Researchers have made the mystery of why life on Earth developed when it did even more complicated.
A research team ruled out the theory that would explained how the Earth was warm enough to support life even in the early days when the Sun was only about three-quarters of the strength it is now, a University of Manchester press release reported.
Life is believed to have popped up on Earth during the Archean, which took place between 2.4 billion and 3.8 billion years ago. This timeframe has puzzled scientists for ages because during that time the planet should have been too cold to support living organisms. Scientists have been working to solve this contradiction, which is often referred to as the "faint, young Sun paradox."
"During the Archean the solar energy received at the surface of the Earth was about 20 to 25 [percent] lower than present," study author, Doctor Ray Burgess, from Manchester's School of Earth, Atmospheric and Environmental Sciences, said. "If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread."
One possible explanation for the phenomenon is that the climate-regulating greenhouse gas levels could have been much higher during the Archean.
"To counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth's atmosphere would need to have been 1,000 times higher than present," lead author Professor Bernard Marty, from the CRPG-CNRS University of Lorraine, said. "However, ancient fossil soils - the best indicators of ancient carbon dioxide levels in the atmosphere - suggest only modest levels during the Archean. Other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect."
The research team looked at the theory that atmospheric levels of nitrogen could have been higher than they are today, which would amplify the "greenhouse gas effect" and melt some of the Earth's surface ice. To test this idea the researchers took samples from tiny air bubbles preserved in Australian quartz.
"We measured the amount and isotopic abundances of nitrogen and argon in the ancient air," Marty said. "Argon is a noble gas which, being chemically inert, is an ideal element to monitor atmospheric change. Using the nitrogen and argon measurements we were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and, from that, the atmosphere that was once in equilibrium with the water."
The team concluded that the partial pressure of nitrogen in the ancient air was either the same or lower than it is today, meaning nitrogen levels could not explain the lack of ice on the frigid young planet.
"The amount of nitrogen in the atmosphere was too low to enhance the greenhouse effect of carbon dioxide sufficiently to warm the planet. However, our results did give a higher than expected pressure reading for carbon dioxide - at odds with the estimates based on fossil soils - which could be high enough to counteract the effects of the faint young Sun and will require further investigation," Burgess said.
