Climbing Mount Everest is a bit like travelling back in time. Earth’s atmospheric pressure 2.7 billion years ago may only have been a quarter of what it is now – equivalent to conditions 2000 metres above the 8848-metre summit of Everest.
The finding challenges the traditional view that a thick atmosphere helped keep Earth warmer than today, at a time when the sun was less active. It could alter the way we assess whether exoplanets are likely to harbour life.
The idea of a thick early atmosphere came from the need to resolve a dilemma. Rocks dating back four billion years show clear evidence of liquid oceans, but at the time the sun was only 70 percent as bright as today. That would have been too faint to keep the oceans from freezing without strong warming from atmospheric greenhouse gases.
In 2009, Colin Goldblatt, now at the University of Victoria in British Columbia, Canada, reported that a massive atmosphere could increase warming by spreading greenhouse absorption across a broader range of wavelengths. To test the model, Sanjoy Som, now at the NASA Ames Research Center in Moffett Field, California, went searching for evidence of ancient air pressure.
Som and his team used holes in frothy lavas deposited 2.7 million years ago to estimate the air pressure at the time. Their results show it was 23 per cent of today’s value, though the uncertainty in the measurement is large. “We are 95 percent sure that the pressure was between 0 and 0.5 atmospheres,” says Som.
The top estimate corresponds to an elevation of 5500 metres, above that of Mont Blanc.
Som believes the changes in pressure reflect the biological use of nitrogen, which makes up roughly 80 per cent of the modern atmosphere. Nitrogen that seeped out of Earth soon after it formed dominated the primeval atmosphere and made it massive
Then, about 3.2 billion years ago, microbes evolved a way to convert the nitrogen into ammonium. That depleted atmospheric nitrogen and deposited it as sediment on the sea floor. It was such a large effect that by 2.7 billion year ago, air pressure was at a fraction of today’s level.
“It’s a surprising and perhaps an extraordinary result,” says Goldblatt, although he thinks validation is essential. “Right now we only have one data point. A time series over a long period would make this really powerful.”
“If it’s right, the nitrogen inventory of the atmosphere is very dynamic,” Goldblatt says. That’s a surprise because geologists had not expected nitrogen levels to change much over geologic time.
Understanding early Earth’s atmosphere is hugely important for astrobiology, says Timothy Lyons of the University of California at Riverside. “It’s really important to understand how greenhouse gases affected habitability early in Earth history and changed in the history of the atmosphere.”
That could reveal what gases to look for in exoplanet atmospheres to find convincing evidence of life.