Climatic conditions continue to leave scientists in a dilemma about how and why life on Earth evolved when it did.
Life on Earth developed during the Archean, approximately 3.4 billion years ago. However, the sun was very weak at that time. This led researchers to wonder how life on Earth evolved as at that time the planet would have been too cold to support life. The earth received solar energy that what about 30 percent lesser than present.
"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," said study author, Dr Ray Burgess, from Manchester's School of Earth, Atmospheric and Environmental Sciences.
Therefore, researchers from the University of Manchester and the Institut de Physique du Globe de Paris conducted a study to find an explanation for this mystery, which is dubbed the "faint, young Sun paradox."
Researchers first tackled one explanation - greenhouse gas levels in the Earth's atmosphere that are responsible for regulating climate. Burgess and his team speculated that since the sun was too weak, high levels of carbon dioxide in the atmosphere may have been the reason behind the evolution of life on Earth. For this, carbon dioxide levels at that time should have been 1,000 times higher than present.
Ancient fossil soils, which are the best indicators of carbon dioxide levels in the atmosphere, were studied. Researchers found that the gas was present only in modest levels during the Archean period, which ruled out the possibility of high levels of carbon dioxide resulting in the evolution of life on Earth.
Researchers then looked at whether high concentrations of other greenhouse gases like methane and ammonia were present in the atmosphere. They soon ruled out this possibility too as both these gases are weak and can be easily destroyed by ultra-violet radiations.
The team then went on to test another previous assumption that high levels of nitrogen in ancient atmosphere may have amplified the greenhouse effect of carbon dioxide and allowed the Earth to remain ice-free. They examined tiny samples of air trapped in water bubbles in rocks from a region of northern Australia that has extremely old and exceptionally well-preserved rocks.
"We measured the amount and isotopic abundances of nitrogen and argon in the ancient air," said lead author, Professor B Marty. "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."
They had to rule out this possibility too as they found that the nitrogen level in ancient atmosphere was similar if not lower than present. However, Burgess and his team are optimistic about solving this mystery as the current study gave a higher than expected pressure reading for carbon dioxide, which may be high enough to counteract the effects of the weak Sun. However, further investigation is required to confirm the same.
