A new and thorough analysis of the chemistry of clouds on Venus has revealed none of the biomarkers indicative of airborne, sulfur-metabolizing life.
For now, that means the question of detectable life in the clouds of Venus is pretty much answered. Until we get new information, it's likely to remain that way. The complex chemistry of the upper Venusian atmosphere simply cannot be explained by the presence of life as we know it.
After a team of scientists controversially announced that they had found phosphine gas in the clouds of Venus back in 2020, speculation about life in the clouds of Venus at temperate altitudes has run pretty rampant.
But the idea is not a new one; indeed, biophysicist Harold Morowitz and astronomer Carl Sagan proposed the idea over 50 years ago, back in 1967.
More recently, scientists have proposed that the chemistry could contain clues – and that life in the clouds of Venus may have developed sulfur-based metabolism, similar to what we have seen in microorganisms here on Earth. The signature of a compound of sulfur, sulfur dioxide (SO2), is very peculiar on Venus: abundant at lower altitudes, but really quite low at higher.
"We've spent the past two years trying to explain the weird sulfur chemistry we see in the clouds of Venus," says astronomer and chemist Paul Rimmer from the University of Cambridge.
"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."
As much as it might be good at weird chemistry, life as we know it isn't very good at hiding its existence, unless it's underground or in a cave or something.
Biological processes extract elements from their environment, and expel different elements into it. Respiration is a good example: we humans breathe in oxygen, and breathe out carbon dioxide. (Trees absorb carbon dioxide, and expel oxygen – it's a good system.)
The chemistry of Venus is very different from that of Earth, with an atmosphere extremely rich in sulfur – it reaches concentrations 100,000 times higher than those in Earth's atmosphere, bound up in compounds such as sulfur dioxide, sulfuric acid, and carbonyl sulfide.
So in the new study, a team of researchers led by astronomer Sean Jordan from the University of Cambridge set out to investigate the chemical reactions we should expect, given the available energy sources in Venus' 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," Jordan says.
"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 strange sulfur dioxide signature was of particular interest. Here on Earth, the compound is produced volcanically, and it's possible that it is produced that way on Venus, too.
However, if organisms with sulfur-based metabolism were living in the upper atmosphere of Venus, they might be responsible for the peculiar lack of sulfur dioxide at those altitudes.
Scientists have previously proposed sulfur-based metabolisms that could theoretically utilize the chemical species available in Venus' atmosphere.
Using these as a basis, Jordan and his colleagues sought to model the chemical reactions that would be taking place were those lifeforms present, to see if they produce the observed composition of the Venusian atmosphere.
They found that sulfur-metabolizing life could produce the observed depletion of sulfur dioxide; but the output of the metabolic processes of a biomass of the size required would produce other compounds in abundances that, in brief, are simply not there.
"If life was responsible for the SO2 levels we see on Venus, it would also break everything we know about Venus's atmospheric chemistry," Jordan says.
"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."
We still don't know how or why sulfur dioxide gets slurped out of the upper atmosphere of Venus, so that remains an open question. It's also possible that a biosphere exists with an unknown metabolism, which we won't know until a probe goes over there to check it out. So that's exciting.
In the meantime, the team said, their research offers a framework that could help simulate the effect of an aerial biosphere on alien worlds, and thereby search for life in the atmospheres of exoplanets. Given that exoplanet atmospheres are the best place to probe for signs of life, this is really quite nifty.
"Even if 'our' Venus is dead, it's possible that Venus-like planets in other systems could host life," says Rimmer.
"We can take what we've learned here and apply it to exoplanetary systems – this is just the beginning."
The research has been published in Nature Communications.