. "The answer to that, is that they've arrived here from space in the form of meteorites."

This theory has come in the last few decades to be held by the majority of scientists as a way of explaining gold's abundance. There may only be 1.3 grams of gold per 1,000 tonnes of other material in the Earth's crust (the rocky shell of the planet that is around 25 miles thick) but that's still too much to fit with the standard models of our planet's formation.

After its birth four-and-a-half billion years ago, the surface of the Earth heaved with volcanoes and molten rock. Then, over tens of millions of years, most of the iron sank down through the outer layer, known as the mantle, to the Earth's core. Gold would have mixed with the iron and sunk with it. Matthias Willbold, a geologist at Imperial College London, likens the process to droplets of vinegar collecting at the bottom of a dish of olive oil.

"All the gold should be gone," he says.

It isn't though. So science has had to come up with an explanation, and the answer currently favoured is - a meteoric shower.

"The theory is that after the core formed there was a meteoric shower that struck the Earth," says Willbold. "These meteorites contained a certain amount of gold and that replenished the Earth's mantle and the continental crust with gold."

Willbold says the theory fits with the pattern of meteorite activity as scientists understand it, climaxing with a huge storm that took place more than 3.8 billion years ago, referred to as the "terminal bombardment". The meteorites punched out the craters we see on the moon and came from an asteroid belt that still exists between Earth and Mars.

This idea of the gold-laden-meteorite "veneer" was first proposed following the Apollo moon landings of the 1970s. Scientists examining rock samples from the moon's mantle found much less iridium and gold than they did in samples from the surface of the moon or from the earth's crust and mantle. It was proposed that the moon and Earth had been battered by iridium-rich meteorites, known as chondrites, from outer space. While the precious fallout from this meteoric shower lay scattered on the surface of the moon, on Earth the planet's internal activity had churned it into the mantle too.

The idea, called the "late veneer hypothesis", has become a fundamental theory in planetary science.

It also helps to explain many other anomalies in the Earth's composition - it is thought that the same meteorites delivered the carbon, nitrogen, water and the amino acids that are vital to all life on the planet.