An image of Saturn’s haze-shrouded moon Titan taken by the Cassini spacecraft. The UW-based Virtual Planetary Laboratory studied records of the haze on early Earth to see how such atmospheric conditions might affect an exoplanet, or one beyond our solar system. They found that such a haze might show the world is habitable, or that life itself is present. Credit: NASA
An atmospheric haze around a faraway planet – like the one which probably shrouded and cooled the young Earth – could show that the world is potentially habitable, or even be a sign of life itself. In new research, UW doctoral student Giada Arney and team chose to study Earth in its Archean era 2 ½ billion years back, because it is, as Arney said, “the most alien planet we have geochemical data for.”
The work builds on geological data from other researchers that suggests the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex hydrocarbons.
“Hazy worlds seem common both in our solar system and in the population of exoplanets we’ve characterized so far,” Arney said. They used photochemical, climate and radiation simulations to examine the early Earth shrouded by a “fractal” hydrocarbon haze, ie the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together not unlike grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature. However, they found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. They saw that this combination would result in a moderate, possibly habitable average global temperature.
Such a haze, the researchers found, also would have absorbed UV light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.
Based on the early Earth data, it’s unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earthlike amounts of CO2 in their atmospheres, Arney said, “organic haze might be a novel type of biosignature. However, we know these hazes can also form without life on worlds like Saturn’s moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones.”
Shawn Domagal-Goldman of the NASA Goddard Space Flight Center said, “Giada’s work shows that the haze could have intertwined with life in more ways than we previously suspected.” Arney added that astronomers often think of Earthlike exoplanets as “pale blue dots” – after a famous photo of Earth taken by the Voyager spacecraft – “but with this haze, Earth would have been a ‘pale orange dot.'” http://www.washington.edu/news/2015/11/12/pale-orange-dot-early-earths-haze-may-give-clue-to-habitability-elsewhere-in-space/
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