Strange gas in Venus’s clouds may be a sign of volcanoes, not life

By Leah Crane

Maat Mons, a large volcano on Venus, in a simulated-color radar image from NASA’s Magellan spacecraft

NASA/JPL

The unexpected discovery of a gas called phosphine on Venus led to speculation that there may be life floating in the planet’s clouds – but it may have come from huge volcanic eruptions instead.

In 2020, a team led by Jane Greaves at Cardiff University in the UK saw evidence of phosphine in Venus’s clouds, which are primarily made of concentrated sulphuric acid. When the team analysed ways to make phosphine on Venus, they didn’t find any that could produce enough of it to explain the signal. They suggested that it may have come from living organisms, which is the main way the gas is made on Earth.

Now, Ngoc Truong and Jonathan Lunine at Cornell University in New York have calculated that if Venus is as volcanically active as some of the most volcanic areas on Earth, that could produce enough phosphine to explain the signal without invoking the possibility of life on Venus.

The thick atmosphere on Venus has made it difficult to study its surface, so we don’t know for certain whether it is volcanically active. “Many of the volcanic eruptions on Earth are things that would escape our attention if they happened on Venus because of this blanket of sulphuric acid clouds,” says Lunine.

However, there are hints that volcanoes may be erupting on Venus. Radar images from orbiting spacecraft have shown features that could be relatively fresh lava, but it isn’t clear that is what they are. And changing amounts of sulphur dioxide in the air could be explained by eruptions tossing particles aloft. Truong and Lunine suggest that phosphorus in the planet’s mantle could erupt from volcanoes in huge plumes, and then interact with sulphuric acid to form phosphine.

Not everyone agrees that this is a viable explanation. “We do not think that deep mantle plume volcanism can produce sufficient amounts of phosphine to explain the observations,” says Janusz Petkowski at the Massachusetts Institute of Technology, a member of Greaves’s team. He says that it isn’t clear whether there is as much phosphorus in the Venusian mantle as Truong and Lunine assumed based on comparisons with Earth.

Additionally, we don’t know enough about the chemistry of Venus’s atmosphere to say for sure what would happen if that phosphorus was erupted into the sky. “I’d expect chemical spikes of other gases if a huge plume had happened,” says Greaves. We haven’t seen such unexplained spikes in the abundances of other chemicals in the atmosphere.

Lunine agrees that we don’t have enough data to say for sure what might be producing the phosphine, but he says volcanism is a less outlandish potential explanation than life in Venus’s toxic clouds. “Unfortunately, we’re sitting here with these little hints of volcanism from all these pieces of circumstantial evidence, phosphine included,” he says. “We don’t know what Venus is capable of.”

Journal reference: PNAS, DOI: 10.1073/pnas.2021689118

Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every FridayMore on these topics:

1. planets
   

2. solar system
   

3. venus
   

Read More