We ate a planet
Study: earth consumed a Mercury-like body early in its history.
AMERCURY-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, the 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?”
To drive Earth’s magnetic field, you need radioactive elements like potassium, thorium or uranium – which give off heat as they decay – to also be in the planet’s churning iron core.
Those elements love getting together with oxygen, making oxides. But oxides are really light and would float toward the planet’s
an artist’s rendering showing the Mercury Surface, Space environment, Geochemistry, and ranging (Messenger) spacecraft around Mercury. The spacecraft will crash into the planet at the end of the month. photo: ap/nasa 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,” 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 – a conundrum for planetary scientists.
But Wood and Oxford colleague Anke Wohlers realised that if you had a source of reduced sulphides – sulphur compounds that don’t have oxygen – in the iron core, it would make it easier for these iron-hating radioactive elements to hang with the metal.
How did Earth, which is full of oxides, get all these reduced sulphides in the first place? It probably came from a body that looked a lot like Mercury, which is rich in sulphur and very poor in oxygen.
The scientists think that, early in the planet’s history, Earth gobbled up a Mercury-like body, and those sulphides allowed the uranium to stay in the core, which is what has allowed it to power our magnetic field for an estimated 3.5 billion years.
This body, by the way, was Mercury-like in composition, but it was not Mercurysized, 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 is what gave birth to the Moon.
It’s possible that this Mercury-like body was in fact that selfsame Earth-shattering missile.
“We think that 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.” — Los Angeles Times/Tribune News Service