Is Venus habitable? A new study fuels ongoing debate

Although Venus ranks low on the list of planets with surface conditions that could be habitable, scientists have been debating whether or not life could survive in the clouds of this scorching hot planet.

In the latest addition to the ongoing debate, a team of scientists suggests the presence of ammonia in the Venusian clouds, which could make for habitable conditions. But as skepticism still looms over Venus’ potential habitability, the latest findings are just one step in understanding the complex atmosphere of the planet.

The study was published Monday in the Proceedings of the National Academy of Sciences.

Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology and lead author of the new study, was also involved in a 2020 study that first proposed habitable conditions in Venus’ clouds due to the purported presence of phosphine.

“It was quite controversial, but it brought a lot of new attention to Venus,” Seager tells Inverse.

HERE’S THE BACKGROUND — Venus is similar to Earth in size, mass, and density, but it is a far more hardcore version of our planet.

Although it is the second closest planet to the Sun, Venus is the hottest one in the Solar System.

The surface of Venus boasts temperatures that reach up to 900 degrees Fahrenheit, thanks to a dense carbon dioxide atmosphere that traps heat and a hellish volcanic landscape to match the dire surroundings.

This scorching world spins slowly in the opposite direction of most planets, but its winds blow as fast as hurricanes, sending Venus’ acidic clouds for a spin around the planet once every five days.

Venus’ atmosphere consists primarily of carbon dioxide and traps heat like greenhouse gases do on Earth.

Evidence suggests that Venus may have had different conditions during its early history, with possible water flowing on its surface. But as the planet heated up, the oceans evaporated and its surface temperature became so hot that any life would have been destroyed.

On the other hand, Venus’ clouds are not such a hopeless case since the temperatures at its clouds are slightly more bearable. The planet’s potential habitability centers on liquid droplets there.

It’s also the location of recent controversy. In September 2020, a potentially groundbreaking discovery on Venus claimed to provide evidence for habitability in Venus’ clouds. Using the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter/submillimeter Array in Chile, scientists detected traces of phosphine gas in Venus’ atmosphere.

When searching for signs of life on other planets, scientists look for traces of these “bio-signature” gases to help them identify if a planet is potentially habitable. Astrobiologists consider phosphine a biosignature gas on Earth, meaning that a living organism typically produces it.

But the results were met with doubt as members of the science community debated whether other conditions on Venus could have created the phosphine or if the signal was produced by phosphine at all.

WHAT’S NEW — Following the initial discovery, some team members wanted to further investigate the possibility of habitable conditions in the Venusian clouds, independent of the phosphine detection, according to Seager.

Instead, the team behind the new study created a chemical model of Venus’ atmosphere with the hypothesis that there is life in the atmosphere generating ammonia gas.

“This sets off a chain of reactions which helps to explain a lot of the observations from the past which haven’t been explained before,” Seager says.

The model set out to explain the presence of oxygen in Venus’ atmosphere and an unusual pattern of sulfur dioxide and water vapor.

The study suggests the presence of ammonia in Venus’ clouds, which exists in some of Earth’s extreme environments that harbor life. Although the scientists are not sure of the source of ammonia, they suggest that it may be produced by biological processes that would also explain the presence of oxygen in Venus’ atmosphere.

“This study is suggesting that there is ammonia in the atmosphere and this chain of chemical reactions is actually happening,” Seager says. “And a really nice consequence of that is that some of the cloud droplets on Venus would be more habitable than previously thought.”

Most of the cloud droplets are made of sulfuric acid, which is dangerous and toxic to life on Earth. But if this new model is correct, then some of the droplets would not be as acidic as pure concentrated sulfuric acid.

WHY IT MATTERS — Although the new study is not claiming life on Venus, it is an important step in figuring out the mysteries of Venus’ atmosphere and whether the planet holds the potential for habitability.

“We’re not saying there’s life on Venus,” Seager says. “We’re just putting forward a hypothesis.”

To prove life on another planet, it would take a lot of data and a series of hypotheses, which are all a part of the scientific process, especially considering how big the claim of alien life is. It would also take a series of in situ measurements.

WHAT’S NEXT — Venus enthusiasts rejoiced earlier this year when NASA announced not one but two missions to the scorching hot planet.

In April, NASA selected both DAVINCI+ and VERITAS missions to study the planet’s atmosphere and geological history.

This would mark a return to Venus after decades of being away from the planet. NASA sent several missions to Venus in the 1960s and 1970s, but the last one arrived at Venus in 1989 and shut down in 1994.

But Seager points out that neither of these missions is equipped to study the cloud particles on Venus and are not astrobiology focused. She suggests a mission to Venus that would investigate the presence of oxygen and ammonia in the planet’s atmosphere and study Venus’ cloud particles directly.

“And in the very far future, if everything goes well, we can try to bring a sample of the cloud material back to Earth and look for life itself,” Seager says.

Abstract — The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O2 in the cloud layers, an unknown composition of large particles in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO2 and H2O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH3), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH3 dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO2 as ammonium sulfite salts. This trapping of SO2 in the clouds, together with the release of SO2 below the clouds as the droplets settle out to higher temperatures, explains the vertical SO2 abundance anomaly. A consequence of the presence of NH3 is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH3 is unknown but could involve biological production; if so, then the most energy-efficient NH3-producing reaction also creates O2, explaining the detection of O2 in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.