For years scientists have debated what could have kept Earth warm enough to prevent the oceans from freezing solid. Now a team of researchers from Tokyo Institute of Technology and University of Copenhagen's Department of Chemistry have coaxed an explanation out of ancient rocks, as reported in the journal PNAS

A perfect greenhouse gas
"The young sun was approximately 30 percent weaker than it is now, and the only way to prevent Earth from turning into a massive snowball was a healthy helping of greenhouse gas," Associate Professor Matthew S. Johnson of the Department of Chemistry explains. He has found the most likely candidate for an Archean atmospheric blanket is carbonyl sulphide, a product of the sulphur disgorged during millennia of volcanic activity.

"Carbonyl sulphide is and was the perfect greenhouse gas. Much better than carbon dioxide. We estimate that a blanket of carbonyl sulphide would have provided about 30 percent extra energy to the surface of the planet, and that would have compensated for what was lacking from the sun," says Johnson.

Strange distribution
To discover what could have helped the faint young sun warm early Earth, Johnson and his colleagues in Tokyo examined the ratio of sulphur isotopes in ancient rocks.They saw a strange signal: A mix of isotopes that couldn't have come from geological processes.

"There is really no process in the rocky mantle of Earth that would explain this distribution of isotopes," says Johnson. "You would need something happening in the atmosphere."

Painstaking experimentation helped the researchers find a likely atmospheric process. By irradiating sulphur dioxide with different wavelengths of sunlight, they observed that sunlight passing through carbonyl sulphide gave them the wavelengths that produced the weird isotope mix.

"Shielding by carbonyl sulphide is really a pretty obvious candidate once you think about it, but until we looked, everyone had missed it," says Johnson. "What we found is really an archaic analogue to the current ozone layer, a layer that protects us from ultraviolet radiation. But unlike ozone, carbonyl sulphide would also have kept the planet warm.

The only problem is, it didn't stay warm."

Life caused Ice Age
As life emerged on Earth, it produced increasing amounts of oxygen. With an increasingly oxidizing atmosphere, the sulphur emitted by volcanoes was no longer converted to carbonyl sulphide. Instead it was converted to sulphate aerosols, which are powerful climate coolants.

Johnson and his co-workers created a computer model of the ancient atmosphere. In conjunction with laboratory experiments, the model suggests that the fall in carbonyl sulphide and rise of sulphate aerosols taken together would have been responsible for creating Snowball Earth, the planet-wide Ice Age hypothesised to have taken place near the end of the Archean era 2.5 billion years ago.

The implications to Johnson are alarming. "Our research indicates that the distribution and composition of atmospheric gasses swung the planet from a state of life-supporting warmth to a planet-wide Ice Age spanning millions of years. I can think of no better reason to be extremely cautious about the amounts of greenhouse gasses we are currently emitting to the atmosphere."

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Tokyo Institute of Technology
University of Copenhagen's Department of Chemistry
Explore The Early Earth at TerraDaily.com

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