Scientists say they’ve detected a chemical associated with biological activity within the clouds of Venus, at a height where airborne life forms could theoretically exist.
The chemical, known as PH3 or phosphine, isn’t the first biomarker to be found in Venus’ atmosphere. But the scientists say they can’t come up with a non-biological process that could produce phosphine at the levels they’re seeing.
This isn’t the smoking gun for life on Venus. Nevertheless, the latest findings — which leaked out over the weekend and were published today in Nature Astronomy — give peer-reviewed weight to an idea that once seemed almost ludicrous: the idea that microbes or other life forms may be perpetually floating in Venus’ acidic air, more than 30 miles above the planet’s searingly hot surface.
The findings are also likely to give a push to several proposed space missions that are already targeting the clouds of Venus.
“It may be that Venus, not Mars, is our best hope for a long-inhabited nearby neighbor,” David Grinspoon, a senior scientist at the Planetary Science Instutute, told me in an email.
The possibility of finding life in Venus’ clouds has been under debate for decades. The late astronomer Carl Sagan surveyed the prospects almost 60 years ago. More recently, Grinspoon and other astrobiologists have revived the case for closer study of Venus, in hopes of finding traces of microbial life in the clouds.
Grinspoon told me it’s been a tough sell. “Folks would roll their eyes at my conference talks, but I was tolerated because I did a lot of good work on other aspects of Venus, writing papers on the clouds, the surface evolution, the climate, and so forth,” he said.
It’s not hard to see why Venus has been upstaged by Mars as well as the icy moons of Jupiter and Saturn when it comes to the search for life elsewhere in the solar system. Although Venus is close to Earth’s size and mass, its average surface temperature of 900 degrees Fahrenheit is hot enough to melt lead, due to a runaway greenhouse-gas effect.
Venus’ dense, surface-obscuring atmosphere consists primarily of carbon dioxide, but it’s also laced with droplets of sulfuric acid that makes it inhospitable to most life on Earth.
Even if amped-up versions of our own planet’s acid-loving microbes were to exist on Venus, the only place astrobiologists can imagine them getting a foothold would be within a temperate band of clouds that lie between 30 and 40 miles above the surface.
Just last month, a team of scientists — including some of the co-authors of the newly published study — proposed a spore-based life cycle for aerial microbes within that cloud band.
What kind of evidence might such creatures leave behind? Researchers at the Massachusetts Institute of Technology zeroed in on phosphine — a smelly, toxic gas given off by anaerobic bacteria on Earth. MIT planetary scientist Clara Sousa-Silva thought the spectral fingerprint of phosphine would be a good biosignature to look for when advanced telescopes analyze the light reflected by planets in alien star systems.
“I was thinking really far, many parsecs away, and really not thinking literally the nearest planet to us,” she said in a news release.
The astronomers who focused in on Venus weren’t expecting to find phosphine, either. When they observed the planet using the James Clerk Maxwell Telescope in Hawaii, they expected to rule out some of the claims surrounding life on Venus.
“This was an experiment made out of pure curiosity, really — taking advantage of JCMT’s powerful technology, and thinking about future instruments,” study lead author Jane Greaves, an astronomer at Cardiff University in Wales, said in a news release.
“I thought we’d just be able to rule out extreme scenarios, like the clouds being stuffed with organisms,” she said. “When we got the first hints of phosphine in Venus’ spectrum, it was a shock!”
What’s more, the phosphine was found precisely in the band of the cloud layer that’s most hospitable to life.
The detection was confirmed with follow-up observations from the Atacama Large Millimeter Array, or ALMA, in Chile. Greaves and her team then turned to other scientists to help interpret the findings.
Researchers considered a wide range of non-biological mechanisms for putting phosphine into the Venusian atmosphere — for example, by cooking other molecules with solar radiation or lightning, or having the wind sweep up minerals from the surface, or having the phosphine expelled by volcanoes, or bringing it in from space via meteors.
Phosphine is created non-biologically at Jupiter and Saturn, due to the abundance of hydrogen and the crushing atmospheric pressure at those gas giants, but the researchers noted that such conditions don’t exist on Venus. “That particular chemistry is definitely not happening at Venus,” MIT’s William Bains said today during a news briefing.
None of the mechanisms that the researchers considered could produce the level of phosphine that the astronomers detected, which amounts to 20 molecules per billion. Their most productive non-biological scenario could make, at most, only one-ten-thousandth of the required amount.
That leaves the biological scenario as the favored explanation, unless someone else comes up with a better explanation that the research team missed.
“It’s very hard to prove a negative,” Sousa-Silva said. “Now, astronomers will think of all the ways to justify phosphine without life, and I welcome that. Please do, because we are at the end of our possibilities to show abiotic processes that can make phosphine.”
