Early Life Fed on Organic Haze, Study Suggests
Richard A. Lovett
for National Geographic News
|November 6, 2006|
A haze of atmospheric chemicals similar to those now found on Saturn's giant moon Titan might have been a major source of food for ancient life on Earth, a new study suggests.
Scientists have long been fascinated with Titan, which is shrouded by a murky orange atmosphere of smog-like chemicals created as sunlight interacts with methane high above the surface. (Related: "Saturn Moon Has Seas of Sand, Images Reveal" [May 4, 2006].)
"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 and a study co-author.
Tolbert's team wondered if similar chemicals might have formed in early Earth's atmosphere, which at various times also contained methane. So they mixed methane and carbon dioxide—a major constituent of the Earth's primordial atmosphere—in a reaction chamber and exposed it to simulated sunlight.
Researchers had previously thought that carbon dioxide—which is not present in Titan's atmosphere—would stifle the reactions that produce complex molecules.
Instead, Tolbert's team found that their simulated atmosphere produced a stew of organic chemicals.
"Contrary to the early predictions, you get more haze with carbon dioxide," she 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."
In other words, the work suggests that Earth's early atmosphere may have been a veritable banquet for the earliest single-celled organisms.
Tolbert's team reports its findings this week in the journal the Proceedings of the National Academy of Sciences.
Titan's Big Chill
Titan's orange smog is also a rich stew of organic chemicals. But that doesn't mean that these chemicals might be feeding primordial microorganisms on the moon's surface.
"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."
But in a science fiction story in the November 2006 issue of Analog Science Fiction and Fact, Carl Frederick, a New York-based physicist and engineer, notes that Titan will warm considerably billions of years from now when the sun expands to become a red giant star.
"When the sun goes into its red giant phase, Titan suddenly becomes habitable," Frederick told National Geographic News. "Then all of these wonderful chemicals that are now largely frozen become active, and it's like primordial Earth."
That, he says, gives Saturn's now-frigid moon all of the elements it needs for life. (Related: Saturn wallpaper.)
The only problem is that the red giant phase will only last a few hundred million years—not long compared to the billions of years that life on Earth has been evolving.
Scientists are still analyzing data from the European Space Agency's Huygens probe—which parachuted to the moon's surface on January 14, 2005—for insights into Titan's geology and chemistry. (Related: "Huygens Sends Images of Titan" [January 17, 2005].)
New Take on Early Life
The new findings may force scientists to revise their ideas about life on early Earth.
Most traditional theories say life existed only in scattered environments, such as underwater hot springs (hydrothermal vents), where concentrated amounts of energy-containing chemicals emerge from Earth's interior.
But the new theory suggests that food might have washed out of the sky with each rainstorm.
"You can envision an environment where life is thriving in every little puddle," Tolbert said. "It's a very different picture."
The haze may even have started forming before life arose, helping to contribute the chemicals needed 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 life appeared, however, early microorganisms began generating methane that could react with the sunlight to produce even more food.
For millions or even billions of years, Tolbert says, this process might have produced "a lot" of organic material—perhaps a hundred million tons a year.
Hot Side Effect?
The organic haze may also have helped warm early Earth at a time when the sun was considerably fainter than it is now, further enhancing conditions for widespread life and supporting a theory proposed in 1997 by Carl Sagan and Christopher Chyba.
"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 and I suggested," said Chyba, now an astrophysicist at Princeton University in New Jersey.
According to the theory, haze would allow 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.
"Some of the authors of this new work have previously argued for cooling, but their new experimental results show that cooling might not have happened after all. It's still too early to know for sure."
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