Every scientific field has its own version of time travel. Physicists look at light emerging from the darkest regions of the universe to peer back at the ancient formation and destruction of stars. Biologists use genetics to work out how one species evolved from another. And geologists use frozen bubbles, which capture the state of the world around it at the time they formed.

Some of these bubbles are found in ice cores drilled from the polar regions, all of which contain detailed records of the density, temperature, and composition of the atmosphere of Earth at the time. Other frozen bubbles called “vesicles” are found in ancient lava flows, some of which can be several billions of years old.

A new Nature Geoscience study has used some 2.7-billion-year-old vesicles to throw a long-held scientific theory into disrepute. For decades, scientists thought that Earth’s atmosphere back then – hundreds of millions of years before the Great Oxygenation Event – was thicker, in order to compensate for a presumably much weaker, younger Sun. These vesicles’ record of the atmosphere at the time reveal that it was in fact fairly thin.

Back then, the atmospheric pressure could have been anywhere between almost zero and 0.5 times what it is today, with the latter being the absolute maximum limit. This is equivalent to the air pressure at the top of the world's highest mountains, where water is present unvaryingly as ice.

“Our result is the opposite of what we were expecting,” lead author Sanjoy Som, CEO of Seattle-based Blue Marble Space, said in a statement. Co-author David Catling, a UW professor of Earth and space sciences, added that “people will need to rewrite the textbooks.”

^{Some of the vesicles revealing the air pressure of the ancient atmosphere. Scale bar is 1 centimeter. Som et al./Nature Geoscience}

Nearly 3 billion years ago, when the Sun was around 1.6 billion years old, its radiative output would have been about a fifth as high as it is today. Many have thought that this meant that a liquid ocean on Earth would have been impossible – it would simply have been frozen. There’s plenty of geological evidence to the contrary, however, and for decades scientists have been trying to resolve how such a cool, dim Sun would allow for the formation of a liquid ocean.

This conundrum is known as the “Faint Young Sun Paradox,” and several solutions have been put forward. One of them is that the output of the Sun was actually greater than we think, and our astrophysical models are wrong. Another is that the ancient Earth had a thicker atmosphere, producing a stronger greenhouse effect and warming the world.

At a field site in Western Australia, within the Beasley River, researchers exposed a 2.7-billion-year-old basalt lava flow that indubitably occurred at the surface; some of it even plunged into seawater and rapidly cooled. This means that at least some of the bubbles within it contain records of the Earth’s atmosphere back then.

The size of the bubbles within the lava flow give an indication as to the atmospheric pressure at the time, and X-ray measurements of their precise sizes revealed something truly revelatory: The air pressure was about half that of today’s, meaning that it was remarkably thin.

^{A modern lava flow pouring into the sea. Alexey Kamenskiy/Shutterstock}

As liquid water was definitely present at the time, this can only mean one of two things. Either the Sun was actually far brighter back then than scientists have suggested, or the early atmosphere contained more greenhouse gases than these vesicles have recorded.

In any case, something is clearly very wrong with our understanding of the evolution of the Solar System, and at present, no one can be sure what it is. The paradox continues to mystify.