A team of astrobiology scientists discovered that, during the Earth's early times, greenhouse gas in the form of methane was not the one to play an important role in keeping the climate warm.
Researchers from the University of California, Riverside, discovered that a billion years in the past our planet could have been frozen, but it miraculously survived. Trying to solve the mystery, the team found that methane was, in fact, not the crucial gas for keeping the planet's temperature above the freezing limit.
While today greenhouse gases cause panic and need to be drastically diminished in order to keep Earth safe, it turns out that 1.8 billion years ago, microscopic ocean dwellers were kept alive by their effects. As the sun used to be 10 to 15 percent less bright than we know it, our planet needed another actor to be responsible for its evolutionary survival. As the paper unveiled, heat-trapping gases had a significant contribution in this process.
For a few decades, the scientific consensus was that methane fulfilled the role of Earth's savior. The thought process behind this theory was that the gas had 34 times more heat-trapping capacity compared to carbon dioxide. Therefore, it could have assured our planet's minimum temperature to survive the sun's lack of power, especially since oxygen was not yet present in our atmosphere.
What the team of scientists did was to show that, while methane indeed has heat-trapping power, it could not have survived the "battle" against sulfate. The paper suggests that sulfate represents an even more powerful enemy of methane than oxygen, therefore proving that methane could not be found in significant quantities, either.
"You can't get significant methane out of the ocean once there is sulfate," explained Stephanie Olson, graduate student at the UCR and author of the study.
However groundbreaking, the theory can be easily explained in terms of chemical processes. While sulfate wasn't an important environmental factor until the appearance of oxygen, even 1 percent of the substance could kill methane.
Olson and her team, the Alternative Earths coauthors, suggested that sulfate in the ocean kept the methane quantity on a very low level, at approximately 1 to 10 parts per million. This finding cancels the previous theory, since there are 300 parts per million assessed by further theoretical analyses.
The lead cause for previous theories that stated methane as responsible for the planet's survival is that they ignored the organic activity of the ocean and its consequences. Though most of the methane in the atmosphere comes from the decomposition of organic matter in the oceans, the sulfate in seawater decreases its amount. Seawater sulfate heavily contributes to the destruction of methane through two separate methods: directly and by limiting its production.
Scientists now have a new puzzle to solve, the researchers said. If not methane, then which greenhouse gases were responsible for keeping the Earth warm all those years ago, and in what quantities? The answer would enable the scientific community to determine whether the Milky Way's other Earth-like planets, which number in the billions, will be able to support life.
“If we detect methane on an exoplanet, it is one of our best candidates as a biosignature, and methane dominates many conversations in the search for life on Mars,” said study coauthor and biogeochemistry professor Timothy Lyons.
