Titan's Skies Shed Light on Early Earth

Nov. 7, 2006 — Scant evidence has been left behind about life's rise and spread on Earth, but scientists re-creating a bit of the planet's primordial atmosphere in laboratory experiments have found a key clue by studying another place in the solar system: Saturn's giant moon Titan.

"It's been our model," said University of Colorado researcher Margaret Tolbert, who headed a team of scientists who brewed a tiny sample of atmospheric gases that could have enveloped early Earth.

Taking a cue from Titan, which is blanketed by a heavy shroud of organic haze, Tolbert and her team believe a similar phenomenon could have been triggered on Earth, even with high amounts of carbon dioxide.

Previous theories discounted the chance of planetary haze, which could have served as building blocks for life on Earth because of the presence of carbon dioxide, Tolbert said.

But an unusual laboratory experiment proved otherwise.

Tolbert and her colleagues recreated Titan's hazy skies by exposing methane gas to an ultraviolet lamp, then added carbon dioxide gas to see if conditions probably present on early Earth would have produced a similar organic haze.

"It turns out that organic haze can form over a wide range of methane and carbon dioxide concentrations," said Tolbert, who reported her findings in this week's Proceedings of the National Academy of Sciences.

"This means that hazy conditions could have been present for many millions or even a billion years on Earth while life was evolving," she said.

The haze circling Titan — and possibly Earth in its early years — is due primarily to ultraviolet radiation from the sun chemically reacting with methane in the atmosphere.

Scientists are not sure how methane is being replenished in Titan's heavy skies — the gas cannot exist long-term even in places as cold as Saturn — but ancient Earth's supply could have been produced by Earth's earliest life forms: microscopic organisms known as methanogens.

Tolbert estimates that more than 100 million tons of organic material could have been generated in the atmosphere, providing a global food source for living organisms.

The study provides an alternative explanation for understanding early life on Earth. Current theories suggest life existed in extreme environments, such as around hydrothermal vents, which would have provided an energy source and nutrients.

Tolbert's work suggests habitable zones might have been much more widespread.

"Once life started you wouldn't necessarily need these special environments," Tolbert said. "It could thrive everywhere, it could thrive in a puddle. (The haze) is biologically high-energy and it's global. It opens up new possibilities."

A chemical record of the haze may be locked in bits of ancient rocks, added Alexander Alexander Pavlov, with the University of Arizona.

The researchers plan to expand their experiments to include other gases that could have been present in Earth's early atmosphere.