Early Earth's air was less than half as thick as it is today. Why does it matter?
Scientists thought they had an explanation for the so-called 'faint young sun paradox' in our solar system's early history. But new research could turn that solution on its head.
It may be time to rewrite the textbooks.
Some 2.7 billion years ago, the Earth's air weighed less than half what it does today, according to a paper published Monday in the journal Nature Geoscience. This finding, if confirmed, would prompt a shift from the dominant viewpoint that that ancient atmosphere was twice as thick as today's, which in turn could change the solution to a longstanding scientific conundrum known as the "faint young sun paradox."
At the time, our sun was about 20 percent dimmer than it is now. That meant that the sun's rays wouldn't have warmed Earth's surface as readily.
"If you took the modern atmosphere of Earth and placed it 2.7 billion years ago, the entire planet would freeze over," study lead author Sanjoy Som, an astrobiologist with NASA's Ames Research Center, tells The Christian Science Monitor.
But the geological record shows clear signs of there having been liquid water. So what, exactly, was keeping our planet warm?
To explain this apparent paradox, scientists had proposed that the atmosphere was was markedly different from today's. Perhaps, they suggested, high atmospheric pressure along with a significant amount of greenhouse gases blanketed the Earth and kept it warm.
But, if this new study is correct, pressure might need to be taken out of the equation and, as study co-author David Catling says in a press release, "People will need to rewrite the textbooks."
With a thick atmosphere out of the picture, the supposed paradox seems to be best resolved by greenhouse gases alone, suggests James Kasting, a geoscientist at Pennsylvania State University who was not part of the study.
"You don't necessarily need a thicker atmosphere," Dr. Kasting tells the Monitor. "You need a bigger greenhouse effect."
Thick to thin, then back to thick again
The researchers aren't suggesting that the Earth's atmosphere evolved linearly. Instead, the team suggests that the atmosphere was indeed quite thick some 3.5 billion years ago. Then something happened to thin the air.
Their hypothesis goes something like this: The atmosphere was thick with nitrogen until microbes figured out how to make use of the gas.
In today's nitrogen cycle, Som explains, nitrogen makes up more than three-quarters of the modern atmosphere, and oxygen helps keep the cycle in balance. But billions of years ago, there was almost no free oxygen in our planet's atmosphere, and little organisms sucked a lot of nitrogen out of the atmosphere, gradually thinning it out.
Then, when the Great Oxygenation Event, as it is known, began about 2.4 billion years ago, that process changed and atmospheric pressure increased again.
Pieter Visscher, founding director for the Center for Integrative Geosciences at the University of Connecticut who was not part of the study, is skeptical of that explanation. "It makes it sexy, but I don't like that part," he tells the Monitor.
Dr. Visscher cautions against assuming similarities between today's nitrogen cycle and the processes of 2.7 billion years ago. "The nitrogen cycle is of course important for productivity today, but there are so many facets of that that may or may not have taken place in the presence of oxygen," he says.
That's just a working hypothesis for now, says Som, and he hopes to find other rocks that can support or refute that idea.
Visscher agrees that future research will help. "The nitrogen story that they try to tell here is maybe the start of that, but I think we will go much further than that."
Previous research has largely focused on computer simulations, but Som and his colleagues found physical evidence – in the form of bubbles.
When the lava erupted at sea level, air bubbles were trapped inside it. Those air bubbles have since filled with minerals that preserve the bubbles' shape and size.
"A lava flow will cool from top down and bottom up," Som explains. "As the lava cools, it will trap the bubbles that are at the top and the bottom. The pressure that's acting on the bubbles at the top is air pressure, and the pressure that's acting on the bubbles at the bottom is air pressure plus the weight of the lava."
After measuring the thickness of the cooled lava, the researchers were able to calculate the air pressure from the difference in size of these bubbles, a technique invented decades ago by Dork Sahagian, a geologist at Lehigh University.
"It’s really bold, really ambitious, and fraught with difficulties," Dr. Sahagian told Science Magazine. "But you’ve got to try it. It’s as good a proxy of the pressure as you can hope to find."
Som acknowledges that these new findings won't be accepted easily.
"It is controversial, and I get that," he says. "However, I think it makes sense. It's not just a measurement that's out of thin air (pun intended). There have been other independent works, including mine, that are pointing in that direction."
Som and his colleagues published a paper in 2012 that reported imprints of raindrops in 2.7-billion-year-old rocks that suggested a thick atmosphere was not the solution to the "faint young sun paradox."
A question of habitability
One element of the "faint young sun paradox" was that Earth supported life at the time.
"To make a planet habitable, well, we don't know what you need," Visscher says, "but one of the ideas is you need liquid water and you need a sort of temperature range in which we know that you find life." So with a less luminous sun, something else had to make early Earth warm enough for life.
Low atmospheric pressure might seem daunting for life as we know it, points out Norman Sleep, a professor of geophysics at Stanford University. Keep in mind that ambitious hikers who climb to the top of Mount Everest enter the "death zone," generally considered to begin at 26,000 feet, where the air pressure is about a third of that at sea level.
This new study suggests the air pressure 2.7 billion years ago may have been even less – about a quarter of what it is at sea level today. "We would suffocate at that atmospheric pressure," Dr. Sleep tells the Monitor. But, he adds, "we haven't evolved to live at that pressure."
But it's unlikely that the primordial slime that made up life on Earth at the time would have died off as a result of low pressure, Sleep says. Not only weren't they breathing oxygen, he says, they also would have been living largely in the water and therefore experienced water pressure instead of air.
"Rocks are the history books of the Earth," Som says, but studying early Earth's atmosphere isn't just about learning to read our own planet's history.
"The early Earth is like an exoplanet proxy," Som says. "The planet was able to sustain life, and yet it was completely unfamiliar to what we have today. Because the early Earth was so markedly different, life could be existing on planets that have a very different environment than modern Earth."