Many believed that the early Earth, billions of years ago, had a thicker atmosphere. But, that turns out to be wrong. New research from the University of Washington (UW) who studied trapped bubbles inside old rocks found evidence that the Earth’s atmosphere was actually thinner before.

The finding contradicts a long-held notion that a thick atmosphere helped keep Earth warmer than today, at a time when the sun was less active. “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. “Our result is the opposite of what we were expecting.”

The finding also has implications for which gases were in that atmosphere and how the interplay of climate and biology affected the young Earth.

The scientists used bubbles trapped in 2.7 billion-year-old rocks to show that air at that time exerted at most half the pressure of today’s atmosphere. The scientists used sophisticated scanning technology to analyze the size and distribution of gas bubbles trapped in ancient basalt rock that formed from ancient lava flows along the shores of Australia’s Beasley River that solidified at sea level. It was then found that the planet back then possessed a much thinner atmosphere; with air pressure half of what it is today. The team used a paleo barometer to measure the weight of ancient air.

As lava cools from top to bottom, trapped bubbles in the bottom become noticeably smaller than the top bubbles. The difference in size is due to the varying degrees of air pressure exerted on the lava as it cools.

“This study doesn’t yield direct knowledge about the air composition,” Sanjoy said. “Nonetheless, because most of the air pressure is nitrogen, and you needed greenhouse gases to compensate for a faint sun, methane – a powerful greenhouse gas – was a likely important constituent, as well as water vapor – another powerful greenhouse gas.”

The research was published online May 9 in the journal Nature Geoscience.