Fossilized Raindrops May Help Resolve Early Earth Paradox

A meerkat perches atop rocks bearing the fossil impressions of raindrops that fell in South Africa 2.7 billion years ago. CREDIT: Wlady Altermann/University of Pretoria View full size image

SAN FRANCISCO — The young Earth may not have been a churning ball of scalding hot water, but a planet slightly cooler than today with more temperate oceans, according to two new studies.

The studies, presented Monday (Dec. 3) here at the annual meeting of the American Geophysical Union, may shed light on the paradox of the faint young sun: Why, despite the sun being 70 percent as bright as it is now, the early Earth during the Archean Eon (about 2.5 billion to 4 billion years ago) wasn't a giant snowball. Rather, it had a vast liquid water ocean filled with primitive microbes, ancestors to modern-day methane-producing and sulfur-eating microbes.

In one study, researchers analyzed fossilized raindrops that fell from the heavens some 2.7 billion years ago, finding the atmosphere from which they fell was not that different from today, suggesting that it didn't have the several-fold increase in greenhouse gases that was thought necessary to keep the planet hot.

Another study found that scientists could resolve the paradox because the young planet didn't actually need to be warm to support liquid water. If you model the Earth as a 3D sphere, even with a dimmer sun and an atmosphere not that different from today's, the Earth could still have supported liquid water around the equator — just not at scalding hot temperatures. [50 Amazing Facts About Earth]

"We think that for the last four decades the community has been making the faint young sun paradox harder than it needs to be," said climate scientist Eric T. Wolf, who conducted the 3D simulation, adding that early Earth "could have been similar in temperature to modern Earth or maybe a little colder."

Faint sun, hot Earth?

Starting in the 1960s, scientists used ocean cores and other fossilized records to determine that the Earth's oceans reached as high as 170 degrees Fahrenheit (77 degrees Celsius) during the Archean period. Meanwhile, scientists ran computer simulations of early Earth with a faint sun and a similar atmosphere to our modern one by simplifying the Earth to a one-dimensional line, rather than a more realistic sphere. That meant an average temperature below freezing caused the entire planet to freeze over in their simulations.

To explain the faint sun paradox, scientists have proposed the early Earth's atmosphere was filled with much greater amounts of greenhouse gases such as carbon dioxide that kept the Earth warm. Pressure rises in direct proportion to the amount of gas in the atmosphere, which gave researchers a way to test this idea.

To find out early Earth's atmospheric pressure (and temperature), Sanjoy Som, an astrobiologist at NASA Ames Research Center in California and his colleagues looked at primeval, fossilized raindrops found in South Africa. During a brief, light rainstorm, the raindrops fell into an ancient river that was blanketed with volcanic ash. The imprints were preserved after another fine veil of ash covered them, immortalizing the divots in the fossil record, Som told LiveScience.

These rocks, found in South Africa, show 2.7 billion year old rain.
CREDIT: Som et al, Nature, 2012

View full size image

To calculate the pressure in the early atmosphere, the researchers dropped water droplets from a seven-story height and measured the size of the imprints they made in a pan of volcanic ash from the Icelandic volcano Eyjafjallajökull. Because a raindrop's top speed, or terminal velocity, depends on the density of the air around it as it falls to Earth, Som's team could calculate the air pressure by calculating the speed at which the 2.7-billion-year-old raindrops hit the surface.

They concluded that the ancient atmospheric pressure was no more than twice what it is today, which suggests ancient Earth couldn't have had anywhere near the level of greenhouse gases as other researchers had suggested. Given that, Som said, "I don't think we have a solid explanation as to how the planet stayed warm." [10 Weird Ways Weather Changed History]

Cooler planet

Wolf and his colleagues, meanwhile, using their 3D computer simulation, found that even given more realistic atmospheric carbon dioxide levels, the Earth would have been about as cold as it was during the last ice age. Even so, it could have supported smaller belts around the poles where temperatures were higher and could support liquid water.

The team also reassessed older geological evidence that scientists used to infer the temperature on early Earth, such as marine sediment cores, finding that for near-boiling oceans much of that evidence was questionable.

For instance, scientists have previously used the absence of ice in the fossil record from that time as proof that Earth was ice-free, when in fact, it could mean we just haven't found any ice, Wolf said. And geological evidence for warm temperatures found at northern latitudes came from unknown ocean depths and may very well have come from closer to the equator; that evidence shifted around with breaking continents and churning oceans in the 2.8 billion years since. That means scientists may have been looking at samples that are more representative of tropical, equatorial regions and using those to infer the average temperature on Earth.

More modern research, they found, supported the notion of a more temperate Earth.

That finding may resolve the faint young sun paradox, Wolf told LiveScience.

"This would allow liquid water and life to survive," Wolf said. "Looking at it from that view, the paradox ceases to become a paradox."

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