The Earth's atmosphere 2.7 billion years ago was possibly more than two-thirds carbon dioxide. The discovery was made during a study of how the ancient atmosphere interacted with particles of cosmic dust falling from the sky.
The carbon dioxide-rich atmosphere may have created a powerful greenhouse effect, the researchers suggest. This could provide an answer to a long-standing mystery known as the "Faint Young Sun Paradox": how the oceans could remain liquid on Earth when the sun was about 30% dimmer than it is now.
Estimates of the carbon dioxide content in the atmosphere 2.5-4 billion years ago vary greatly. “Current estimates span roughly three orders of magnitude: 10 to 1,000 times more than they are now,” says astrobiologist Owen Lehmer of the University of Washington in Seattle. Therefore, scientists tried to somehow reduce the spread.
The answer came from 59 micrometeorites found in 2.7 billion-year-old limestone in the Pilbara region of northwestern Australia. They were first described in a 2016 study and are still the oldest meteorite fossils ever found.
Tiny pieces of stone of iron and nickel, no wider than a human hair, swept through the atmosphere of the ancient Earth and fell into the ocean, to the seabed. There they slowly sank into limestone.
During their short flight and due to their partially molten state, micrometeorites entered into a chemical reaction with the Earth's atmosphere. Atmospheric gas, be it oxygen or carbon dioxide, oxidizes iron, capturing its electrons and transforming the original minerals into new ones.
Based on chemical analyzes of more than a dozen micrometeorites, a 2016 study showed surprisingly oxygen-rich upper layers of the atmosphere. That is, 2.7 billion years ago, there was 20% oxygen, as on modern Earth. But the results of that study did not satisfy many scientists, says Lehmer: “It's hard to imagine an atmosphere like this. Any atmosphere that we see on planets is well mixed."
Therefore, Lehmer and his colleagues conducted a new study and linked the oxidation of meteorites to carbon dioxide, not oxygen. Both gases can be oxidizing agents, although free oxygen reacts much faster than oxygen bound in CO2. To test how well carbon dioxide can oxidize fast-moving micrometeorites, the team simulated a fall in the atmosphere of about 15,000 bits of cosmic dust ranging from 2 to 500 microns. The concentration of carbon dioxide varied from 2% to 85% of the total volume.
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An atmosphere of at least 70% carbon dioxide could oxidize micrometeorites. This conclusion is consistent with other data obtained during the analysis of ancient soils.
A similar composition of the atmosphere, and even with the addition of methane, could create a warm world in which the oceans could not freeze, despite the cold young sun.
Kirill Panov