Tiny meteorites suggest ancient Earth had a carbon dioxide–rich atmosphere

Earth’s atmosphere 2.7 billion years ago may have been more than two-thirds carbon dioxide. That finding comes from a new study that simulates how the ancient atmosphere may have interacted with bits of cosmic dust falling through the sky.

Such a carbon dioxide–rich atmosphere may also have created a powerful greenhouse gas effect, researchers suggest January 22 in Science Advances. That, in turn, could help answer a decades-old conundrum known as the “faint young sun paradox:” how liquid oceans could have existed on Earth when the sun was about 30 percent dimmer than it is now (SN: 4/18/13)

Estimates for atmospheric carbon dioxide during the Archean Eon, which lasted from 4 billion to 2.5 billion years ago, vary widely. “Current estimates span about three orders of magnitude, from about 10 times more than now to a thousand times more,” says Owen Lehmer, an astrobiologist at the University of Washington in Seattle. So scientists have hunted for data that can shrink that range.

Enter a group of 59 micrometeorites found embedded in 2.7-billion-year-old limestone from the Pilbara region of northwest Australia. These carefully preserved meteorites were first described in a 2016 study in Nature, and are still the oldest fossil meteorites ever found, by about 900,000 years. As such, they offer a rare glimpse into the atmosphere of a lost world.

The tiny bits of rock, no wider than a human hair, zoomed through the atmosphere of ancient Earth. Made of iron and nickel, the micrometeorites heated up as they plummeted, melting and then refreezing before landing in the ocean and sinking to the seafloor. There, they became slowly entombed in limestone.

During their brief, partially molten state, the micrometeorites chemically reacted with Earth’s atmosphere. Some atmospheric gas — whether oxygen or carbon dioxide — oxidized the iron, snagging its electrons and transforming the original minerals into new minerals.

Based on chemical analyses of over a dozen of the micrometeorites, the original 2016 study suggested that the degree of iron oxidation points to a surprisingly oxygen-rich upper atmosphere 2.7 billion years ago, not dissimilar to today’s 20 percent oxygen.  

But that answer was never wholly satisfying, Lehmer says.

Based on data gleaned from Archean outcrops, scientists generally agree that there was very little oxygen in the atmosphere right at Earth’s surface during the Archean. So a lot of oxygen much higher up would mean a layer cake–like stratification, with two very different atmospheric compositions at different altitudes.

“It’s not clear that’s impossible, but it’s difficult to imagine an atmosphere in that state,” Lehmer says. “Every atmosphere that we can see on terrestrial planets is well-mixed,” stirred together by swirls and eddies and chaotic flows of air. “Turbulent mixing prevents that stratification from occurring.”

So Lehmer and his colleagues decided to tackle the elephant in the room. What if carbon dioxide, rather than oxygen, was responsible for oxidizing the iron? Both can be oxidizers, although free oxygen reacts much more quickly than oxygen bound up in CO₂. Still, Lehmer says, “if you can’t have a stratified atmosphere, it’s reasonable to think there was little to no oxygen.”

To test how well carbon dioxide could oxidize fast-moving micrometeorites, the team simulated the journeys of about 15,000 bits of cosmic dust, ranging in size from two to about 500 microns, as they entered Earth’s atmosphere and arced groundward. The tiny bits of rock swooped in from various angles and moved at different speeds, altering how much they might melt. And the team also had the rocks pass through atmospheres with a range of carbon dioxide concentrations, from 2 to 85 percent by volume.

The simulations suggest that an atmosphere made up of at least 70 percent carbon dioxide could have oxidized the micrometeorites, rather than a stratified atmosphere with an upper atmospheric layer enriched in oxygen. That’s also consistent with other lines of evidence suggesting a carbon dioxide-dominated atmosphere during the Archean, including analyses of ancient soils weathered from rocks, the team says.

Such a CO₂-enriched atmosphere, along with a healthy dose of the even stronger greenhouse gas methane, also could have created a warm, greenhouse world. That could make it the long-sought answer to the faint young sun paradox.

“It perhaps doesn’t solve the whole puzzle. But it puts an important piece in place,” Lehmer says.

“They do have a point,” says planetary scientist Matthew Genge of Imperial College London, a coauthor of the 2016 Nature study. Genge acknowledges that the idea that there might have been a layered atmosphere was surprising even then. But “I think the jury is still out” on whether oxygen or carbon dioxide was responsible for oxidizing the cosmic dust, he says.

Lehmer’s team’s simulations suggest CO₂ could have reacted quickly enough with the iron to oxidize outer layers of the rocks, or even fully oxidize them. But such simulations of reaction times “are an ideal case,” Genge says. “Under these conditions, reactions are as fast as possible,” but such speedy reactions may not be realistic. More chemical analyses of actual micrometeorites may help scientists put realistic bounds on the simulations.

The wonder of it is “that there are little rocks that let us do geology on the atmosphere so far above the ground,” Genge says. “It is exciting that these tiny particles, which still fall all around us, allow us to peer so far back in time.”