November 12, 2015 – 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.
Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.
In new research from the University of Washington-based Virtual Planetary Laboratory, UW doctoral student Giada Arney and co-authors chose to study Earth in its Archean era, about 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 and scientific data from other researchers, including scientists from the University of Colorado Boulder, 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, organic compounds of hydrogen and carbon.
“Hazy worlds seem common both in our solar system and in the population of exoplanets we’ve characterized so far,” Arney said. “Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself.”
Arney and co-authors presented their findings this week at the American Astronomical Society’s Division of Planetary Sciences conference in National Harbor, Maryland.
The researchers used photochemical, climate and radiation simulations to examine the early Earth shrouded by a “fractal” hydrocarbon haze, meaning that 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 also 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 ultraviolet 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.
The researchers also found that, 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 carbon dioxide 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.”
Co-author Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland, 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.’”
The research was funded through the NASA Astrobiology Institute.
Arney’s UW co-authors are Victoria Meadows, professor of astronomy and director of the Virtual Planetary Laboratory, and doctoral student Edward Schwieterman and postdoctoral researcher Benjamin Charnay. Other co-authors are Domagal-Goldman, Eric Wolf of the University of Colorado at Boulder and Mark Claire of the University of St. Andrews in the UK and Seattle’s Blue Marble Space Institute of Science.
The theory of early Earth being shrouded by a gaseous blanket containing methane and ammonia first arose in the 1960s and was subsequently discarded by scientists. In the 1970s and 1980s some scientists suggested the early Earth atmosphere was similar to those on Mars and Venus with lots of carbon dioxide, another theory that eventually went by the wayside. Since CO2-rich atmospheres do not produce organic molecules easily, scientists began looking in deep-sea volcanic vents and at wayward asteroids to explain early Earth life.
A 1997 paper by the late Carl Sagan of Cornell University and Christopher Chyba, then at the University of Arizona, proposed that an organic aerosol shield in early Earth’s atmosphere would have protected the ammonia wafting beneath it, allowing heating to occur at Earth’s surface. But the authors proposed the haze particles were spherical and did not consider methane to be the driver of the system, eventually sinking that theory.
In 2010, a study by then CU-Boulder doctoral student Eric Wolf and Professor Brian Toon of the atmospheric and oceanic sciences department published a study that showed a thick organic haze that enshrouded early Earth several billion years ago may have been similar to the haze now hovering above Saturn’s largest moon, Titan, and would have protected primordial life on the planet from the damaging effects of ultraviolet radiation.
The CU Boulder scientists believed the haze was made up primarily of methane and nitrogen chemical byproducts created by reactions with light. Not only would the haze have shielded early Earth from UV light, it would have allowed gases like ammonia to build up, causing greenhouse warming and perhaps helped to prevent the planet from freezing over. The researchers determined the haze of hydrocarbon aerosols was probably made up of fluffy, microscopic particles shaped somewhat like cottonwood tree seeds that would have blocked UV but allowed visible light through to Earth’s surface.
CU-Boulder researchers estimated there were roughly 100 million tons of haze 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.