For decades, astrobiologists have speculated about the possibility of bacterial life in Venus’ atmosphere. Although the planet is similar in size and mass to Earth, its closer proximity to the Sun and the composition of Venus’ atmosphere create a massive greenhouse effect: Venus’ bone-dry surface has temperatures 450 degrees and pressures a hundred times higher than Earth’s surface .

At an altitude of 50 to 60 kilometers, however, conditions prevail in the Venusian atmosphere in terms of temperature and pressure that are similar to those on the earth’s surface and are therefore quite habitable. Bacteria, the astrobiologists say, could live there. But a team of researchers from Great Britain came to a different conclusion: The metabolism of such microbes does not match the observed composition of the Venusian atmosphere, according to the scientists in the journal “Nature Communications”.

“We spent two years trying to explain the strange sulphur-based chemistry in the clouds of Venus,” explains Paul Rimmer of the University of Cambridge. “Life is pretty good at creating weird chemical makeup.”

The earth is a good example of this: Without the oxygen-producing plants, the high oxygen content of the earth’s atmosphere would be inconceivable. “That’s why,” Rimmer continues, “we looked for ways to explain the observation data with the help of bacteria.”

The abundance of sulfur dioxide in the atmosphere of Venus is particularly puzzling. On Earth, most of the sulfur dioxide comes from volcanoes. Venus also has active volcanoes that could emit sulfur dioxide into the atmosphere. But the proportion of this gas is only high in the lower cloud layer, above which it decreases rapidly with increasing altitude. According to the researchers, a process there must therefore consume the sulfur dioxide.

The idea of ​​Rimmer and his colleagues: The sulfur dioxide serves as an energy source for bacteria in the local, life-friendly temperature-controlled region of the atmosphere. The scientists compiled a list of possible bacterial metabolisms based on sulfur dioxide to see whether this could explain the reduction in the gas. In fact, bacteria could explain the decrease in the proportion of sulfur dioxide with increasing altitude.

But the supposed reference to life has a catch. Metabolism always produces excretions, i.e. other molecules. And, as Rimmer and his colleagues discover, these are substances that are not present in the Venusian atmosphere. “We would have liked to show that bacteria are a possible solution,” says Rimmer’s colleague Sean Jordan. “But our models show that it doesn’t work. They go against everything we know about the atmosphere of Venus.”

The mystery of the sulfur dioxide in the Venusian atmosphere thus remains unsolved. The researchers now want to explore non-biological approaches to the strange chemistry in the planet’s clouds. They also hope to soon be able to apply their method to planets around other stars. With the new James Webb telescope, for example, sulfur compounds could be detected there. “We can apply what we learned from Venus to exoplanets,” says Rimmer.