How Early Earth Got Warm and Hospitable

Our planet might have kept warm in the super-ancient past when thesun was substantially dimmer than it is today because of a complex brew ofglobal warming gases much like that now enveloping Saturn's moon Titan,scientists reveal.

These new findings could also shed light on how the buildingblocks of life might have formed on Earth.

When the sun was young, models suggest it was just 70 percent asbright as it is now. However, during the first two billion years or so ofEarth's history, the surface of the planet was warm enough for glaciers to notform and earlylife to emerge.

Scientists including Carl Sagan have proposed a number of possiblesolutions to this apparent "faintyoung sun paradox." These generally involve atmospheres withgreenhouse gases that trap heat to insulate the Earth, ones far more powerfulthan the carbon dioxide and water vapor that help keep our planet warm today.

However, these ideas have had various drawbacks ? ultraviolet rayswould quickly destroy the greenhouse gas ammonia, for instance, making itirrelevant, while a nitrogen-methane mix seemed to prevent enough visible lightfrom substantially heating the Earth.

Nowresearchers propose that a haze of nitrogen and carbon-loaded organic compoundssimilar to that currently seen on Titanmight have done the job if a significant portion of the organic particlesclumped together into larger, complex structures. The smallest, sphericalparticles would interact with the shortwave, ultraviolet radiation, while thelarger, fluffy structures made out of the smaller particles would affectlonger, visible wavelengths.

The end result of this arrangement, dubbed a fractal sizedistribution, would be an aerosol haze opaque enough to block the shortwaveultraviolet radiation that would have hindered or prevented life from arising.At the same time, it would have proven transparent enough in longer, visiblewavelengths to let them keep the atmosphere warm and the planet wet enough forlife to emerge.

"It's surprising that molecules with complex shapes couldmake such a difference," said researcher Eric Wolf, an atmosphericscientist 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 potentgreenhouse gas. Intriguingly, ammonia could also play an important role increating the primordial soup from which life originated ? experiments nearly 60years ago revealed that when ammonia and methane were exposed to electricsparks, amino acids and other building blocks of life could form.

"This idea fell out of favor because ammonia is unstable inthe presence of ultraviolet rays, and astrobiologists have in the last 15 to 20yearsbeen looking more at hydrothermal vent systems for the creation of complexorganic compounds and thus life," Wolf said. "Our model allowsammonia to exist, which could have permitted interesting organic chemistry totake place in the atmosphere."

"Theidea of ammonia in the atmosphere to help solve the faint young sun paradoxgoes back almost 40 years now, and this research suggests it is still a viableidea," said planetary scientist Christopher Chyba at Princeton University,who did not take part in this study. "The fact that it took us until 2010to model the nature of the organic haze carefully is a little sobering, andit's a reminder that a little humility is in order here if we think we've gotthe theory down now. There are probably some other theoretical surprises instore for us."

Anothermodel that could help explain the faint young more of the earlyEarth's surface was covered with ocean

"Whenit comes to the big picture, this is a reminder of the value of planetaryexploration, since a better understanding of Titan and its haze could helpprovide a model for the early Earth," Chyba noted.

Wolf and his colleague Brian Toon detailed their findings in theJune 4 issue of the journal Science.

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