The idea that the young Earth had a thicker atmosphere turns out to be wrong. Researchers used bubbles trapped in rocks to show that the air 2.7 billion years ago exerted at most half the pressure of today’s atmosphere.
The findings reverse the commonly accepted idea that the early Earth had a thicker atmosphere to compensate for weaker sunlight—and also have 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,” says lead author Sanjoy Som, who did the work as part of his doctorate in Earth and space sciences at the University of Washington. “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 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 site where truly ancient lava had undisputedly formed at sea level.
Their 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.
“People will need to rewrite the textbooks.”
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 suggest 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 believed. A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids.
“We’re still coming to grips with the magnitude of this,” Buick says. “It’s going to take us a while to digest all the possible consequences.”
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 new study, published in the journal Nature Geoscience, 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 coauthor David Catling, professor of earth and space sciences. “People will need to rewrite the textbooks.”
The researchers will next look for other suitable rocks to confirm the findings and learn how atmospheric pressure might have varied through time.
While clues to the early Earth are scarce, it is still easier to study than planets outside our solar system, so this will help understand possible conditions and life on other planets where atmospheres might be thin and oxygen-free, like that of the early Earth.
Researchers from the University of Alaska, Fairbanks; Scotland’s James Hutton Institute; and the Denver Museum of Nature and Science are coauthors of the study, which was funded by NASA.
Source: University of Washington