A paradox about the climate of the early Earth that has been plaguing scientists for nearly 50 years may have a new solution.

The so-called 'young' sun paradox — first proposed by Carl Sagan and his colleague George Mullen in 1972 — refers to the fact that the Earth had liquid oceans for the first half of its more than 4-billion-year existence despite the fact that the sun was likely only 70 percent as bright in its youth as it is now.

A lower solar luminosity should have left Earth's oceans frozen over, but there is ample evidence in the Earth's geological record that there was liquid water — and life — on the planet at the time.

Over the past few decades, scientists have proposed several possible mechanisms that may have kept the Earth toasty enough to keep water from freezing during our planet's early history – a period of time called the Archaean. But just when scientists think they have the paradox solved, other researchers come up with alternate explanations or reasons why a previous proposal doesn't work.

"It keeps resurfacing," said atmospheric scientist Jim Kasting of Penn State University, who put forward his own explanation for the young sun paradox in 1980s and '90s. That explanation involved a greenhouse gas effect that would have kept the planet warm — similar to the human-driven effect that is warming the Earth today. The early greenhouse, first proposed by other scientists in the 1970s, would have been on a much larger scale than current climate warming, with theoretical calculations suggesting that about 30 percent of the Earth's atmosphere at the time consisted of carbon dioxide. For comparison, today, Earth's atmosphere is about 0.038 percent carbon dioxide.

A powerful greenhouse effect on the early Earth is "the obvious solution" to the paradox, said Minik Rosing of the University of Copenhagen in Denmark. Rosing and his colleagues have offered up a new explanation for the seeming paradox that is detailed in the April 1 issue of the journal Nature.

Carbon dioxide constraints

To see what carbon dioxide (CO2) concentrations might have actually been in the Archaean, Rosing and his team analyzed samples of 3.8-billion-year-old mountain rock from the world's oldest  sedimentary rock, called Isua, in western Greenland.

The samples contain features called banded iron formations (BIFs) that formed in abundance when the Earth was young, but not since. These BIFs contain certain iron-rich minerals that give clues as to the atmospheric environment in which they formed.

"The analyses of the CO2 content in the atmosphere, which can be deduced from the age-old rock, show that the atmosphere at the time contained a maximum of one part per thousand of this greenhouse gas. This was three to four times more than the atmosphere's CO2 content today. However, not anywhere in the range of the 30 percent share in early Earth history which has hitherto been the theoretical calculation," Rosing said.

So Rosing and his colleagues looked at another avenue that could explain the paradox.

All about albedo

One of the factors that partly determines the temperature of the Earth is the amount of incoming sunlight the Earth's surface and atmosphere reflect back to space, called the planet's albedo. Different types of surfaces reflect or absorb different amounts of light — for example, ice is highly reflective, while the open ocean is highly absorptive.

Rosing and his team looked at two possible influences on the early Earth's albedo: the amount of land on the planet's surface and the amount of cloud cover in the atmosphere.

Geologists haven't yet determined when the Earth's continents first formed, but radioactive tracers in the hot rock of the Earth's mantle can help determine the rate at which the crust of the planet formed, hinting at how much land was exposed above the oceans.

Rosing and his colleagues suggest that there was less continental area on the early Earth, and because oceans are more absorptive of sunlight than land, the Earth's albedo would have been slight lower, meaning the Earth's surface would have absorbed slightly more sunlight than it does today.

A bigger effect might have been the thinner cloud cover of the early Earth, which could have allowed more sunlight through the atmosphere to reach the surface.

"The reason for the lack of cloud [cover] back in Earth's childhood can be explained by the process by which clouds form," Rosing said.

The droplets of water that make up clouds form by glomming on to tiny particles, called cloud condensation nuclei, many of which are chemical substances produced by algae and plants, which weren't present on the Earth at that time.

Rosing and his team came to this conclusion by observing areas of the present-day ocean that have very little biological activity and thin cloud cover, which "shows that the clouds are different in such places" and therefore were likely the same for the early Earth.

Any clouds that did form would have had larger drops — as happens when cloud condensation nuclei are in low supply ­— which are more transparent to sunlight and so would have allowed more through to reach the Earth's surface, keeping it warm.

So the combination of less continental area and an atmosphere more transparent to sunlight could explain why the Earth didn't freeze over, despite the smaller amount of sunlight.

But this explanation may not settle the paradox for all scientists who have looked into the problem.

Potential controversy

Kasting, who wrote an accompanying editorial piece to the new study also appearing in Nature, had several critiques of the explanation to the paradox.

The part of the study he found most interesting was the analysis of the BIFs to determine the amount of carbon dioxide in the ancient atmosphere.

"But I think that's going to be controversial," Kasting told SPACE.com, as other researchers have looked at the same rock and come to the completely opposite conclusion about the carbon dioxide content, suggesting that it contained substantially more than Rosing and his team concluded.

To figure out the issue once and for all, geochemists need to come up with a model that explains how the BIFs formed, something that has been missing from the equation up until now.

Kasting also wasn't sure that thinner cloud layer could explain the paradox.

"I'm not that sold on the cloud-feedback mechanism," he said. In part this is because the temperature that the thinner clouds would boost Earth up to isn't as warm as scientists think the Earth was during the Archaean, he said. "It just barely gets you up to the freezing point."

Rosing counters though that not all scientists agree with the evidence that has been used to suggest that the early Earth was a very warm place.

So while the new research provides a plausible explanation for what kept the early Earth from freezing over, the paradox isn't likely to be declared solved anytime soon.

"We keep solving it, and someone comes along and tells you that you haven't solved it right," Kasting said. Still, other studies are already in the works with other possible explanations for the young sun paradox, he added.

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