‘Weird’, long-lost rocks could explain how a hellish Earth became habitable

Early Earth is often described as “Hadean” for good reason. Born from the ashes of a collision that gave us our Moon, the Primordial Aeon was characterized by hellish heat trapped under a thick blanket of carbon dioxide and water vapor.

Strangely, these conditions should have been inhospitable much longer than they were. About 4 billion years ago – after only a few hundred million years of cooling – our planet was already beginning to look remarkably habitable.

Any explanation for Earth’s dramatic transformation should take into account the rapid loss of its greenhouse gases, allowing the planet to cool and its water vapor to condense into the oceans.

The only problem is that this period in the history of our planet has left few traces of its geology. Crusts of crystallized minerals floating on oceans of magma would have long since sunk into the abyss, taking with them evidence of the planet’s surface conditions.

Thus, any hypotheses we propose to solve the mystery of the missing gas must rely on mostly circumstantial forms of evidence.

Two Yale University researchers recently calculated the numbers for a rather speculative scenario involving “strange” rocks that no longer exist on the Earth’s surface, soaking up all that CO2. And the idea seems to hold true.

“Somehow a massive amount of atmospheric carbon had to be removed,” said planetary scientist Yoshinori Miyazaki, who now works at the California Institute of Technology.

“Because there are no preserved rock records of early Earth, we set out to build a theoretical model for the very first Earth from scratch.”

Much of what we know about the Hadean eon on Earth comes from astrophysical and geochemical models of planetary formation.

Our Earth-Moon system was most likely the product of a collision between two proto-planets, one roughly the size of Mars and the other more or less the mass of Earth today.

What was deposited from this mess of volatiles and rocks would have been a molten lump of swirling minerals and gases that were kept warm by a constant downpour of rubble from space.

From these origins, one could imagine a long period of heat and chaos, perpetuated by a greenhouse atmosphere of carbon dioxide and water. Just look at our neighbor, Venus, to get an idea of ​​what that might look like.

Among the rare mineral evidence we have of the Hadean are signs that it already hosted oceans after only a few hundred million years of cooling.

At the end of the eon, about 4 billion years ago, the carbon cycle appears to have stabilized temperatures to the point that life could exist rather happily.

One possibility is that carbon in the atmosphere dissolved in the oceans, turning into solid carbonates, which could have sunk and become embedded in mantle currents.

It’s a neat idea, but even on second thought, it helps to know if the numbers add up.

Miyazaki and his colleague Jun Korenaga therefore put together models on fluid mechanics, heat movement and atmospheric physics to see if they could make the hypothesis work.

The results suggest it could… if some type of rock were exposed on our planet’s surface.

“These rocks would have been enriched in a mineral called pyroxene, and they probably had a dark greenish color,” said Miyazaki.

“More importantly, they were extremely enriched in magnesium, with a level of concentration rarely seen in current rocks.”

A crust of wet, molten molten rock filled with pyroxene could account for a rapid loss of all that carbon dioxide in a stabilization process that would take millions rather than billions of years.

And then, following a cooling that gave us a regenerating crust made up of a handful of slow-moving plates, all that magnesium-rich rock would have remained far below our feet.

As the crust rapidly turned over, the waterlogged minerals would have quickly dehydrated, filling the oceans to the levels we see today.

The scenario is intriguing, especially because such a phenomenon would have helped revive life in other ways.

“As a bonus, these ‘strange’ rocks from early Earth would readily react with seawater to generate a large flux of hydrogen, which is widely believed to be essential for the creation of biomolecules,” said Korenaga.

It’s the kind of science that only demands hard evidence, which is both buried deep in time and far below the surface.

There is no doubt that the “hellish” period of the Earth will still keep its mysteries a little. But little by little we understand why our planet has become the paradise we see today.

This research was published in Nature.

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