Scientists calculate life expectancy of Earth’s atmospheric oxygen

An artist’s interpretation of the atmosphere of Archean Earth, prior to 2.4 billion years ago. A new study suggests it may return to this oxygen-poor, methane-rich atmosphere in about 1 billion years’ time
NASA’s Goddard Space Flight Center/Francis Reddy

The Earth has an expiry date – in about five billion years, the Sun will expand and swallow up our home world. But it turns out life on Earth could have a much earlier end point. A new study has found that in about a billion years’ time, the atmosphere will lose most of its oxygen rapidly, which may have important implications in the search for life on other planets.

An oxygen-rich atmosphere is one of the defining features of Earth, with the vital gas making up around 20 percent of the air. That of course is good news for humans and most other animals that call this planet home, and we have plants to thank for it.

But how long can an oxygen-rich atmosphere stick around? For the new study, researchers from Georgia Tech and Toho University set out to investigate the long-term stability of Earth’s atmospheric oxygen.

The team conducted simulations of the Earth’s systems, including its climate, biological and geological processes, and even the brightness of the Sun, and observed how the oxygen levels changed as it was shuttled between the air, water and rock. While other studies have simulated some of these systems in the past, these new models were more complex than usual.

The researchers found that the Earth’s oxygenated atmosphere will most likely last another billion years, before it plummets relatively rapidly. By about 1.1 billion years from now, the team says, oxygen levels will likely drop to just one percent of the present atmospheric level.

The leading cause of this deoxygenation, according to the models, is the Sun. As it ages, our parent star is expected to brighten and heat up, which will increase Earth’s surface temperature and break down carbon dioxide in the atmosphere. These two factors would kill off plant life, depriving the planet of its main oxygen source.

This future atmosphere would end up bearing a striking similarity to that of the ancient past – after all, even Earth wasn’t always this pleasant a place. It only began around 2.4 billion years ago, when photosynthesizing microbes and later plants began pumping out oxygen in high amounts. That paved the way for multicellular life to arise.

While this find may not seem to have a direct impact on our lives today – humans will no doubt be long gone in a billion years anyway – it does complicate the search for signs of alien life. Since oxygen is tied so inextricably to life on Earth, astronomers have long considered it a useful signature to look for in the atmospheres of exoplanets, where it could indicate the existence of extraterrestrial life.

Now, it’s becoming clearer that it’s not enough to look for worlds that are in the right point in space to be habitable – the right point in time will be key too. The team calculated that oxygen might only be detectable in an atmosphere for about 20 to 30 percent of the planet’s overall lifetime.

It’s still worth looking for, of course, but the team suggests that there are other potential biosignatures that we should keep an eye out for too. One of those might be a methane-rich, organic haze that they predict will hang in the air of far-future Earth, post-oxygen. And most intriguingly, that sounds an awful lot like the atmosphere of modern-day Titan.

Source: Springer Nature

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