Air bubbles trapped in 2.7 billion-year-old lava flows in the Pilbara suggest the Earth's atmosphere weighed less than half that of today and was far thinner than previously thought.
The discovery has forced a rethink of how the Earth managed to stay relatively ice-free during that period, despite the sun being much cooler than it is today.
In a study published today in Nature Geoscience, researchers analysed the relative sizes of air bubbles across ancient lava flows along the Beasley River in Western Australia's Pilbara region.
The lava flows - discovered by Australian researcher Dr Tim Blake, then at the University of Western Australia - bear distinctive features such as lava "toes" with cracked, glassy rinds, which suggested the molten rock once flowed over a beach.
Astrobiologist Professor David Catling, one of the study's authors from the University of Washington, said the Sun was roughly 20 percent less luminous 2.7 billion years ago, so if Earth had an atmosphere similar to today it would have been covered in ice. Yet similar conditions as today prevailed.
"All the signs are that it wasn't that different, in terms of the climate system from today; there was rainfall and rivers, there might have been polar ice caps, but it was warm enough ... for the whole world (not) to be covered in ice," Professor Catling said.
Until now, the prevailing hypothesis to explain these warmer-than-expected conditions was that the atmosphere at that time had been much thicker, which would have insulated the Earth.
But the bubbles preserved in the rock at what would have been sea level paint a very different picture of the Earth's atmospheric pressure during the Archean period, which extends from the formation of the Earth's crust 4 billion years ago to around 2.5 billion years ago.
The researchers compared the size of air bubbles trapped at the surface of the lava - whose size is constrained only by air pressure - and those at the base, which are constrained both by atmospheric pressure and the weight of the lava itself, to work out the air pressure at the time the lava flow happened.
"We found the atmosphere basically was a lot thinner back then and weighed less than half of what it does today," Dr Catling said.
He said the finding raised the question of what exactly was keeping the Earth warm enough for liquid water to flow.
"The first thing is the air must have had a lot of proportions of greenhouse gases like carbon dioxide or methane, so even though the air is thin if you stuff it full enough of greenhouse gases it can still warm the Earth," he said.
Dr Catling said there was also likely to have been a much lower concentration of nitrogen - which was and still is mostly produced by microbes - than previously thought.
"What our result means about the low pressure is that the way that nitrogen was modulated back then was very different and it didn't allow so much nitrogen in the air as today," he said.