A thick organic haze cloaked early Earth several billion years ago and may have kept the planet from freezing over, protecting primordial life from the damaging effects of the sun's ultraviolet rays, a new study suggests.
The haze, made from methane and nitrogen chemistry in the upper atmosphere, would have been analogous to the cloudy curtain hovering above Saturn's largest moon, Titan, the researchers say.
The results help solve a longstanding mystery called the faint young sun paradox: While geological evidence suggests early Earth was ice-free, climate models haven't been able to get the planet warm enough for such a wet, toasty world.
"Since climate models show early Earth could not have been warmed by atmospheric carbon dioxide alone because of its low levels, other greenhouse gases must have been involved," said lead researcher Eric Wolf, a doctoral student at the University of Colorado at Boulder. "We think the most logical explanation is methane, which may have been pumped into the atmosphere by early life that was metabolizing it."
The findings are published in the June 4 issue of the journal Science.
During the Archean period some 3.8 billion to 2.5 billion years ago, the sun's output may have been between 20 percent and 30 percent fainter than today, meaning fewer rays, Wolf said. However, previous work by other scientists suggests Earth's surface temperatures were as warm or warmer than today.
Scientists have long tried to figure out how our planet was warmed. In the 1970s, Carl Sagan and George Mullen proposed early Earth's atmosphere was full of methane and ammonia and that did the warming trick. This idea fell out of favor in the 1980s and early 1990s, when scientists figured it was actually a carbon dioxide-rich atmosphere that warmed Earth. That also fell out of favor when geological evidence began showing there was a limit to how much carbon dioxide could've been in the atmosphere.
Then, about a decade ago scientists suggested a methane-rich atmosphere kept Earth toasty. The problem: A mix of methane and nitrogen produces a haze that was first thought to cause significant cooling. But in this "cooling" model, the haze particles were assumed to be spheres.
That probably wasn't the case, as Wolf and CU colleague Owen B. Toon found out. They ran computer simulations using a climate model from the National Center for Atmospheric Research and concepts about Titan's odd haze learned from lab studies by another CU group.
Laboratory studies show that the haze enshrouding early Earth was made up of irregular "chains" of aggregate particles whose geometric sizes were larger than spheres. The particle shapes actually seemed to match those of aerosols believed to populate Titan's dense atmosphere.
They were likely fluffy looking fractals. The fractal nature of the particles means the haze would have sufficiently shielded Earth from UV light and allowed gases like ammonia to build up, causing greenhouse warming and perhaps helping to keep the planet from freezing over.
"Without a UV shield ammonia is destroyed quickly from high-energy photons," Wolf told LiveScience.
The fractal particles also would've let visible wavelengths pass through to warm the planet.
In order for the fractal haze to pull this warming off, Wolf's team estimated about 100 million tons of haze were produced annually in the atmosphere of early Earth during the Archean.
"If this was the case, an early Earth atmosphere literally would have been dripping organic material into the oceans, providing manna from heaven for the earliest life to sustain itself," Toon said.
"In our model the haze requires methane. It is easy to get methane in our model if you assume we are starting at post-biotic Earth," Wolf said. "If we step back into the pre-biotic Earth it is a bit harder to find where this methane is coming from."
That's the "big question mark now," Wolf said. If they do find an abiotic source of methane, such as from volcanoes or deep-sea ridges, that would be "very exciting," he added.
Even while looking into the future, Wolf points out the findings bring scientists back to Sagan's ideas.
"This study is a step forward but it also has led us back to our original ideas," Wolf said.