Earth used to be a bully, absorbing Mercury-like bodies at will

16 Apr 2015

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The mystery of Earth’s magnetic field may have been solved, with researchers theorising a pretty weird moment in our planet’s infant years.

Back in the day, Earth was feeling peckish and devoured a substantial rock that had all the properties of today’s planet Mercury.

That’s what two Oxford geochemists claim, in a report that goes some way to finding a solution to one of our toughest questions.

Basically, Earth’s magnetic field is an absolute mystery. The power of it is driven by radioactive elements such as potassium, thorium and uranium, which give off heat as they decay.

This is a love/hate relationship: these elements love getting together with oxygen and making oxides – but oxides are really light and would float toward the planet's surface; they wouldn't be heavy enough to stay in the core. These elements also hate getting together with iron.

"They love oxygen so much and they hate being metals so much that they shouldn't go into the Earth's core," says Bernard Wood, co-author of the report that features in Nature.

Then there’s the wonder of samarium. Earth’s core is full of the stuff, and we don’t know how. It seems Earth is the only rock around with such a high proportion of it.

Wood and Anke Wohlers performed a series of tests after thinking of using reduced sulfides, which are oxygenless sulfuric compunds . They realised that using substances like this in the iron core could make life a lot easier for iron-hating radioactive elements forced to hang out with it.

"We said, 'OK, we'll re-create those conditions in our high-pressure apparatus and we'll look and see whether the radioactive elements uranium and thorium, and also some of the so-called rare earth elements, would partition into the sulfur-rich metal under those conditions'," Wood said.

"And we found much to our pleasure and surprise that uranium very strongly partitions into sulfur-rich metal under those very oxygen-poor-or-reducing conditions."

The duo's paper essentially suggests that Earth, billions of years ago, may have engulfed a sulfur-rich Mercury-like body – this in turn could have seen its uranium dissolve better in iron sulfide.

Asteroids and Earth image, via Shutterstock

Gordon Hunt is senior communications and context executive at NDRC. He previously worked as a journalist with Silicon Republic.

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