It's a Good Thing Early Earth's Magma Oceans Only Melted Half the Planet

A giant collision threatened to "melt the entire planet," but ended up only melting about half of it—and helped form the moon at the same time.

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Jun 10 2014, 8:15pm
Image: NASA

Several billion years ago, a giant, Mars-sized planet collided with our poor little Earth, breaking off a chunk of both—a conglomeration of which eventually turned into the moon. The heat and energy generated from the impact was so much that it easily could have vaporized the planet. But it didn't—it only formed extremely hot, magma oceans on roughly half the planet. Which is a very good thing. 

At least, that's what scientists now think happened, thanks to new research by Sujoy Mukhopadhyay at Harvard University. The magma ocean hypothesis has been floated before, but there was one problem with it: If the entire mantle had turned into molten magma, the planet would have vaporized and "melted," according to Mukhopadhyay. He's put forward another hypothesis that suggests that the impact formed "multiple magma oceans," but that it was generally limited to one half of the hemisphere, while the other side of the Earth was just chillin', experiencing "a modest heating that [did] not result in complete melting."

That's a new hypothesis—previous ones suggest that, perhaps, most of the Earth had magma oceans after the giant collision, between an ancient Earth and a giant, Mars-sized planet or celestial body known as Theia. For a relatively unknown, extremely ancient planet, Theia has been in the news quite a bit lately—last week, scientists confirmed that at least part of the moon is likely made out of Theia.

That discovery lends some more credence to the Great Impact Hypothesis, the idea that the moon formed out of this giant impact. Mukhopadhyay's work, presented at the Goldschmidt Conference in California this week, tells us more about what happened after that impact.

"The prevailing view was that the moon-forming impact would have completely melted the whole Earth," he told me. "But, the fact that it didn't, and that the Earth appears to keep a 'memory' of these collisions means we have a better opportunity to learn about what happened."

Mukhopadhyay's lab, where he analyzes the makeup of volcanic rocks. Image: Sujoy Mukhopadhyay

Mukhopadhyay says this "memory" comes from an analysis of the ratio of helium to neon isotopes in the Earth's mantle, which were recovered form rocks shot out from Earth's volcanoes—he noticed that, in the shallower parts of the mantle, there is a much higher ratio of helium to neon than there is in deeper parts of the mantle, which suggest that "the magma oceans didn't extend all the way to the core," he told me.

"In the Earth's interior, we see evidence of multiple impacts and multiple magma oceans—in fact, at one time, the interior of the Earth could have been liquid," he said. "But the evidence suggests these magma oceans didn't extend all the way to the core, which could have caused the planet to melt completely."

That sounds like a potentially not-so-great outcome for the planet. He says that, had that happened, the Earth wouldn't have been totally destroyed, but it would have had major implications for the future—more specifically, life probably would have never evolved here.

"It wouldn't cease to exist, but the Earth would be almost entirely liquid, all the way down to the core," he said. "You'd effectively have almost a liquid planet, not the solid planet we have today."