Photograph by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington 
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The planet Mercury, shown here in 2008, may resemble an object scientists say could have smashed into the young Earth, bringing with it the radioactive elements that have kept our planet’s core molten.

Photograph by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington 

Ancient Cosmic Smack-Up May Have Made Earth’s Molten Core

Ancient collision explains presence of radioactive elements that keep planet’s center from solidifying.

Our planet’s hot, gooey center may be the result of a collision between a young Earth and a Mercury-like protoplanet billions of years ago, a study published Wednesday in Nature suggests.

This is “an intriguing conclusion,” Richard Carlson of the Carnegie Institution of Washington writes in an accompanying commentary. Before this study, he writes, “there was only limited (and controversial) experimental evidence” to support such ideas.

Geologists can’t observe Earth’s core directly, but they know from the way seismic waves travel through the planet that the core is made mostly of molten iron and nickel. Those hot substances power the planet’s magnetic field as well as the phenomenon known as plate tectonics. But why the core remains molten some 4.5 billion years after Earth’s formation has been a mystery.

If the core were only iron and nickel, it should have solidified eons ago, as the heat created by the asteroid impacts that formed the planet gradually leaked into space. Scientists suspected that radioactive elements, through their decay, provide the heat that keeps the core liquified and the magnetic field intact, but they did not know which elements were responsible.

The new study indicates that uranium and thorium are the most likely suspects—and they may have come from a Mercury-like protoplanet that smashed into Earth while our planet was still taking shape.

A Solution for Earth Enigmas

There were three main suspects for the identity of the core’s radioactive elements—potassium, uranium, and thorium. “One of our aims,” says lead author Anke Wohlers, an Earth scientist at Oxford University, “was to figure out which of the three … is responsible for the molten core.”

His team also hoped to solve another, much more obscure mystery: why the rocky outer layers of the planet have an odd mix of two rare-earth elements called samarium and neodymium. On the outer Earth, these two elements appear in a different ratio from what is found in asteroids, which are believed to be the leftover building blocks from the formation of the planets.

The reasoning that led Wohlers and his co-author to posit an ancient impact as an explanation for both phenomena involves a complicated mix of planet-formation theory, data on Mercury and other planetary bodies, and high-temperature laboratory experiments that would be hard for anyone but a geochemist to follow.

But the best explanation that fits all the data, says Wohlers, is that in the early days of the solar system, asteroid-like bodies slammed together to form a fledgling Earth. Then something similar in composition to Mercury smacked into the young planet, adding its own unusual chemistry to the mix.

This wouldn’t be the first time scientists have posited a collision between the young Earth and a planet-size body. Most experts think that an object the size of Mars smacked into our planet and spewed molten rock out into space, where it cooled to form the moon.

But the new research adds a twist to the story. The moon-forming object might not have been quite as large as Mars, says Wohlers. It could have been about the size of Mercury. That means that the projectiles that created the moon and supplied the radioactive elements that keep Earth’s core molten could in theory have been the same object.

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