Earth
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The origins of life on Earth is a mystery as the Sun, at the time, was too weak to sufficiently warm the planet but there is plenty of evidence of liquid water during the Archean. Increased levels of greenhouse gas could have aided in sufficiently heating the Earth but a new study has eliminated that theory as a possible solution for the "faint young sun paradox."

As the researchers from the University of Manchester explain, the Sun was 20 to 25 percent less bright during the Archean eon, approximately four billion to 2.5 billion years ago, than it is at present, the Sun has increased in brightness over its lifetime. With a weaker Sun, and if greenhouse gas levels were around the same as they are in the present day, liquid water would not have been possible on Earth but fossil records indicate otherwise as life first appeared on earth during the Archean eon, approximately 3.8 billion years ago..

According to Ray Burgess, from the University of Manchester, "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."

To explain the presence of liquid water on Earth with a weaker Sun, researchers have theorized that higher levels of greenhouse gas, such as carbon dioxide, heated up the planet. That's not the case according to the new study, published in the journal Science, which states that while greenhouse gas levels were slightly higher than they are at present, the levels were not significantly higher to make up for the weak sun.

Lead author Bernard Marty, from the CRPG-CNRS University of Lorraine, says greenhouse gas levels during the Archean had to be 1,000 times than they are at present to solve the paradox. To test greenhouse gas levels during the time period, the researchers analyzed fossil soil samples. Marty said in a statement, "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."

After ruling out an increase in carbon dioxide levels, the researchers then tested nitrogen levels in the Archean atmosphere as another possible solution to the young sun paradox. The researchers measured nitrogen levels in air bubbles that were trapped in ancient quartz. Nitrogen levels were roughly the same during the Archean as they are in the present day, ruling out that gas as a solution. Interestingly, the researchers discovered the nitrogen isotope ratio, same element with different number of neutrons, was similar to what it is at present, indicating a significant magnetic field, at least 50 percent of the current magnetic field. The magnetic field, notes the university, would have prevented the gases from escaping into space.

The researchers did report higher carbon dioxide levels in the trapped air bubbles that what was estimated and what was found in the soil samples. Further research would be needed to determine the level of carbon dioxide trapped in the ancient quartz and if these levels would have been enough to warm the Earth.

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