The primordial haze might have fed and protected early life

Nov 8, 2006 15:50 GMT  ·  By

A haze of atmospheric chemicals like those found on Saturn's giant moon Titan might have been a major source of chemical reactions that lead to life burst on Earth.

Titan is wrapped in a smog like orange atmosphere (photo) formed by a cocktail of chemicals created by UV sunlight interacting with methane and nitrogen high above the surface. Scientists think that early Earth supported a similar atmosphere. "This thick haze completely shrouds Titan, so all you can see in the photographs is orange," said Margaret Tolbert, a chemist at the University of Colorado at Boulder.

In lab experiments, researchers exposed methane to ultraviolet light, like in the hazy atmosphere of Titan, but added carbon dioxide, a major constituent of the Earth's primordial atmosphere, to see if early Earth conditions could produce a similar haze. "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."

Researchers had previously believed that carbon dioxide, absent in Titan's atmosphere, would impede the reactions to complex molecules. Instead, it diversified the final product. "Contrary to the early predictions, you get more haze with carbon dioxide," Tolbert said.

"You also get a richer broth of chemicals, including oxygen-containing molecules. They have more energy," she added. "Microorganisms can eat them, so they can be a food source."

Thus, Earth's early atmosphere may have been a rich food source for the earliest organisms. Titan's orange smog is also a rich stew of organic chemicals. But in Titan, the organic chemicals are not used by a biotic system. "Titan is just too far away from the sun to be warm," Tolbert said. "The way we understand life is that there needs to be liquid water, and there's just no possibility of liquid water on Titan."

This new observation shifts once again our ideas about life origins on Earth. The main theories say early life concentrated in scattered environments, like underwater hot springs (hydrothermal vents), where high quantities of energy-containing chemicals lifted up from Earth's interior. But this theory suggests that food might have arrived from the sky with any rainfall. "You can envision an environment where life is thriving in every little puddle, it's a very different picture." Tolbert said.

The haze had started forming before life appeared, thus it added chemicals necessary for life's creation. "But that's tricky to determine, Tolbert says, because scientists aren't sure how much methane was around at the time."

Once appeared, however, early microbes were able to generate methane, that autofueled their food chain. The primordial haze could have delivered food to the biosphere more than 100 million tons of organic material yearly during millions or maybe billions of years. "As these particles settled out of the skies, they would have provided a global source of food for living organisms," said Melissa Trainer, also from the University of Colorado at Boulder.

But the haze also protected early Earth's life from harmful UV radiation and regulated climate by warming at a time when the sun was much weaker than today, thus enhancing conditions for life. "These new results show that a haze layer might have been formed on Earth before the origin of life, and that that layer could have shielded the lower atmosphere and Earth's surface from ultraviolet light-as Carl Sagan and I suggested in 1997" said Christopher Chyba, an astrophysicist at Princeton University in New Jersey.

This theory claims that primordial haze allowed ultraviolet-sensitive, planet-warming gases (in particular, methane and ammonia) to persist in the lower atmosphere. "But the question of whether the overall outcome was to warm the surface or cool it depends on the size of the haze particles," Chyba said.

Photo credit: NASA