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The atmosphere today weighs more than double the early Earth's air, according to new research.

The study shows that the idea that the young Earth had a thicker atmosphere was wrong.

Researchers used bubbles trapped in 2.7 billion-year-old rocks to show that air at that time exerted half the pressure of today's atmosphere at most.

The results, published online by the journal Nature Geoscience, reverse the commonly accepted idea that the early Earth had a thicker atmosphere to compensate for weaker sunlight.

Researchers said their findings also have implications for which gases were in that atmosphere, and how biology and climate worked on the early Earth.

(Image: NASA)

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Study lead author Doctor Sanjoy Som, of NASA's Ames Research Centre in California, said: "For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter.

"Our result is the opposite of what we were expecting."

The idea of using bubbles trapped in cooling lava as a "paleobarometer" to determine the weight of air in Earth's youth occurred decades ago to study co-author Professor Roger Buick.

Others had used the technique to measure the elevation of lavas a few million years old.

(Image: Getty)

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To flip the idea and measure air pressure farther back in time, researchers needed a site where truly ancient lava had undisputedly formed at sea level.

Their field site in Western Australia was discovered by co-author Tim Blake of the University of Western Australia.

There, the Beasley River has exposed 2.7 billion-year-old basalt lava. The lowest lava flow has "lava toes" that burrow into glassy shards, proving that molten lava plunged into seawater.

The team drilled into the overlying lava flows to examine the size of the bubbles.

A stream of molten rock quickly cools from top and bottom, and bubbles trapped at the bottom are smaller than those at the top.

(Image: SPL / Barcroft Media)

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The size difference records the air pressure pushing down on the lava as it cooled, 2.7 billion years ago.

Rough measurements in the field suggested a surprisingly lightweight atmosphere. More rigorous x-ray scans from several lava flows confirmed the result: the bubbles indicate that the atmospheric pressure at that time was less than half of today's.

Earth 2.7 billion years ago was home only to single-celled microbes, sunlight was about one-fifth weaker, and the atmosphere contained no oxygen.

A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids.

Prof Buick said: "We're still coming to grips with the magnitude of this.

"It's going to take us a while to digest all the possible consequences."

(Image: Tommy Eliassen)

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Other geological evidence clearly shows liquid water on Earth at that time, so the early atmosphere must have contained more heat-trapping greenhouse gases, such as methane and carbon dioxide, and less nitrogen.

The new study is an advance on the researchers' previous work on "fossilised raindrops" that first cast doubt on the idea of a far thicker ancient atmosphere.

The result also reinforces Prof Buick's 2015 finding that microbes were pulling nitrogen out of Earth's atmosphere some three billion years ago.

Study co-author Professor David Catling, of the University of Washington, added: "The levels of nitrogen gas have varied through Earth's history, at least in Earth's early history, in ways that people just haven't even thought of before.

"People will need to rewrite the textbooks."