Phosphine gas found in Venus’ atmosphere may be ‘a possible sign of life’

Venus’ clouds appear to contain a smelly, toxic gas that could be produced by bacteria, a new study suggests.

Chemical signs of the gas phosphine have been spotted in observations of the Venusian atmosphere, researchers report September 14 in Nature Astronomy. Examining the atmosphere in millimeter wavelengths of light showed that the planet’s clouds appear to contain up to 20 parts per billion of phosphine — enough that something must be actively producing it, the researchers say. 

If the discovery holds up, and if no other explanations for the gas are found, then the hellish planet next door could be the first to yield signs of extraterrestrial life — though those are very big ifs.

“We’re not saying it’s life,” says astronomer Jane Greaves of Cardiff University in Wales. “We’re saying it’s a possible sign of life.”

Venus has roughly the same mass and size as Earth, so, from far away, the neighboring planet might look like a habitable world (SN: 10/4/19). But up close, Venus is a scorching hellscape with sulfuric acid rain and crushing atmospheric pressures.

Still, Venus might have been more hospitable in the recent past (SN: 8/26/16). And the current harsh conditions haven’t stopped astrobiologists from speculating about niches on Venus where present-day life could hang on, such as the temperate cloud decks.

“Fifty kilometers above the surface of Venus, the conditions are what you would find if you walk out of your door right now,” at least in terms of atmospheric pressure and temperature, says planetary scientist Sanjay Limaye of the University of Wisconsin–Madison, who was not involved in the new study. The chemistry is alien, but “that’s a hospitable environment for life.”

Previous work led by astrochemist Clara Sousa-Silva at MIT suggested that phosphine could be a promising biosignature, a chemical signature of life that can be detected in the atmospheres of other planets using Earth-based or space telescopes.

On Earth, phosphine is associated with microbes or industrial activity — although that doesn’t mean it’s pleasant. “It’s a horrific molecule. It’s terrifying,” Sousa-Silva says. For most Earthly life, phosphine is poisonous because “it interferes with oxygen metabolism in a variety of macabre ways.” For anaerobic life, which does not use oxygen, “phosphine is not so evil,” Sousa-Silva says. Anaerobic microbes living in such places as sewage, swamps and the intestinal tracts of animals from penguins to people are the only known life-forms on Earth that produce the molecule.  

Still, when Greaves and colleagues searched Venus’ skies for signs of phosphine, the researchers didn’t expect to actually find any. Greaves looked at Venus with the James Clerk Maxwell Telescope in Hawaii over five mornings in June 2017, aiming to set a detectability benchmark for future studies seeking the gas in the atmospheres of exoplanets (SN: 5/4/20), but was startled to find the hints of phosphine. “That’s a complete surprise,” Greaves says. When she was analyzing the observations, “I thought ‘Oh, I must have done it wrong.’”

So the team checked again with a more powerful telescope, the Atacama Large Millimeter/submillimeter Array in Chile, in March 2019. But the signature of phosphine — seen as a dip in the spectrum of light at about 1.12 millimeters — was still there. The gas absorbs light in that wavelength. Some other molecules also absorb light near that wavelength, but those either couldn’t explain the whole signal or seemed improbable, Greaves says. “One of those is a plastic,” she says. “I think a floating plastic factory is a less plausible explanation than just saying there’s phosphine.”

Phosphine takes a fair amount of energy to create and is easily destroyed by sunlight or sulfuric acid, which is found in Venus’ atmosphere. So if the gas was produced a long time ago, it shouldn’t still be detectable. “There has to be a source,” Greaves says.

Greaves, Sousa-Silva and colleagues considered every explanation they could think of apart from life: atmospheric chemistry; ground and subsurface chemistry; volcanoes outgassing phosphine from the Venusian interior; meteorites peppering the atmosphere with phosphine from the outside; lightning; solar wind; tectonic plates sliding against each other. Some of those processes could produce trace amounts of phosphine, the team found, but orders of magnitude less than the team detected.

“We’re at the end of our rope,” Sousa-Silva says. She hopes other scientists will come up with other explanations. “I’m curious what kind of exotic geochemistry people will come up with to explain this abiotically.”

The idea of searching for life on Venus “has been regarded as a pretty out-there concept,” says Planetary Science Institute astrobiologist David Grinspoon, who is based in Washington, D.C. Grinspoon has been publishing about the prospects for life on Venus since 1997, but was not involved in the new discovery.

“So now I hear about this, and I’m delighted,” he says. “Not because I want to declare victory and say this is definite evidence of life on Venus. It’s not. But it’s an intriguing signature that could be a sign of life on Venus. And it obligates us to go investigate further.”

Because of the planet’s acidic atmosphere, extreme pressures and lead-melting temperatures, sending spacecraft to Venus is a challenge (SN: 2/13/18). But several space agencies are considering missions that could fly in the next few decades.

In the meantime, Greaves and colleagues want to confirm the new phosphine detection in other wavelengths of light. Observations they had planned for the spring were put on hold by the coronavirus pandemic. And now, Venus is in a part of its orbit where it’s on the other side of the sun.

“Maybe when Venus comes around on the other side of the sun again,” Greaves says, “things will be better for us here on Earth.”