You can't rise from the primordial ooze if that ooze is frozen. But about three billion years ago the sun was around thirty percent dimmer, meaning our planet should have been a snowball. The puzzle has haunted scientists for decades, but a study in Science has a new answer: It argues that a dense cloud of "fractal haze" enveloped the Earth. Old Theories This isn't the first attempt to solve the early Earth conundrum. Carl Sagan, for one, had a few ideas. First, in 1972, he speculated that the atmosphere had ammonia which could trap heat, but later work showed that the sun's ultraviolet radiation would have broken that ammonia down. In 1996 he tried again, saying that Earth might have had a thick haze, perhaps a nitrogen-methane mix, that blocked the ultraviolet but let in enough of the sun's then-meager rays to warm the planet. Unfortunately, that too was a no go:
Early models assumed the haze particles were spheres, and that when individual particles collided, they globbed together to make bigger spheres. These spheres blocked visible light as well as ultraviolet light, and left the Earth’s surface even colder. “It basically led us to a dead end where we couldn’t have a warm early Earth,” said Eric Wolf, a graduate student in atmospheric sciences at the University of Colorado at Boulder and the first author of the new study. [Wired]
This Theory The perfect haze was not too sparse (since it needed to provide some UV-protection for developing life), and not too dense (because then the planet would have been dark and cold). Just right, the new study suspects, might have been hydro-carbon clouds of what the study's authors call fractal haze. Unlike the spherical haze particles, fractal haze is made of long chains of particles stuck together.
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 [Space.com]
This theory also allows Sagan's ammonia, protected from the UV, to exist in the atmosphere. The famous Miller-Urey experiment--in which scientists sparked what they believed comprised the earth's early atmosphere and made amino acids, life's building blocks--assumed that the young planet had ammonia to work with. Alternate Theories? But in April, scientists proposed another answer: that the young earth had smaller land masses and so reflected fewer of the sun's rays, absorbing more heat. But it's possible that the theories can work together.
Rather than being an alternate explanation to last month’s theory about how Earth stayed warm under a faint young sun, the newly proposed haze layer may actually be a complement to it, says Wolf. Researchers who conducted that study didn’t include a haze layer, which probably would have helped keep their darker world warm enough to prevent water at Earth’s surface from freezing. Future research could clarify the issue, Wolf notes. [Science News]
This all means that Earth's baby picture probably looked a lot like a current shot of Saturn's moon Titan (shown above). Related content: 80beats: Our Alien Atmosphere? Earth's Gases May Have Arrived Here Aboard Comets Bad Astronomy: When did Earth's Oxygen atmosphere appear? Bad Astronomy: Titan's shadow
Image Credit: NASA/JPL/Space Science Institute