Phosphine is a colorless, flammable, toxic gas that smells like rotting fish. Humans manufacture it to use in pest control and the production of computer chips. But it’s also a waste product from a certain kind of “abiotic” microbe that lives in oxygen-free environments. Its presence is a potential sign that there’s something alive.
The gas with the chemical formula PH3 has been at the center of a passionate debate among scientists concerned with, well, life: what it is, what it needs to survive, and where it could be located elsewhere in the universe.
On one side are are scientists and their supporters who, a year ago, claimed they had detected signs of phosphine in the practically unlivable atmosphere of Venus—the second planet from the sun best known for its boiling, 800-degree-Fahrenheit surface and thick clouds made not of water, but acid. Whether intentionally or not, these researchers set off the alarms that perhaps we have discovered signs of extraterrestrial life on another world.
On the other side are critics who have credibly questioned the science behind the original phosphine claim. And between the two camps is a powerful mediator: NASA’s top scientist, who recently penned a paper to address the increasingly heated argument over a very stinky gas and its possible presence on Venus, and to urge scientists searching for alien life to be a little more careful.
Now a new group of scientists—including some members of the team that first posited phosphine on Venus—is stepping back, taking a deep breath and trying to make sense of what they describe as an important and ongoing argument. A preprint of their paper appeared online last week. “One year after the original announcement, the tentative discovery of PH3 in the clouds of Venus continues to bring much interest and controversy,” they wrote.
“People might think that the Venus phosphine story is over, that the discovery is debunked or that it is wrong, that the signal is not there,” Janusz Petkowski, an expert in so-called “biosignature gases” at MIT and a co-author of both the original phosphine paper and the latest one, told The Daily Beast. “That is not the case,” Petkowski added. “The Venusian phosphine story is very much alive and a topic of an intense scientific debate.”
Back in 2019, a team led by astronomer Jane Greaves at Cardiff University in the U.K. had a hunch. Sifting through images of Venus gathered by colleagues peering through the James Clerk Maxwell Telescope in Hawaii, the team saw what it suspected might be the visual signature of phosphine in Venus’s dense, toxic atmosphere.
So Greaves’ team commandeered the Atacama Large Millimeter/submillimeter Array in Chile and pointed it at Venus specifically to look for phosphine. A year later in September 2020, the team announced to the world the shocking news: It had found phosphine. And the implications were revolutionary.
There are, Greaves and company wrote in Nature, “no currently known abiotic production routes [for phosphine] in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery.” While phosphine “could originate from unknown photochemistry or geochemistry,” the better explanation might be “the presence of life.”
Greaves did not immediately respond to a request for comment.
The astronomical community was abuzz with the news. But the excitement quickly soured into skepticism. “The community still considers Venus to be the least likely abode of life,” Petkowski and his co-authors explained. Europa and Enceladus, respectively moons of Jupiter and Saturn, seem to be more friendly to microbes, as do Mars and Titan, another moon of Saturn.
“This skepticism results from the fact that the Venusian environment has many challenges for life as we know it,” Petkowski’s team wrote. Namely, there’s the fact that Venus is wrapped in clouds of sulfuric acid.
The doubt deepened in some quarters as scientists checked Greaves’ team’s work. Some pointed out that ALMA isn’t the best instrument for inspecting an object that’s as bright as Venus is. The scrutiny compelled Greaves and company to recheck their data and publish a couple of minor corrections to their original paper.
“Our discovery of phosphine in Venus’s clouds has sparked much debate,” they acknowledged. But they stood by their conclusion that there could be phosphine on Venus, and it might be evidence of “life in a hyperacidic aerial biosphere.”
The argument raged on, to the point where Jim Green, the chief scientist at NASA, felt compelled to step in. In a paper published in Nature on October 27, Green proposed what he described as a “framework for reporting evidence for life beyond Earth.”
“Our generation could realistically be the one to discover evidence of life beyond Earth,” Green wrote. “With this privileged potential comes responsibility.” To avoid premature declarations based on cursory research, the NASA scientist urged his colleagues—as well as members of the press—to hold claims about extraterrestrial life up to a seven-step evidentiary scale ranging from level one (interesting) to level seven (definitive).
Venusian phosphine is still a “level one measurement,” Green told The Daily Beast. “Why would we promote it or talk about it like it was a level five event?”
“The magnitude of the question of whether we are alone in the universe, and the public interest therein, opens the possibility that results may be taken to imply more than the observations support, or than the observers intend,” Green wrote. In other words, scientists should think long and hard before insisting they’ve found evidence of aliens, even if they are just microbes. It’s too important to get wrong.
It’s not at all clear Greaves and her team did get it wrong, Petkowski and his colleagues stressed. After examining Greaves’ data a couple different ways, they concluded “there is strongly suggestive evidence from two independent methods that there is phosphine in the cloud decks of Venus.”
The next question, of course, is where the phosphine came from. There’s no consensus whether phosphine “can be made by natural processes,” Avi Loeb, a Harvard University physicist, told The Daily Beast.
Some scientists think the gas Greaves and her team detected might, in part, be a byproduct of volcanic eruptions. “Although the original phosphine study by Greaves [and her team] estimated that the amount of phosphine detected is much too high to be solely contributed by volcanic eruptions, they would certainly be a contributor, if currently occurring on Venus,” Siddharth Krishnamoorthy, a planetary scientist at NASA’s Jet Propulsion Laboratory in California, told The Daily Beast.
In addition to volcanoes, Petkowski and his co-authors assessed all the other major, possible non-biological sources of phosphine on Venus: lightning, meteor impacts, solar wind, and weird chemical processes in the atmosphere.
None make a lot of sense, they wrote. “None of the examined processes produce sufficient amounts of PH3 to explain the observed... abundance.” That jibes with what the Greaves investigation originally concluded last year.
That doesn’t necessarily mean there’s got to be life on Venus. It does mean that, despite the skepticism in certain corners of the astronomy world, we can’t yet rule out the possibility of life on Venus. “A chemical explanation seems more likely, but biology should not be dismissed as a possible explanation,” Dirk Schulze-Makuch, an astrobiologist at the Technical University Berlin, told The Daily Beast.
Yes, Venus is nasty for life as we know it. But it might be just right for some lifeform we don't currently know about. Life might have found a way to adapt to that planet’s acid atmosphere, Schulze-Makuch said.
It’s going to take a lot more exploration, and much closer observation, to get a handle on what’s going on in the acidic clouds of the second planet from the sun. Petkowski and his team don’t think we’ll get any real resolution to the phosphine question until we actually study it directly. In other words, we need to send a probe to Venus.
Fortunately, there are two in development. The Deep Atmosphere Venus Investigation of Noble gases, Chemistry and Imaging mission, or DAVINCI+, aims to scan Venus’ atmosphere with a spherical probe that will plunge through the planet’s atmosphere, sampling the unbreathable air “to understand why Venus’ atmosphere is a runaway hothouse compared the Earth’s,” according to NASA.
Around the same time, the separate Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy probe—or VERITAS—will orbit Venus and use its on-board radar and infrared sensor to scan the planet.
The probes are scheduled to launch around 2029 and arrive at Venus a couple years later. Maybe then we can finally begin to settle the argument that began a year ago with a bold claim about a curious gas on a seemingly inhospitable planet.