Researchers from the University of Cambridge have concluded: “If life was responsible for the sulphur (SO2) levels we see on Venus, it would break everything we know about Venus’s atmospheric chemistry.”
The unusual behavior of sulfur (SO2) in Venus’ atmosphere cannot be explained by an “aerial” –”life in the clouds”–form of extraterrestrial life, according to a new study by the researchers, who used a combination of biochemistry and atmospheric chemistry to test the hypothesis, which astronomers have speculated about for decades.The team found that life cannot explain the composition of the Venusian atmosphere.
Any life form in sufficient abundance is expected to leave chemical fingerprints on a planet’s atmosphere as it consumes food and expels waste. However, the Cambridge researchers found no evidence of these fingerprints on Venus.
Even if Venus is devoid of life, the researchers say their results, reported in the journal Nature Communications, could be useful for studying the atmospheres of similar planets throughout the galaxy, and the eventual detection of life outside our solar system.
“We’ve spent the past two years trying to explain the weird sulfur chemistry we see in the clouds of Venus,” said co-author Dr. Paul Rimmer from Cambridge’s Department of Earth Sciences. “Life is pretty good at weird chemistry, so we’ve been studying whether there’s 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 are expected to occur, given the known sources of chemical energy in Venus’s atmosphere.
“If food is being consumed by life, we should see evidence of that through specific chemicals being lost and gained in the atmosphere.”
“We looked at the sulfur-based ‘food’ available in the Venusian atmosphere—it’s not anything you or I would want to eat, but it is the main available energy source,” said Sean Jordan from Cambridge’s Institute of Astronomy, the paper’s first author. “If that food is being consumed by life, we should see evidence of that through specific chemicals being lost and gained in the atmosphere.”
The models looked at a particular feature of the Venusian atmosphere—the abundance of sulfur dioxide (SO2). On Earth, most SO2 in the atmosphere comes from volcanic emissions. On Venus, there are high levels of SO2 lower in the clouds, but it somehow gets “sucked out” of the atmosphere at higher altitudes.
“If life is present, it must be affecting the atmospheric chemistry,” said co-author Dr. Oliver Shorttle from Cambridge’s Department of Earth Sciences and Institute of Astronomy. “Could life be the reason that SO2 levels on Venus get reduced so much?”
The models, developed by Jordan, include a list of metabolic reactions that the life forms would carry out in order to get their “food,” and the waste by-products. The researchers ran the model to see if the reduction in SO2 levels could be explained by these metabolic reactions.
They found that the metabolic reactions can result in a drop in SO2 levels, but only by producing other molecules in very large amounts that aren’t seen. The results set a hard limit on how much life could exist on Venus without blowing apart our understanding of how chemical reactions work in planetary atmospheres.
“If life was responsible for the SO2 levels we see on Venus, it would also break everything we know about Venus’s atmospheric chemistry,” said Jordan. “We wanted life to be a potential explanation, but when we ran the models, it isn’t a viable solution. But if life isn’t responsible for what we see on Venus, it’s still a problem to be solved—there’s lots of strange chemistry to follow up on.”
Although there’s no evidence of sulfur-eating life hiding in the clouds of Venus, the researchers say their method of analyzing atmospheric signatures will be valuable when JWST, the successor to the Hubble Telescope, begins returning images of other planetary systems later this year. Some of the sulfur molecules in the current study are easy to see with JWST, so learning more about the chemical behavior of our next-door neighbor could help scientists figure out similar planets across the galaxy.
“To understand why some planets are alive, we need to understand why other planets are dead,” said Shorttle. “If life somehow managed to sneak into the Venusian clouds, it would totally change how we search 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 host life,” said Rimmer, who is also affiliated with Cambridge’s Cavendish Laboratory. “We can take what we’ve learned here and apply it to exoplanetary systems—this is just the beginning.”
The Last Word –Martin Ferus, Sean Jordan and Paul Rimmer
Sean Jordan: “I would say that, given that life likely cannot be responsible for the observed SO2-depletion via the proposed energy-metabolisms, this would strengthen the in-droplet cloud chemistry hypothesis from Rimmer et al., 2021 as a possible explanation to the SO2-depletion. The hypothesis relies on a flux of mineral dust to the cloud layer and remains unconfirmed, however it may be possible to confirm when we return to Venus at the end of decade.”
If there are abiotic sources of significant disequilibrium chemistry, then it turns out that disequilibrium chemistry may not provide good evidence for the presence of life.
Paul Rimmer: “What chemistry is involved depends a lot on what is in the clouds. For context: Historically, there have been claims of PH3, NH3, H2S, CH4 and O2, based on probe measurements and ground-based measurements (in the case of PH3). The amounts of these range from ppb to ppm concentrations. If all of these are present in the clouds in ppm concentrations, then there is significant redox disequilibrium chemistry in the clouds, and either life is producing this disequilibrium chemistry, or significant disequilibrium chemistry can be produced without life. If there are abiotic sources of significant disequilibrium chemistry, then it turns out that disequilibrium chemistry may not provide good evidence for the presence of life.
I have no idea what abiotic source could explain ppm concentrations of all these species. I also don’t know what kind of biology could survive in the clouds of Venus.
“If it’s just one or a couple of these things, maybe just PH3, then I still don’t know what abiotic process exactly could be producing this, but my bet is on heterogenous chemistry in the clouds, maybe something involving photochemical sources of reducing species, like Martin Ferus‘s group suggests (full disclosure: I’m collaborating with Martin on these experiments).”
Martin Ferus: “Photochemistry on surfaces can mimic life by producing false positive biosignature gases. For instance, acidic surfaces of aerosol can produce methane from carbon dioxide and maybe also phosphine from oxidized phosphorus compounds upon UV light. Both are highly related to unknown chemistry in upper Venus clouds. But, this must be explored and we are working on that. However, not only photochemistry, but complex Venus chemistry must be explored and its history must be well understood. Venus is a type of a planet representing maybe one of Earth evolution stages, maybe hot stage in Earth’s very early history, but for sure the Earth’s future, because solar power is slowly increasing and Earth will be as hot as Venus some day in distant future.”
More information: Sean Jordan, Proposed energy-metabolisms cannot explain the atmospheric chemistry of Venus, Nature Communications (2022). DOI: 10.1038/s41467-022-30804-8. www.nature.com/articles/s41467-022-30804-8
Image credit: NASA
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