Something deadly might be wafting through the clouds shrouding Venus—a smelly, flammable gas called phosphine that annihilates life-forms reliant on oxygen for survival. Ironically, though, the scientists who today announced sightings of this noxious gas in the Venusian atmosphere say it could be tantalizing—if controversial—evidence of life on the planet next door.
As far as we know, on rocky planets such as Venus and Earth, phosphine can only be made by life—whether human or microbe. Used as a chemical weapon during World War I, phosphine is still manufactured as an agricultural fumigant, is used in the semiconductor industry, and is a nasty byproduct of meth labs. But phosphine is also made naturally by some species of anaerobic bacteria—organisms that live in the oxygen-starved environments of landfills, marshlands, and even animal guts.
Earlier this year, researchers surmised that finding the chemical on other terrestrial planets could indicate the presence of alien metabolisms, and they suggested aiming the sharpest telescopes of the future at faraway exoplanets to probe their atmospheres for signs of the gas.
Now, we may have found signs of phosphine on the planet next door, astronomers report in the journal Nature Astronomy.
“I immediately freaked out, of course. I presumed it was a mistake, but I very much wanted it to not be a mistake,” says study co-author Clara Sousa-Silva, a postdoctoral researcher at the Massachusetts Institute of Technology (MIT) who initially identified phosphine as a potential biosignature.
Put simply, phosphine shouldn’t be in the Venusian atmosphere. It’s extremely hard to make, and the chemistry in the clouds should destroy the molecule before it can accumulate to the observed amounts. But it’s too early to conclude that life exists beyond Earth’s shores. Scientists caution that the detection itself needs to be verified, as the phosphine fingerprint described in the study could be a false signal introduced by the telescopes or by data processing.
“It’s tremendously exciting, and we have a sort of obligatory response of first questioning whether the result is real,” says David Grinspoon of the Planetary Science Institute. “When somebody comes up with an extraordinary observation that hasn’t been made before, you wonder if they could have done something wrong.”
But if phosphine really is floating through the Venusian cloud deck, its presence suggests one of two intriguing possibilities: that alien life-forms are deftly linking together phosphorus and hydrogen atoms, or that some completely unanticipated chemistry is crafting phosphine in the absence of life.
Life on a “blasted hellhole”
Venus, the second world from the sun, has long been considered Earth’s twin. It’s about the same size as our home planet, with similar gravity and composition. For centuries, hopeful humans thought its surface might be covered in oceans, lush vegetation, and verdant ecosystems, providing a second oasis for life in the solar system.
Then reality intruded.
Early science observations of the planet next door revealed that it is a menace of a world that could kill Earthlings in multiple ways. Its surface can reach a sweltering 900 degrees Fahrenheit. Tucked beneath as many as 65 miles of cloud and haze, those roasted rocks are smothered by a bone-crushing amount of pressure, more than 90 times what’s felt on Earth’s surface. Plus, the planet’s atmosphere is primarily suffocating carbon dioxide populated by sulfuric acid clouds.
Even so, scientists have considered the possibility that life might exist in the Venusian cloud deck for nearly 60 years, perhaps thriving where conditions are a bit friendlier.
“While the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether,” Carl Sagan and Harold Morowitz wrote in the journal Nature back in 1967.
Despite the acid, the clouds carry the basic ingredients for life as we know it: sunlight, water, and organic molecules. And near the middle of the cloud layer, temperatures and pressures are rather Earthlike. “It’s shirt-sleeve weather, with all these tasty things to eat,” says Martha Gilmore, a Wesleyan University planetary scientist and leader of a proposed mission to Venus, referring to molecules in the planet’s air that microbes could metabolize.
Early observations of the planet revealed that parts of its atmosphere absorb more ultraviolet light than expected, an anomaly that scientists hypothesized could be the work of aerial microbes. While the phenomenon is more likely due to the presence of sulfur-containing compounds, a handful of scientists have since elaborated on the possibility of airborne Venusians, laying out scenarios in which microbes might metabolize sulfur compounds, stay afloat among the ever-present clouds, and even develop life cycles enabled by periods of dormancy at varying altitudes.
“When I first started talking about it, there was a lot of resistance, mostly because it’s such a harshly acidic environment,” says Grinspoon, who has pushed the idea of cloud-borne life on Venus since the mid-1990s.
But everything we’ve learned about life on Earth suggests that it will move into every available nook and cranny. Here, we find microbes thriving in hostile, corrosive environments such as hot springs and volcanic fields. We also know that microbes regularly hitch a ride on cloud particles, and scientists have found organisms flying more than six miles above the Caribbean. Clouds are ephemeral on Earth, so it’s unlikely that they support permanent ecosystems, but on Venus, cloudy days are in the forecast for millions or even billions of years.
“On Venus, that puddle never dries up,” Grinspoon says. “The clouds are continuous and thick and globe-spanning.”
Although Venus is a roasting world today, observations suggest that it once had a liquid water ocean. For most of its history, Venus could have been as habitable as Earth—until sometime in the last billion years, when ballooning greenhouse gases transformed the planet from an oasis into a death trap. Perhaps, as the scorched surface became less hospitable, life-forms migrated into the clouds to avoid certain extinction.
Any life there now is “much more likely to be a relic of a more dominating early biosphere,” says Penelope Boston, a NASA astrobiologist who specializes in studying microbes in weird places on Earth. She’s skeptical, though. “I think it’s a blasted hellhole now, so how much of that ancient signal could have held up?”
