Hazy antidote to a faint young sun

How do you keep a chilly youngster warm? Wrap it in a blanket, of course. Turns out that a blanket of haze might have warmed Earth during its infancy, when the sun was substantially dimmer. That might have kept the planet from freezing, as some studies suggest its fate would have been otherwise.

Theories abound about why Earth didn’t freeze during the first half of its history, even though the sun was about 30 percent dimmer then than it is today. Just over a month ago, one team suggested that the planet kept warm billions of years ago by absorbing a larger fraction of the sun’s radiation (SN: 4/24/10, p. 11). In that early era, the researchers argued, light-colored continents were much smaller and light-scattering clouds covered less of Earth’s surface, so that, on the whole, Earth was darker.

Now comes a totally different idea — a planet-cloaking layer of hydrocarbon haze. Previous studies have suggested that such haze, generated by light-driven chemical reactions in the methane-rich atmosphere, would actually cool Earth, but this haze is different, says Eric T. Wolf, an atmospheric scientist at the University of Colorado at Boulder. Earlier teams have mistakenly assumed that individual haze particles would be spherical droplets, but Wolf and CU-Boulder colleague Brian Toon suggest in the June 4 Science that the haze particles wafting through Earth’s early atmosphere actually were large clumps of small particles arranged in long, branching chains — the same sort of haze that might swaddle Saturn’s moon Titan today.

Each of the small particles in a clump was probably about 50 nanometers across, about the size of a large airborne virus. Particles of that size don’t block visible or infrared light efficiently but do a really good job of blocking ultraviolet light, so the layer of haze — which probably sat at an altitude of around 20 kilometers — slowed photochemical reactions in the lower atmosphere. That, in turn, allowed ammonia, a strong greenhouse gas, to accumulate at low altitudes, where it could block outgoing infrared radiation, thereby warming the atmosphere.

Chemical reactions in Earth’s early atmosphere — predominantly nitrogen and methane — could have generated enough ammonia to keep Earth from freezing, says Wolf. A concentration as small as one ammonia molecule for each 100,000 molecules of gas in the atmosphere would have been enough to do the job.

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.