Finding oxygen in an exoplanet’s atmosphere is a clue that life might be at work. On Earth, photosynthetic organisms absorb carbon dioxide, sunlight and water and produce sugars and starches for energy. Oxygen is the by-product of this process, so if we can detect oxygen elsewhere, it will generate excitement. But researchers have also pushed the idea that oxygen in an exoplanet’s atmosphere indicates life. It’s only proof of life if we can rule out other pathways that created oxygen.
But scientists can’t rule them out.
The Earth is saturated with oxygen. It makes up 46% of the crust and about the same percentage of the mantle, and the atmosphere contains about 20% oxygen. The presence of oxygen comes from the Great Oxygenation Event (GOE) about 2 billion years ago. Ancient cyanobacteria evolved pigments that absorb sunlight and use it in photosynthesis. Oxygen is a waste product of photosynthesis, and life has had a few billion years to accumulate oxygen in the atmosphere, mantle, and crust.
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So if scientists find oxygen in an exoplanet’s atmosphere, that’s a strong indication that life might be at work. Simple life may be bubbling up in the planet’s oceans, soaking up sunlight and spewing out oxygen. But new research has identified a source of oxygen that doesn’t depend on life.
The research article is “Production of Abiotic Molecular Oxygen – Ionic Pathway from Sulfur Dioxide”, published in Science Advances. The lead author is Måns Wallner, a PhD student in physics at the University of Gothenburg in Sweden.
Researchers have found an abiotic source of oxygen that comes from sulfur dioxide. Sulfur is not uncommon in celestial bodies, and since volcanoes produce sulfur and pump it into the atmosphere, terrestrial volcanic exoplanets may have oxygen in their atmosphere. And life doesn’t need to be involved.
Instead, high-energy radiation from a star can ionize the sulfur dioxide molecule. The formula for sulfur dioxide is SO2, and when ionized, the molecule reorganizes. It becomes a “double positive charge system”. Then it has a linear shape with the two oxygen atoms adjacent to each other and the sulfur at the other end. This is called roaming because the oxygen atoms are free to drift in chaotic orbits until they settle into new compounds.
![This figure shows how solar radiation, when energetic enough, can ionize SO2 and produce oxygen. Image credit: University of Gothenburg.](https://oponame.com/wp-content/uploads/2022/10/Scientists-Discover-New-Way-Exoplanets-Could-Make-Oxygen-Unfortunately-it.png)
“During double ionization, two of the molecule’s bound electrons are ejected and can cause changes in the angle between atoms in the molecule,” lead author Wallner said in a press release. “Alternatively, as crucial in the present case, roaming can occur, i.e. the atoms change places and the molecule takes on a whole new shape.”
But the constituents of the molecule might not reform into SO2 again. Instead, the sulfur can break apart and a single, positively charged oxygen molecule can remain. Then the positive charge can be neutralized by attracting an electron from another molecule. Some molecular oxygen (O2) remains and is vital for life on Earth.
This pathway to oxygen may explain some of the oxygen we find elsewhere. Io, Ganymede, and Europa all have oxygen in their atmospheres, and roaming could be the cause. Io is a volcanic place – the most volcanic world in the solar system – so life is excluded there. Ganymede and Europa have underground oceans, so they could potentially harbor life. But this life cannot build an atmosphere of oxygen like earthly life. Another explanation is needed to account for the oxygen found on these moons.
![Astronomers using the Hubble Space Telescope found evidence of a thin oxygen atmosphere on Ganymede in 1996. The atmosphere is far too thin to support life as we know it. The fact that there is oxygen in its atmosphere means there must be an abiotic source. Juno spacecraft captured this image of Ganymede in 2021. Image Credit: By NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill - Ganymede - Perijove 34 Composite, CC BY 2.0, https://commons. wikimedia.org/w/index.php?curid=106499339](https://oponame.com/wp-content/uploads/2022/10/Scientists-Discover-New-Way-Exoplanets-Could-Make-Oxygen-Unfortunately-it.jpg)
This oxygen pathway could also occur on Earth, the researchers say. “We also suggest in our paper that this occurs naturally on Earth,” said Raimund Feifel, co-author of the paper reporting the findings.
This pathway of ionic oxygen formation may also work for other molecules, and this is the next step for the researchers. They want to know if other molecules like carbon diselenide are subject to double ionization. “We want to see if it also happens at that time or if it was just a happy coincidence with sulfur dioxide,” Feifel said.
Other researchers have tackled abiotic sources of O2. A 2014 paper presented evidence of molecular oxygen being produced from CO2 when exposed to high energy UV light. In a 2015 paper, Japanese researchers showed that near-ultraviolet light could produce O2 on exoplanets when interacting with water using Titania (titanium dioxide) as a catalyst.
These findings help explain how Earth had a small amount of oxygen in its atmosphere before the GOE. Since oxygen is so reactive, there must have been a source of replenishment, and these pathways could be responsible.
The James Webb Space Telescope is part of the backdrop for this research. Studying exoplanet atmospheres is one of the telescope’s scientific goals, and with its powerful infrared instruments, it is poised to reveal the chemical composition of exoplanet atmospheres.
If he finds oxygen, there will be excitement. But as this research shows us, there is more to oxygen than life.
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