Something is making Venus’s clouds less acidic

Life as we know it shouldn’t be able to survive in Venus’s swirling atmosphere of carbon dioxide and sulfuric acid. Its clouds are so acidic, in fact, that they don’t even register on the regular pH scale. But a new study published in the Proceedings of the National Academy of Sciences suggests that under the right circumstances, certain lifeforms could make a home for themselves by producing ammonia in the planet’s toxic clouds. 

The clouds of Venus contain a few tantalizing anomalies—and the unanswered questions for why these anomalies exist leave open the possibility for life on the planet. Researchers have found, for instance, that the concentration of sulfur dioxide dramatically drops higher up in the atmosphere without a known explanation. 

To understand why this depletion occurs, Paul Rimmer, a postdoctoral researcher in astrochemistry at the University of Cambridge and co-author of the new study, examined the chemical makeup of the sulfuric acid droplets in the high clouds. There, “instead of being like pure battery acid, it’s a bit more like stomach acid,” Rimmer says. “Still very acidic, but not as acidic.” 

Researchers had theorized that acid-neutralizing salts could be swept up into the clouds from the planet’s surface–but the amount of mineral salt needed is too extreme, according to Rimmer’s previous calculations. Now, the research team has proposed a new hypothesis to explain the acidity anomaly: What if the cause wasn’t a mineral from the surface, but a substance produced in the clouds? 

The researchers created a model using ammonia, NH₃, as the neutralizing agent. Ammonia had been unexpectedly detected in the cloud layers in the 1970s by the Venera 8 and Pioneer Venus probes. This ammonia could be a sign of metabolic activity naturally occurring on Venus—meaning that the atmosphere is home to some form of life, according to Janusz Petkowski, an astrobiology research scientist at MIT and co-author of the paper.

When Rimmer and his colleagues added ammonia into their model of chemical reactions in Venus’s clouds, the ammonia explained the known abundance of water vapor and oxygen in its atmosphere. Chemical pathways involving this compound can also explain previously detected sulfite salts in the cloud droplets.

The harsh conditions in this atmosphere are not unlike extreme environments on Earth, such as the Dallol sulfur pools of Ethiopia, where luminescent yellow pools harbor a few hardy species of bacteria. What’s more, microorganisms on Venus might be shaping their own habitat to be less harsh by producing the ammonia that neutralizes acid. “If life on Venus exists, and if it indeed is responsible for making ammonia, then it makes its own environment in which it lives. It adapted,” says Petkowski.

While ammonia provides a neat solution in this model, this doesn’t necessarily mean that the ammonia is a result of biological processes. 

“The responsible thing to do as scientists and astrobiologists interested in this is that we have to cultivate the habit of mind where we assume it’s everything but life first,” says David Grinspoon, a senior scientist at the Planetary Science Institute who was not involved with this research. “There are other things that can happen” in an atmosphere to produce these gases, he says, calling for experts to “rule out everything else” before making claims about life on Venus.

[Related: These scientists spent decades pushing NASA to go back to Venus. Now they’re on a hot streak.]

The study authors acknowledge that, while ammonia is a byproduct of life on Earth, the ammonia found on Venus may not be created in the same way. There might be non-biological ways of producing ammonia on other planets that we’re not currently aware of, Rimmer explains.

And even if life on Venus isn’t responsible for ammonia, Grinspoon says, there is still some exotic chemistry at play that is worth trying to discover.

For Rimmer, the key to making further assessments is to have new atmospheric probe data with more advanced technology to make sure that existing data from the 1970s and 1980s did not include anomalous results or false positives. 

Interpreting data from the old instruments on the probes is actually quite tricky, Grinspoon adds, posing another problem for making inferences from those initial measurements.

In about a decade, scientists may receive a bounty of data on this cloud chemistry. NASA has scheduled the DAVINCI+ atmospheric probe, planned to launch in 2029, to carry out the first complete compositional study of the entire cross-section of Venus’ atmosphere. Another initiative by private spaceflight company Rocket Lab aims to send a probe to look for biosignatures in Venus’ clouds by 2023.

Meanwhile, Petkowski and the other authors are pushing their colleagues to rethink what it means for a planet to be habitable. 

“We should not overextend our understanding of life’s adaptations to absolutely every planetary body everywhere, because our life has never had an environment like the clouds of Venus to even consider adapting to,” says Petkowski. “And so life on Venus, if it exists, is not like life on Earth. It’s life as we don’t know it. The only question is, to what degree it is different?”