Laughing Gas Likely Warmed Early Earth, Made Conditions Suitable For Life

A new study looking at the evolutionary history of Earth has suggested laughing gas could have played a critical role in warming up our world, making conditions suitable for the proliferation of complex life.

For years, scientists have known the earliest microbes came to be billions of years ago, while multicellular organisms didn’t appear on the geologic time scales until the end of Proterozoic Eon — the time from 2.5 billion years ago to 541 million years ago. Seemingly, the changes occurring during this period made conditions on Earth suitable enough to host complex life.

But, as the sun was much dimmer than today during that period, many wondered how our planet stayed warm enough. This was the famous “Faint Young Sun Paradox” suggested by astronomer Carl Sagan.

Though it has been posited a combination of greenhouse gases and astrophysical factors probably contributed toward making our planet warm, nobody could figure out the right balance of gases prevailing in the atmosphere during that time. The decades-old mystery remained unsolved until the research team from Georgia Institute of Technology determined the role of laughing gas or nitrous oxide (N2O).

After running a series of atmospheric experiments and computer models, the team found nitrous oxide, which is known to be a powerful greenhouse agent, would have worked alongside atmospheric carbon dioxide, methane, and other gases to keep the ancient Earth warm. The presence of CO2 and methane was determined in previous studies, but this particular work reaffirms they would have needed N2O, despite being available at high levels.

The gas, as the researchers described, was likely pumped into the atmosphere through a series of chemical processes occurring in oxygen-less oceans of the mid-Proterozoic, which is some 1.8 to 0.8 billion years ago. Essentially, it was the presence of soluble ferrous iron (Fe2+) in ocean waters that drove the processes required for the production of N2O.

"The ocean chemistry was completely different back then," Jennifer Glass, the study's principal investigator, said in a statement. "Today's oceans are well-oxygenated, so iron rapidly rusts and drops out of solution. Oxygen was low in Proterozoic oceans, so they were filled with ferrous iron, which is highly reactive."

Taking note of this property, Glass and team conducted lab experiments to witness how Fe2+ in seawater would have reacted with nitrogen molecules, particularly nitric oxide. The work, backed by NASA’s Astrobiology Institute, yielded the production of nitrous oxide, which probably bubbled up and bolstered the amount of nitrous oxide in the atmosphere.

On running a series of simulations using the bubbled up levels, they found atmospheric nitrous oxide would have been 10 times higher than present-day levels if ancient atmospheric oxygen was at 10 percent of today’s levels. Moreover, as some modern-day microbes breathe nitrous oxide, the researchers think same might have been used by early-Earth microbes to survive way before oxygen levels started to increase and complex life evolved.

The study was published Aug. 20 in the journal Geobiology.