What can fossilized raindrop impressions preserved in 2.7-billion-year-old volcanic ash tell us about the ancient Earth's atmospheric pressure? Can they help resolve one of the great astrophysical puzzles? A study released Wednesday in the journal Nature suggests they can at least bring us a step closer to understanding the primordial Earth.
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A meerkat perches atop rocks bearing the fossil impressions of raindrops that fell in South Africa 2.7 billion years ago. Photo by Wlady Altermann/University of PretoriaThe puzzle is known as the "faint young sun" paradox, and was first proposed by Carl Sagan and his colleague George Mullen in 1972.
Here's how it goes: As our sun has aged, its inner core has gotten hotter and more dense, causing it to release more energy and heat. We know that on Earth 2 billion years ago, the sun was fainter -- 85 percent as strong as it is now. With such a faint sun, you'd expect an Earth frozen solid, but instead, scientists have found evidence of liquid water: "sedimentary deposits that preserve ripple marks and mud cracks, glassy 'pillow' lavas that were rapidly quenched, and alga-like microfossils," according to an article that accompanies the study in Nature.
That the Earth wasn't frozen over then has mystified scientists, and it's unclear whether stronger air pressure caused by greater concentrations of nitrogen or a heavier concentration of greenhouse gases like methane or sulfur dioxide was responsible for trapping heat and insulating the planet. Until now though, no one has found a reliable way to calculate air pressure from the time.
The study concludes that atmospheric pressure on Earth 2.7 billion years ago was similar to that of today, indicating that likely played little role in warming the planet.
In 1851, renowned geologist Charles Lyell surveyed raindrop imprints preserved in a bed of volcanic ash and suggested that they be used to calculate air density. Such fossilized raindrops from South Africa had fallen onto volcanic ash 2.7 billion years ago, where they hardened and were preserved.
"The raindrop imprints are proofs of showers of rain, the drops of which resembled in their average size those which now fall from the clouds," Lyell wrote in an 1851 paper in the Quarterly Journal of the Geological Society. "From such data, we may presume that the atmosphere of one of the remotest periods known in geology corresponded in density with that now investing the globe."
"Nobody bothered to take him up on that," said David Catling, co-author of the study and a professor of earth and space sciences at the University of Washington. "Until now."
The team studied these ancient raindrop impressions, which fell during the Archaean Eon, when the air was thicker with methane and teeming with slimy microbial life.
The research team operated on a few key pieces of information. Central to their study was a simple fact about physics: no raindrop could swell bigger than 7 millimeters in diameter, and the maximum size of a raindrop is independent of air pressure.
"If you're caught in a rainstorm on Earth, the size of the raindrop would be the same as if you were caught in a rainstorm 2.7 million years ago," said lead author Sanjoy Som.
But the atmosphere exerts a drag on raindrops, so if the air density were greater, raindrops would have fallen slower. Greater air density would exert a drag, causing the same size drop to make a smaller imprint and thinner air density would fall faster, making a larger imprint. So any changes in size should be a result of air density, they concluded.
"If the air density were thinner or thicker, the maximum size of the crater would change," Catling said.
To determine how the ancient raindrop impressions compared to impressions made by today's raindrops, they conducted a simple experiment in the Atmospheric Science and Geophysics Building at the University of Washington, near Som's lab.
From the seventh-floor stairwell, a student used a pipette to release large drops of water onto a baking tray full of ash collected from the 2010 Eyjafjallajökull volcano. The drops fell 90 feet -- more than double the height necessary to reach terminal velocity.
"The stairwell experiment allowed us to create a relationship between impact momentum and imprint size." Som said. "It shows how low tech you can get and still get some exciting science out of it."
Then they hardened the imprints with hairspray and liquid plastic.
"What we found is that air density couldn't have been more than a couple times what it is today. Based on statistics, it's less than today, probably," Catling said. "One of the theories as to why the Earth's climate was able to support liquid water is that you just had a much, much thicker atmosphere -- three or four or five times the air pressure it is today. What this experiment does is it rules that out."
Ralph Lorenz, a planetary scientist at the Johns Hopkins Applied Physics lab who is familiar with the work, agreed that the study helps to exclude a high-pressure atmosphere to explain how the paradox. "If you had a thick nitrogen atmosphere -- several times thicker than today -- then raindrops would fall more slowly. They wouldn't leave the fossil imprints that were measured."
Lorenz has studied how methane raindrops fall on Saturn's moon, Titan, which is often seen as a sort of prototype for the early Earth. The early Earth may have had an organic haze formed by methane that is similar to Titan. Lorenz found that in the colder, denser, oxygen-free Titan, which is also lower in gravity, methane drops fall slower, at about the speed of a snowflake on Earth.
"If you teleported a human being, they would last longer on Titan than anywhere else," Lorenz said. "On Venus, they'd be crushed, suffocated and scorched all in one go. On Mars, the air would be sucked out of their lungs, and they'd collapse in a few seconds. But on Titan, you could hold your breath for a minute or two, and then you'd eventually freeze to death. "You'd only need a warm suit and an oxygen mask to survive on Titan."
As to whether it indicates that greenhouse gases were instead responsible for warming the Earth?
So little is known about the Earth billions of years ago, that it's a leap to make that conclusion, Lorenz said. "There are so many unknowns that it's hard to explain the evidence we have of liquid water. But in the realm of the blind, the one-eyed man is king. Any information is valuable."
Knowing more about the ancient Earth could inform our understanding of planets outside of our solar system, Som said.
"One alive planet to compare to extrasolar planets is not terribly satisfying," he said. "Early Earth was a completely different planet -- no grass, no trees, no animals ... But there were a lot of microbes. It was microbially alive. If we could nail down what the composition was, it would increase our database of known alive atmospheres."
