Air bubbles trapped in 2.7 billion-year-old rocks suggest that early Earth's air weighed less than half of today's atmosphere, researchers including one of Indian-origin have found.
The research from the University of Washington 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, researchers said.
"For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter," said Sanjoy Som, who did the work as part of his UW doctorate in Earth and space sciences.
"Our result is the opposite of what we were expecting," said Som.
Researchers used bubbles trapped in cooling lava as a "paleobarometer" to determine the weight of air in our planet's youth.
To measure air pressure farther back in time, researchers needed a site where truly ancient lava had undisputedly formed at sea level.
In the field site in Western Australia, discovered by Tim Blake of the University of Western Australia, 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 that forms a lava 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 different than previously thought, researchers said.
A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids, they said.
The study was published in the journal Nature Geoscience.