Early Earth’s Atmosphere Could Have Been Half As Thick As Today
Did the young Earth really have a more dense atmosphere, as the current theory goes? Researchers recently used bubbles trapped in rocks to demonstrate that the air 2.7 billion years ago exerted at the most only half the pressure of today’s atmosphere.
According to a study published this week in the journal Nature Geoscience, the commonly accepted idea that the early Earth had a thicker atmosphere to compensate for weaker sunlight is wrong.
Lead author Sanjoy Som, an astrobiologist at NASA, 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 concept of using bubbles trapped in cooling lava as a kind of “paleo-barometer” to measure the weight of air in Earth’s youth occurred decades ago to coauthor Roger Buick, professor of earth and space sciences.
Others had used the technique to measure the elevation of lavas a few million years old. To flip the idea and measure air pressure farther back in time, researchers needed a volcano site where truly ancient lava had undisputedly formed at sea level.
The winning field site in Western Australia was discovered by coauthor 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.
The layers on this 2.7-billion-year-old rock, a stromatolite from Western Australia, show evidence of single-celled, photosynthetic life on the shore of a large lake. The new result suggests that this microbial life thrived despite a thin atmosphere. (Credit: Roger Buick/University of Washington)
A stream of molten rock forming lava rapidly cools from top and bottom, but 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 field measurements suggested a surprisingly lightweight atmosphere, and 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.
Even More Otherworldly
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 believed.
This is one of the lava flows analyzed in the study, from the shore of Australia’s Beasley River. Gas bubbles that formed as the lava cooled, 2.7 billion years ago, have since filled with calcite and other minerals. The bubbles now look like white spots. Researchers compared bubble sizes from the top and bottom of the lava flows to measure the ancient air pressure. (Credit: Sanjoy Som/University of Washington)
A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids. The claim is sure to be controversial, other Earth scientists say. Even Buick sounds taken off-guard.
“We’re still coming to grips with the magnitude of this,” he says. “It’s going to take us a while to digest all the possible consequences.”
The new study is an advance on researchers’ previous work on “fossilized raindrops” that first cast doubt on the idea of a far thicker ancient atmosphere. The results also reinforce Buick’s 2015 finding that microbes were pulling nitrogen out of Earth’s atmosphere some 3 billion years ago.
“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,” says co-author David Catling, professor of earth and space sciences. “People will need to rewrite the textbooks.”
Whatever the conditions back then, it’s hard to dispute that early Earth would have seemed pretty foreign to living things today, Som said, speaking to Reuters. The planet rotated more quickly on its axis so the days were shorter. The moon was closer so tides were stronger.
The researchers will next look for other suitable rocks to confirm the findings and learn how atmospheric pressure might have varied through time.