Earth ate a Mercury-like body early in its history, study finds

By Amina Khan From The Canberra Times

A Mercury-like body smashed into a young Earth and gave our planet’s core the radioactive elements necessary to generate a magnetic field, two Oxford geochemists say.

Without that magnetic field, there would be no shield to protect us from the onslaught of radiation constantly bombarding Earth from space, making the existence of life as we know it impossible, scientists say.

The study, published in the journal Nature, offers insight into how Earth’s magnetic field, and perhaps its moon, came to be.

Our planet is thought to have formed from small rocky bodies like the ones in the asteroid belt today, study co-author Bernard Wood, a geochemist at the University of Oxford, said in an interview. It’s a theory that fits quite well with what’s been studied on Earth, though it’s not a perfect fit, he said.

“That sort of roughly works, but there are all kinds of little questions that don’t quite work,” Wood said, “and one of them is, what is the energy source that drives the Earth’s magnetic field?”

Here’s the problem. To drive Earth’s magnetic field, you need radioactive elements like potassium, thorium or uranium – elements that give off heat as they decay – in the planet’s churning iron core. Those elements are attracted to oxygen and combine with it to make 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 avoid contact with iron.

“They love oxygen so much and they hate being metals so much that they shouldn’t go into the Earth’s core,” Wood said.

So there’s no good way, under current models, to keep enough radioactive material in Earth’s centre to power our vital magnetic field, which creates a conundrum for planetary scientists.

But Wood and Oxford colleague Anke Wohlers realised that if you had a source of reduced sulfides (sulfur compounds that don’t have oxygen) in the iron core, it would make it easier for these iron-hating radioactive elements to co-exist with the metal.

“We said OK, we’ll recreate 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 theory also explains why the ratio of samarium to neodymium (two rare earth elements), is higher in the crust and mantle than it is in the rest of the solar system, he added. Because neodymium mixes with iron sulfides more easily than samarium, it more easily sinks into the core, leaving relatively more samarium behind in Earth’s upper layers.

But how did Earth, which is full of oxides, get all these reduced sulfides in the first place? It probably came from a body that looked a lot like Mercury, which is rich in sulfur and very poor in oxygen.

The scientists think that, early in the planet’s history, Earth gobbled up a Mercury-like body and those sulfides allowed the uranium to stay in the core, powering our magnetic field for an estimated 3.5 billion years.

“Before Wohlers’ and Wood’s experiments, there was only limited (and controversial) experimental evidence that either uranium or potassium can be incorporated in iron metal at the high temperatures and pressures of core formation,” Richard Carlson of the Carnegie Institution for Science in Washington, who was not involved in the study, wrote in a commentary on the paper.

However, he added, “a more stringent test” of whether uranium made it into the core in this way would be to study the ratio of different isotopes of neodymium in Earth’s crust and mantle.

This body, by the way, was Mercury-like in composition, but it was not Mercury-sized, Wood said. It was probably closer to the mass of Mars.

That’s interesting, because scientists think that a Mars-sized body’s dramatic collision with Earth created its moon. It is possible that this Mercury-like body was that Earth-shattering missile.

“We think that is quite conceivable,” Wood said. “It’s kind of exciting to think that this reduced body could actually be the thing which caused the moon.”

IMAGE: The earth’s electromagnetic field, visible in an aurora borealis, may have been created when Earth gobbled up another planet. Photo: Brenda Ahearn

TNS

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