On Earth, microbes are routinely lofted into upper levels of the atmosphere and eventually drift back down. But on Venus, such organisms would be killed off if they sank too low. Such an exclusively aerial biosphere might have evolved from an earlier age when Venus was far more hospitable to life, Sousa-Silva said.
“A long time ago, Venus is thought to have oceans, and was probably habitable like Earth,” she said. “As Venus because less hospitable, life would have had to adapt, and they could now be in this narrow envelope of the atmosphere where they can still survive.”
So what’s next? Sousa-Silva and MIT’s Jason Dittman are leading an effort to confirm the phosphine findings with data from other telescopes, and map the distribution of phosphine across the Venusian atmosphere over time. If there are daily or seasonal variations, that could provide additional evidence for biological activity.
“The experiment must and will be repeated,” Grinspoon told me. “Laboratory studies will be undertaken to see how PH3 behaves in a Venus-like environment and what else could possibly produce it. But the best test, and the one I’m most excited about, is to go back to Venus and investigate the atmosphere in situ.”
Last month, a panel of scientists presented a 222-page report laying out the possibilities for a flagship mission to Venus, as part of the astronomy community’s 2020 decadal survey of science priorities.
One mission concept, advanced by Northrop Grumman, calls for sending an instrument-laden, solar-powered aircraft called VAMP into the Venusian atmosphere.
Another concept, known as DAVINCI+, is one of four proposals vying for funding through NASA’s Discovery Program. The DAVINCI+ spacecraft would map Venus and its atmosphere from orbit. It’d also drop a spherical probe through the atmosphere, all the way to the surface, to sniff out the molecules making up each layer.
“Our vision for DAVINCI+ is to send a chemistry lab and orbiter to Venus to put the planet into its appropriate context in our solar system,” principal investigator Jim Garvin, who is chief scientist at NASA’s Goddard Space Flight Center, said in a news release.
If DAVINCI+ is selected for full funding next year, Garvin and his teammates propose launching the mission in 2026.
Yet another Discovery Program finalist, the proposed VERITAS mission, would concentrate on creating three-dimensional maps of Venus’ surface features and geology. NASA is also considering a CubeSat mission to study Venus’ atmosphere.
Meanwhile, California-based Rocket Lab is making plans to send a probe to Venus within three years or so.
“I’m working very hard to put together a private mission to go to Venus in 2023,” Rocket Lab CEO Peter Beck said last month during a webcast. “We’re going to learn a lot on the way there, and we’re going to have a crack at seeing if we can discover what’s in that atmospheric zone. And who knows? You may hit the jackpot.”
MIT’s Sara Seager said she and her colleagues have been talking with Rocket Lab about putting together the scientific payload for such a mission. The requirements are challenging: Such a payload would have to weigh no more than 3 kilograms (6.6 pounds), Seager said.
Details about potential funding for Rocket Lab’s mission haven’t yet come to light, but Russian-Israeli tech billionaire Yuri Milner is known to have Venus on his short list for a privately funded mission.
Back in 1985, the twin Soviet Vega probes deployed two balloon explorers in the Venusian atmosphere. Instruments on the balloons sent back data for 46 hours before their batteries ran out. Today, Seager was asked about that mission concept and said “a balloon is certainly the best way” to study what’s in the clouds.
“We have a long list of things we’d like, actually,” she said.
Over the past 30 years, NASA, the European Space Agency and the Japan Aerospace Exploration Agency have sent probes to Venus. In light of the findings published today, Grinspoon thinks it’s high time for the next visit.
“Now that we’ve found a genuine candidate biosignature, we absolutely must go,” he said. “And even if this turns out to be a false alarm, it could be productive, in the way that the ‘Mars rock’ (ALH84001) was. That turned out — probably — to be a false alarm, but it got everyone to think about it in a fresh way and ask, ‘Why not?’ ”
Update for 8:50 a.m. PT Sept. 14: NASA’s associate administrator for science, Thomas Zurbuchen, tweeted that the findings are “intriguing” but added that NASA will defer further comment until the post-publication discussion has run its course:
Update for 1:10 p.m. PT Sept. 14: Later in the day, NASA Administrator Jim Bridenstine tweeted that “it’s time to prioritize Venus” — which will probably lift the spirits of the folks working on the aforementioned proposals for missions to Venus:
In addition to Greaves, Sousa-Silva, Bains and Seager, the authors of the Nature Astronomy paper, “Phosphine Gas in the Cloud Decks of Venus,” include Anita Richards, Paul Rimmer, Hideo Sagawa, David Clements, Janusz Petkowski, Sukrit Ranjan, Emily Drabek-Maunder, Helen Fraser, Annabel Cartwright, Ingo Mueller-Wodarg, Zhuchang Zhan, Per Friberg, Iain Coulson, E’lisa Lee and Jim Hoge.