Turning the traditional knowledge on its head that young Earth had a thicker atmosphere, scientists, including an Indian-origin researcher, have found that air at that time exerted at most half the pressure of today's atmosphere.
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The new finding reverses the commonly accepted idea that the early Earth had a thicker atmosphere to compensate for weaker sunlight.
The finding also has implications for which gases were in that atmosphere and how biology and climate worked on the early planet.
"For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter," said lead author Sanjoy Som, who did the work as part of his doctorate in earth and space sciences at University of Washington.
The team used bubbles trapped in 2.7 billion-year-old rocks to reach this conclusion. "Our result is the opposite of what we were expecting," he added in a paper published in the journal Nature Geoscience. Som is currently doing astrobiology research at NASA's Ames Research Centre in California.
The idea of using bubbles trapped in cooling lava as a "paleobarometer" to determine the weight of air in our planet's youth occurred decades ago to co-author Roger Buick, professor of earth and space sciences.
A potential site in western Australia was discovered by co-author Tim Blake of University of western Australia. There, the Beasley River has exposed 2.7 billion-year-old basalt lava.
A stream of molten rock quickly cools from top and bottom, and bubbles trapped at the bottom are smaller than those at the top. 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.
But this finding points to conditions being even more otherworldly than previously thought.
A lighter atmosphere could affect wind strength and other climate patterns and would even alter the boiling point of liquids.
Other geological evidence clearly shows liquid water on Earth at that time so the early atmosphere must have contained more heat-trapping greenhouse gases like methane and carbon dioxide and less nitrogen.
The result also reinforces Buick's 2015 finding that microbes were pulling nitrogen out of Earth's atmosphere some three billion years ago.
"People will need to rewrite the textbooks," the authors noted.
The researchers will now look for other suitable rocks to confirm the findings and learn how atmospheric pressure might have varied through time.