How Early Earth Got Warm and Hospitable
Our planet might have kept warm in the super-ancient past when the sun was substantially dimmer than it is today because of a complex brew of global warming gases much like that now enveloping Saturn's moon Titan, scientists reveal.
These new findings could also shed light on how the building blocks of life might have formed on Earth.
When the sun was young, models suggest it was just 70 percent as bright as it is now. However, during the first two billion years or so of Earth's history, the surface of the planet was warm enough for glaciers to not form and early life to emerge.
Scientists including Carl Sagan have proposed a number of possible solutions to this apparent "faint young sun paradox." These generally involve atmospheres with greenhouse gases that trap heat to insulate the Earth, ones far more powerful than the carbon dioxide and water vapor that help keep our planet warm today.
However, these ideas have had various drawbacks ? ultraviolet rays would quickly destroy the greenhouse gas ammonia, for instance, making it irrelevant, while a nitrogen-methane mix seemed to prevent enough visible light from substantially heating the Earth.
Now researchers propose that a haze of nitrogen and carbon-loaded organic compounds similar to that currently seen on Titan might have done the job if a significant portion of the organic particles clumped together into larger, complex structures. The smallest, spherical particles would interact with the shortwave, ultraviolet radiation, while the larger, fluffy structures made out of the smaller particles would affect longer, visible wavelengths.
The end result of this arrangement, dubbed a fractal size distribution, would be an aerosol haze opaque enough to block the shortwave ultraviolet radiation that would have hindered or prevented life from arising. At the same time, it would have proven transparent enough in longer, visible wavelengths to let them keep the atmosphere warm and the planet wet enough for life to emerge.
"It's surprising that molecules with complex shapes could make such a difference," said researcher Eric Wolf, an atmospheric scientist at the University of Colorado, Boulder.
The smaller particles, by shielding against ultraviolet rays, would also end up protecting ammonia, which could then serve as a potent greenhouse gas. Intriguingly, ammonia could also play an important role in creating the primordial soup from which life originated ? experiments nearly 60 years ago revealed that when ammonia and methane were exposed to electric sparks, amino acids and other building blocks of life could form.
"This idea fell out of favor because ammonia is unstable in the presence of ultraviolet rays, and astrobiologists have in the last 15 to 20 years been looking more at hydrothermal vent systems for the creation of complex organic compounds and thus life," Wolf said. "Our model allows ammonia to exist, which could have permitted interesting organic chemistry to take place in the atmosphere."
"The idea of ammonia in the atmosphere to help solve the faint young sun paradox goes back almost 40 years now, and this research suggests it is still a viable idea," said planetary scientist Christopher Chyba at Princeton University, who did not take part in this study. "The fact that it took us until 2010 to model the nature of the organic haze carefully is a little sobering, and it's a reminder that a little humility is in order here if we think we've got the theory down now. There are probably some other theoretical surprises in store for us."
Another
model that could help explain the faint young more of the early
Earth's surface was covered with ocean
"When
it comes to the big picture, this is a reminder of the value of planetary
exploration, since a better understanding of Titan and its haze could help
provide a model for the early Earth," Chyba noted.
Wolf and his colleague Brian Toon detailed their findings in the
June 4 issue of the journal Science.
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