How Early Earth Cooled After Moon-Forming Impact

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Billions of years ago, the Earth's atmosphere was opaque and the planet's surface was a vast magma ocean devoid of life.

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This scenario, says Stanford University professor of geophysics Norman Sleep, was what the early Earth looked like just after a cataclysmic impact by a planet-size object that smashed into the infant Earth 4.5 billion years ago and formed the moon. The moon, once fully formed, which would have appeared much larger in the sky at the time, since it was closer to Earth.

Hundreds of millions of years later, he added, the first forms of life appeared, possibly having hitched a ride on a rock from Mars. The scenario is one presented by Sleep at a recent Royal Society conference here called Origin of the Moon. A paper detailing Sleep’s study was submitted to the symposium volume. [The Moon: 10 Surprising Lunar Facts]

Although many elements of the theory have been around for some time, Sleep's synthesis is "like putting together a jigsaw puzzle with some pieces already known and some that are speculative and have new aspects," said Dave Stevenson, a Caltech professor of planetary science who was not involved with Sleep's study.

One of these new aspects is how Earth cooled down to the temperatures necessary for life to evolve, following the — presumed — giant impact that formed the moon.

The processes Sleep discussed took place in the period called Hadean, about 4 billion to 4.5 billion years ago — before the first organisms came into being, and well before more complex life-forms, including dinosaurs, started roaming the Earth.

Back then, the Earth was nothing like the blue Earth we know today.

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Scorching world

Instead, the entire Earth was hot and molten all the way to its inner core, a mixture of molten rock and liquid.

No life would have been able to survive these brutally high temperatures, which reached 2,000degrees Celsius (more than 3,600 degrees Fahrenheit). Liquid water had no chance to form.

The Earth's atmosphere at this time was also much heavier. Its mass was similar to that of today's oceans, and it pushed down on Earth's surface with a pressure of hundreds of bars. (For comparison, the average pressure at the Earth's surface today is 1 bar). It was also opaque — "you would not have been able to see much, just clouds covering everything," Stevenson said.

Beneath the clouds, a magma ocean swayed, with partially molten rock pushed around by tides, Sleep hypothesizes.

These tides were due to the mutual attraction of the Earth and the moon, and were much stronger than those in today’s watery oceans, as the moon was sitting much, much closer to the Earth back then.

The tides constantly stirred the ocean, causing the mantle to lose heat, similar to stirring and blowing on a bowl of soup. But once released from the Earth's depths, the heat was trapped at the surface, held back by the thick, opaque primordial atmosphere.

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The heat could only escape the planet (and cool it down) at so-called cloud-top temperature levels — where it would be as cold as on a modern high mountain summit. But for the first 10 million years, the temperatures were much, much higher, Sleep said.

The energy loss caused by the mutual attraction of the Earth and the moon was also making the moon gradually pull away. This made the tides progressively weaker, so the molten rock was being stirred less and less, and the Earth's mantle began to solidify in stages.

"While at the top of the Earth there was still partially molten slurry with a bit of liquid left, in the middle there was a mushy layer, but the deep mantle was becoming solid,” Sleep said. "Lava was probably still coming up and erupting and freezing at the top, and then falling back in large, kilometer-size pieces that were sinking into the Earth."