No sign (yet) of life on Venus

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

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

Any kind of life in sufficient abundance is expected to leave chemical footprints in a planet’s atmosphere as it consumes food and displaces waste. However, Cambridge scientists found no evidence of these fingerprints on Venus.

Although Venus is devoid of life, scientists confirm their findings, reported in the journal Nature communicationcould be useful for studying the atmosphere on similar planets throughout the galaxy and the possible detection of life outside our solar system.

“We have spent the last two years trying to explain the strange sulfur chemistry we see in the clouds of Venus,” said co-author Dr. Paul Rimmer from the Department of Soil Science at Cambridge. “Life is pretty good at strange chemistry, so we explored if there was a way to make life a potential explanation for what we see.”

The scientists used a combination of atmospheric and biochemical models to study the chemical reactions that are expected to 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 want to eat, but it’s the main source of energy available,” said Sean Jordan of the Cambridge Institute of Astronomy . first author. “If this food is taken alive, we should see evidence of it in specific chemicals lost and gained in the atmosphere.”

The models looked at a special feature of the Venusian atmosphere – the abundance of sulfur dioxide (SO)2). On Earth, most SO2 in the atmosphere comes from volcanic emissions. On Venus, there are high levels of SO2 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 Dr. Oliver Shorttle, of the Department of Earth Sciences and the Cambridge Institute of Astronomy. “Could life be the reason SO2 are the levels on Venus so low? »

The models, developed by Jordan, include a list of metabolic reactions that life forms would perform to obtain their “food” and waste by-products. The researchers ran the model to see if the reduction in SO2 can be explained by these metabolic reactions.

They found that metabolic reactions can lead to a decrease in SO2 levels, but only by producing other molecules in very large quantities that are not visible. The results set a hard limit on the amount of life that could exist on Venus without exploding our understanding of how chemical reactions work in planetary atmospheres.

“If life was responsible for SO2 levels we see on Venus, it would also ruin everything we know about Venus’ atmospheric chemistry, “Jordan said.” We wanted life to be a potential explanation, but since we ran the models, it’s not a viable solution. . But if life is not responsible for what we see on Venus, it is still a problem to be solved – there is a lot of strange chemistry to follow. »

Although there is no evidence that sulfur-bearing life lurks in Venus’ clouds, scientists say their method of analyzing atmospheric signatures will be useful when JWST, the successor to the Hubble Telescope, begins returning images of other planetary systems later in the year. year. . Some of the sulfur molecules in the current study are easy to see with JWST, so knowing more about the chemical behavior of our next door can 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 sneak into the Venusian clouds, it would totally change the way we look for chemical signs of life on other planets.”

“Even though ‘our’ Venus is dead, it is possible that Venus-like planets in other systems may contain life,” said Rimmer, who is also affiliated with the Cambridge Cavendish Laboratory. “We can take what we’ve learned 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 the UK Research and Innovation (UKRI).

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