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Metal Rain Could Explain Why the Earth Made of Different Stuff Than the Moon

A new study shows that iron-rich asteroids could have vaporized when they hit the early Earth

An artist’s interpretation of an object slamming into the early Earth (Walter Myers/Stocktrek Images/Corbis)
smithsonian.com

We already know that Earth’s formation would have been a rough time for the planet — asteroids and even other young planets from the Solar System’s birth slammed into our planet repeatedly. Now, though, researchers suspect that that bombardment may have included mists of molten iron and other metals raining down from the sky. The finding might also explain why the Moon and the Earth have such different chemistry, according to Simon Redfern, at the Conversation

About 4.6 billion years ago, the swirls of hot dust and gas around our Sun finally coalesced and collided; the Earth and other planets were born. Or so we think. Those events happened so long ago that piecing them together is tricky. For example, the Moon may have come from a collision between young Earth and a Mars-sized protoplanet, whose dense elements joined Earth’s blisteringly-hot iron core. But there are other theories too. Another mystery: why is the Earth’s composition so different from the Moon’s? 

The Moon has less iron and precious metal than the Earth does, and the Earth really has too many of those too close to the surface. As the core formed, it should have "sucked" all the heavier elements out of the mantle. But researchers’ measurements show a lot more iron, gold and other metals in the mantel than we’d expect. The new research solves that problem: Instead of just modeling possibilities, a team of researchers used the Sandia National Laboratory’s Z machine—which can generate enormous pulses of electromagnetic radiation—to figure out what might have happened to iron-bearing asteroids that bombarded early Earth.

They found that when the planet was super hot, like it was during formation, iron would have vaporized easily on impact. 

"Rather than the iron in the colliding objects sinking down directly to the Earth's growing core, the iron is vaporized and spread over the surface within a vapor plume. After cooling, the vapor would have condensed into an iron rain that mixed into the Earth's still-molten mantle," says Lawrence Livermore National Laboratory scientist Richard Krauw in a statement.

The Moon, on the other hand, doesn’t have strong enough gravity to hold on to the fast-moving metal vapor plume. It would have lost any potential iron rain. The researchers published their findings in Nature Geoscience. Gold, platinum and other heavy metals could have been delivered to the Earth and lost by the Moon in the same way.

The findings also change the timeline that scientists have for the core’s formation. Since researchers typically used a technique that involves assumptions about how iron mixes through the mantle, they may now have to revise those estimates, Kraus explains. The core may have formed earlier in Earth’s history than we thought.

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