The search for extraterrestrial life should not disregard planets with zero oxygen, as these other worlds may just be teeming with microbial life.
A team of researchers has found huge fluctuations in oxygen levels of ancient Earth that took place before the Great Oxidation Event.
They believe the same fluctuations could be happening in other planets, which means it is possible that planets with undetectable oxygen could be home to simple microorganisms that dominated the Earth a few billion years ago.
The Great Oxidation Event
Researchers at the University of Washington have found that, for hundreds of millions of years before the Great Oxidation Event, oxygen levels varied tremendously.
Around 3.5 billion years ago, the only life forms on the planet were one-celled microorganisms, such as the blue-green algae called cyanobacteria.
These simple microbes evolved a way to make their own food by using sunlight to create sugar out of water and carbon dioxide. The process, called photosynthesis, created oxygen as a waste product, which the bacteria released into the atmosphere.
Over a billion years, some experts believe the one-celled cyanobacteria became multicellular, allowing it to pump more oxygen into the atmosphere in an event that changed the course of the planet.
A Minor Whiff Before The Major Event
The Great Oxidation Event, which is believed to have happened between 3 and 1 billion years ago, is one of the defining moments in Earth's history. Without the rise of oxygen levels, plant and animal species seen today, including humans, would not be around.
However, the researchers say they have found an instance of rising oxygen levels before the Great Oxidation Event took place. This ancient "whiff" shows that oxygenation did not happen in a single geological breath. It was a complex process characterized by ebbs and flows in oxygen levels.
"The production and destruction of oxygen in the ocean and atmosphere over time was a war with no evidence of a clear winner, until the Great Oxidation Event," says lead author Matt Koehler, PhD student in Earth and space sciences.
Implications For The Search For Alien Life
The findings, published in the Proceedings of the National Academy of Sciences, could keep researchers involved in the search for alien life from discounting false negatives, such as planets that may not have detectable oxygen levels.
When searching for signs of life in other planets, experts look for biosignatures that increase the possibility of success. One of these is the presence of oxygen. However, just because a planet does not have oxygen does not mean life cannot exist on it.
Ancient Earth, after all, was teeming with bacteria at least a billion years before the planet's oxygen levels rose. The researchers believe it is entirely possible that a planet with no oxygen may just be in the ebb phase of its own oxygenation event.
"One of the strongest atmospheric biosignatures is thought to be oxygen," says coauthor Roger Buick, "but this study confirms that during a planet's transition to becoming permanently oxygenated, its surface environments may be oxic for intervals of only a few million years and then slip back into anoxia."
Rock Samples Show Signs Of Oxygen
By analyzing sediment samples collected at the Jeerinah Formation in Western Australia, Koehler's team found rising oxygen levels that took place 2.66 billion years ago.
The minor oxygenation event lasted 50 million years, which is hardly a blink in geological time, and spanned hundreds of miles across a wide body of water.
The researchers looked for traces of nitrogen isotopes and selenium in the rock layers. Both are indications of the presence of oxygen.
They found a gradual rise in nitrogen isotopes ended by an abrupt disappearance, leading the researchers to believe that oxygen levels gradually rose before immediately dropping off.
They also found traces of selenium in a shallow part of the marine basin, which means the selenium must have come from the land. When oxygen levels are high, more selenium gets drained into the sea through the rusting of rocks and is deposited into ocean sediments.
The samples collected by the researchers show high selenium levels in the sediments, suggesting that there was an oxygen spike in the atmosphere at the time.
In 2007, an international team of researchers also discovered a similar whiff that took place 50 to 100 million years before the Great Oxidation Event.