No signal (but) of life on Venus

The unusual behavior of sulfur in Venus’ atmosphere cannot be explained by an “airborne” extraterrestrial life form, a new study has found.

Researchers at the University of Cambridge used a combination of biochemistry and atmospheric chemistry to test the ‘life in the clouds’ hypothesis, which astronomers have speculated about for decades, and found that life cannot explain the composition of the Venusian atmosphere.

Any form of life in sufficient abundance is expected to leave chemical footprints in a planet’s atmosphere as it consumes food and expels waste. However, the Cambridge researchers found no evidence of such fingerprints on Venus.

Even though Venus is devoid of life, the researchers affirm their findings, reported in the journal Nature’s Communicationcould be useful for studying the atmospheres of similar planets across the galaxy and the possible detection of life outside our solar system.

“We’ve spent the last two years trying to explain the strange sulfur chemistry we see in clouds on Venus,” said co-author Dr. Paul Rimmer of the Department of Earth Sciences at Cambridge. “Life is very good at weird chemistry, so we investigated if there is a way to make life a potential explanation for what we see. »

The researchers used a combination of atmospheric and biochemical models to study the chemical reactions that must occur, given the known sources of chemical energy in Venus’ atmosphere.

“We looked at the sulfur-based ‘food’ available in the Venusian atmosphere – it’s not something you or I would want to eat, but it’s the main source of available energy,” said Sean Jordan of the Cambridge Institute of Astronomy. first author. “If this food is consumed by life, we should see evidence of it in specific chemicals lost and gained in the atmosphere. »

The models examined a particular feature of the Venusian atmosphere – the abundance of sulfur dioxide (SOtwo). On Earth, most SOtwo in the atmosphere comes from volcanic emissions. On Venus there are high levels of OStwo lower in the clouds, but is somehow “sucked” out of the atmosphere at higher altitudes.

“If life is present, it must affect atmospheric chemistry,” said co-author Oliver Shorttle of the Department of Earth Sciences and the Cambridge Institute of Astronomy. “Could life be the reason why SOtwo are the levels on Venus that low? »

The models, developed by Jordan, include a list of metabolic reactions that life forms would carry out to obtain their “food” and waste by-products. The researchers ran the model to see if the reduction in OStwo can be explained by these metabolic reactions.

They found that metabolic reactions can lead to a drop in SOtwo levels, but only producing other molecules in very large amounts that are not visible. The results set a hard limit on how much life could exist on Venus without exploding our understanding of how chemical reactions work in planetary atmospheres.

“If life were responsible for SOtwo levels we see on Venus, would also destroy everything we know about Venus’ atmospheric chemistry,” Jordan said. “We wanted life to be a potential explanation, but when we run the models, it’s not a viable solution. But if life isn’t responsible for what we see on Venus, that’s still a problem to be solved – there’s a lot of weird chemistry to follow. »

While there’s no evidence that sulfur-eating life lurks in Venus’ clouds, the researchers say their method of analyzing atmospheric signatures will come in handy when the JWST, the successor to the Hubble Telescope, begins returning images of other planetary systems later this year. year. . Some of the sulfur molecules in the current study are easy to see with the JWST, so learning more about our neighbor’s chemical behavior could help scientists discover similar planets across the galaxy.

“To understand why some planets are alive, we need to understand why other planets are dead,” Shorttle said. “If life somehow managed to infiltrate the Venusian clouds, it would totally change the way we look for chemical signs of life on other planets. »

“Even if ‘our’ Venus is dead, it’s possible that Venus-like planets in other systems could harbor life,” said Rimmer, who is also affiliated with Cambridge’s Cavendish Laboratory. “We can take what we learn here and apply it to exoplanetary systems – this is just the beginning. »

The research was funded by the Simons Foundation and the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).

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