The deadly gas of life
In June 2017, Cardiff University’s Jane Greaves and colleagues took a look at Venus using the James Clerk Maxwell Telescope, which scans the sky in radio wavelengths from its perch atop Mauna Kea in Hawaii. They were looking for rare gases or molecules that might be biological in origin. Among the signatures they spotted was that of phosphine gas, a pyramidal molecule comprising three hydrogen atoms joined to a single phosphorus atom.
Not long after, Greaves got in touch with Sousa-Silva, who spent her years in graduate school working out whether phosphine could be a viable extraterrestrial biosignature. She had concluded that phosphine could be one of life’s beacons, even though paradoxically, it’s lethal to everything on Earth that requires oxygen to survive.
“I was really fascinated by the macabre nature of phosphine on Earth,” she says. “It’s a killing machine ... and almost a romantic biosignature because it was a sign of death.”
In 2019, Greaves, Sousa-Silva, and their colleagues followed up on the initial phosphine observation using ALMA, an array of telescopes on a high Chilean plateau. More sensitive than the Hawaii-based telescope, ALMA also observes the sky at radio frequencies, and it can detect the energy emitted and absorbed by any phosphine molecules spinning in the Venusian atmosphere.
Again, the team detected phosphine. This time, scientists could narrow down the molecule’s signal to equatorial latitudes and an altitude between 32 and 37 miles, where temperatures and pressures aren’t too harsh for life as we know it. Based on the signal’s strength, the team calculated that phosphine’s abundance is roughly 20 parts per billion, or at least a thousand times more than we find on Earth.
In the outer solar system, phosphine is made deep in the interiors of Jupiter and Saturn. Near the giant planets’ cores, the temperatures and pressures are extreme enough to craft the molecule, which then rises through the atmosphere. But on rocky planets, where conditions are significantly less extreme, there’s no known way to make phosphine in the absence of life—it’s just too energetically demanding. In other words, if the observation of phosphine on Venus is right, something must be continually replenishing the molecule in the planet’s atmosphere.
“Life is the only thing that will put energy into making molecules,” Sousa-Silva says. “Otherwise, in the universe, chemistry only happens when it’s energetically favorable.”
Astrobiologist Dirk Schulze-Makuch of Technical University Berlin, who has considered cloud-based Venusian life, agrees a biological explanation for the phosphine is possible, but he thinks other unknown geologic or light-induced chemical reactions might yet account for the signal. “Venus is basically still an alien planet,” he says. “There are a lot of things we don’t understand.”
The study team set out to determine whether phosphine could be made on Venus in the absence of biology. Among the scenarios the scientists investigated were volcanic outgassing, intense lightning strikes, tectonic plates rubbing together, bismuth rain, and cosmic dust. Based on the team’s calculations, none of those events could produce the molecule in such abundance.
“Whether it’s life or not, it has to be a really exotic mechanism,” Sousa-Silva says. “Something weird is happening.”
Getting back to Venus
Still, ALMA observatory scientist John Carpenter is skeptical that the phosphine observations themselves are real. The signal is faint, and the team needed to perform an extensive amount of processing to pull it from the data returned by the telescopes. That processing, he says, may have returned an artificial signal at the same frequency as phosphine. He also notes that the standard for remote molecular identification involves detecting multiple fingerprints for the same molecule, which show up at different frequencies on the electromagnetic spectrum. That’s something that the team has not yet done with phosphine.
“They took the right steps to verify the signal, but I’m still not convinced that this is real,” Carpenter says. “If it’s real, it’s a very cool result, but it needs follow-up to make it really convincing.”
Sousa-Silva agrees that the team needs to confirm the phosphine detection by finding additional fingerprints at other wavelengths. She and her colleagues had planned such observations using the Stratospheric Observatory for Infrared Astronomy, a plane-mounted telescope, and with NASA’s Infrared Telescope Facility in Hawaii. But COVID-19 got in the way, and the team’s attempts have been put on hold.
“It’s disappointing that we don’t have this proof,” Sousa-Silva says.
Even so, Sanjay Limaye, a planetary scientist at the University of Wisconsin-Madison, says the discovery is exciting enough to continue searching, and preferably from a much closer vantage point. “It is intriguing that it may point to something strange going on in the atmosphere of Venus, but is it exotic chemistry, or is it life?” he says. “We need to go explore and find out.”
The tentative detection of phosphine is likely to fuel calls for a return to Venus—a trip that some say is long overdue, given that the last time NASA sent a probe to the planet was in 1989. Schulze-Makuch says it’s completely within the realm of possibility to do an atmospheric sample-return mission, sending a spacecraft to swoop through the clouds and gather gas and particles to bring back to Earth.
Several proposed missions are moving through review, including an elaborate, multi-spacecraft concept led by Gilmore of Wesleyan University, which will be evaluated by the planetary science community as it sets its priorities for the next decade of solar system exploration. Gilmore’s concept includes several orbiters and a balloon that would closely study the Venusian atmosphere and look for signs of life.
On the more immediate horizon, a smaller mission to study the deep atmosphere of Venus, named DAVINCI+, is one of the four finalists in NASA’s Discovery program competition. The next mission selection is scheduled to take place in 2021.
“Venus is such a complex, amazing system, and we don’t understand it. And it’s another Earth. It probably had an ocean for billions of years, and it’s right there. It’s just a matter of going,” Gilmore says. “We have the technology right now to go into the atmosphere of Venus. It can be done.”