7 November 2006

Hazy origins of life on Earth

Cosmos Online
An organic, hazy sky may have given rise to life on Earth, according to U.S. scientists inspired by the atmosphere on Titan, Saturn's largest moon.
Hazy origins of life on Earth

The hazy, organic sky of Saturn's largest moon, Titan ... a similar atmosphere may have given rise to life on Earth. Credit: NASA

SYDNEY: An organic, hazy sky may have given rise to life on Earth, according to U.S. scientists inspired by the atmosphere on Titan, Saturn’s largest moon.

“We found that an organic haze might have been a dominant feature of the early Earth atmosphere,” said Margaret Tolbert, from the University of Colorado, co-author of the study published today in the U.S. journal Proceedings of the National Academy of Sciences.

About 2.3 billion years ago, atmospheric conditions on Earth changed drastically when oxygen-releasing organisms evolved. However, the question of what conditions were like prior to this change has always sparked scientific debate.

In this new study, the atmosphere on Saturn’s largest moon, Titan, inspired the scenario proposed for early Earth.

In 1980, U.S. space agency NASA’s Voyager spacecraft sent back the first close-up images of Titan. They showed a thick, orange haze obscuring the surface and further analysis revealed methane, nitrogen and organic constituents in the atmosphere.

Tolbert and colleagues mimicked Titan’s hazy skies in their lab by exposing methane gas to an ultraviolet lamp. The UV light breaks down methane, leaving its components free to react with other molecules, forming larger organic molecules. The researchers studied the chemical composition, shape and size of the particles formed.

They then added carbon dioxide gas to the mix to recreate conditions on early Earth and determine whether they could have produced an organic haze similar to Titan’s.

“It turns out that organic haze can form over a wide range of methane and carbon dioxide concentrations,” said Tolbert. “This means that hazy conditions could have been present for many millions or even a billion years on Earth while life was evolving.”

After the large, organic molecules formed out of methane, they would have floated down to the Earth’s surface, according to the researchers.

They calculated that 100 million kilograms of organic material could have been produced each year on the early Earth. These molecules could have been the primary food source for the life that was beginning to emerge.

According to the scientists, this amount of organic material would have allowed life to flourish globally, not just in localised environments as predicted by other theories for the origin of life.

“Previous efforts to understand early life on Earth have focused on extreme environments like hydrothermal vents, where energy and nutrients are plentiful,” said Tolbert. In her paper, Tolbert explains how an atmospheric haze would have delivered more organic material to the early Earth than thermal vents or asteroid collisions.

In addition to serving as a source of organic material, a haze layer over Earth could have shielded living organisms from harmful UV rays and helped to regulate Earth’s early climate, according to the study.

The organic haze would not have existed beyond the rise of oxygen. As the amount of oxygen in the atmosphere increased, methane would have become much less abundant, and eventually the production of organic molecules would have stopped.

Although Titan inspired this investigation into the origins of life on Earth, the moon could not support life itself, according to the researchers. The temperature on Titan is a chilly minus 180 degrees Celsius, so any water would be completely frozen.

“Most scientists agree that life as we know it would not be possible on Titan,” Tolbert said.


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