Gas spotted in Venus’s clouds could be a sign of alien life
by Neel V. Patel / September 14, 2020

“If you ever found yourself on Venus, you’d be destroyed in moments. The pressure at the surface is thought to be up to 100 times greater than what is found on Earth, temperatures are around 464 °C, and the air is more than 96% carbon dioxide. And yet, life on Venus suddenly isn’t the most unimaginable possibility. A new paper published in Nature Astronomy today reveals that Venus’s clouds contain traces of phosphine.

The new findings are far from evidence that there once was or still is life on Venus (an extremely inhospitable planet in more ways than one), but its presence nonetheless suggests there’s some sort of unknown activity happening there, biological or otherwise. The new findings suggest that if life ever existed on Venus, either now or in the past, it might actually be present in the air itself.

“Biology in the atmosphere could be the last surviving members of a prior Venusian biosphere,” says Stephen Kane, an astronomer at the University of California, Riverside, who was not involved with the study. “This result would be an extraordinary lesson in how life really can adapt to all available niches within an environment.” Airborne life on Venus would be unusual, but perhaps not as strange one might think.

Just last month, inspired by the upcoming phosphine findings, MIT astronomer Sara Seager and some of the other coauthors of this new study published a paper about a possible life cycle on Venus that could sustain organisms in the Venusian clouds, emphasizing the fact that the clouds present more temperate and habitable conditions for life.

She suggests that life on Venus could exist in droplets at high altitudes that evaporate and leave dried-up spores hanging in the atmosphere. Unlike Earth, Venus’s clouds are permanent—providing a more stable environment where these spores would dry out and fall to lower altitudes, rise back up in growing droplets in the cloud layer, and rehydrate to continue their life cycle. The goal, says Seager, was to help “plug a hole” in thinking about this environment.

The phosphine in Venus’s clouds was found by Jane Greaves, a planetary scientist with Cardiff University, and her team. They studied the planet using the James Clerk Maxwell Telescope (JCMT) in Hawaii, and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Both observe in submillimeter wavelengths that stretch from far infrared to microwave, which allows scientists to more closely characterize the chemical composition of the atmosphere.

The team found traces of phosphine at a concentration of about 20 parts billion. The data suggests the gas is present in regions closer to the equator and at altitudes of about 55 kilometers, where temperatures are relatively cool (about 30 °C) and the pressure is actually similar to Earth’s. “That suggests it’s part of the global circulation pattern of the atmosphere, where gas sinks before it travels as far as the poles,” says Greaves.

Phosphine is created from phosphorus with three hydrogen atoms. On Earth it is primarily produced naturally by life in oxygen-poor ecosystems, says Clara Sousa-Silva, a molecular astrophysicist at MIT and a coauthor of the new study. “We don’t know why life on Earth produces phosphine—just that it does,” she says.

Anaerobic bacteria produce it in places such as sewage, swamps, marshlands, and rice fields, and in the intestines of most animals. It’s actually an extremely dangerous molecule for oxygen-breathing life. In the absence of life you need exceptionally high temperatures and large amounts of energy to make phosphine (like the conditions found deep inside Jupiter’s atmosphere).

On Earth it is also a product of human industrial activity. The researchers have so far ruled out any known natural routes for phosphine production on Venus, including lightning, volcanism, or meteoritic delivery. So where’s the phosphine coming from? Is it life? Greaves and her team have no clue yet. “All the theories are pretty challenging,” she says. It could be some kind of “exotic chemistry” not seen on Earth, or some hardy organisms capable of surviving very acidic environments on the surface and heating up available phosphorus (though that raises new questions about how phosphorus actually got there).

The team still doesn’t know if the gas actually originates at the “temperate” heights observed in the Venusian clouds, or whether it’s produced closer to the surface and then rises. And the study’s analysis uses models of phosphine behavior based on what we see on Earth; it could be radically different on another planet. “We are not claiming we found life on Venus,” Seager emphasizes.

On their own, the findings inspire more interest in Venus. But they present opportunities for scientists to understand possible biological activity on other worlds as well. “We now understand that Venus has everything to do with habitability,” says Kane. Though Venus is pretty inhospitable today, “Earth and Venus likely had very similar starting conditions, and recent work has shown that Venus may have been habitable, with surface liquid water oceans, as recently as a billion years ago,” he says.

Ultimately, the researchers want to find out more about how phosphine is distributed in the atmosphere, and see whether they can pinpoint a more local source. Other ground-based observations would be useful, but they’re still limited in what they can observe. “We hope our work will motivate future space missions that will go to Venus and directly measure the atmosphere,” says Seager. Unfortunately, there are no new missions to Venus slated for the future. But NASA is currently debating two proposals—both orbiters that could help in this sort of investigation. The new findings could help support the case to move forward with either or both of them.”

“Venus clouds sport surprisingly hospitable temperatures and pressures. Researchers have proposed a hypothetical life cycle for microbes surviving within those clouds.”

How Floating Microbes Could Live in the Acid Clouds of Venus
by Kate S. Petersen  /  August 13, 2020

“Venus, with its sulfuric acid clouds and hellish surface temperatures, is often ignored as a potential abode for life. But some planetary scientists have suggested that atmosphere-dwelling microbes could survive in its lower cloud layers, possibly explaining Venus’ mysterious atmospheric phenomena. The cloud layers in question — hovering roughly 30 to 37 miles (48 to 60 kilometers) above Venus’ sweltering surface — feature arguably livable temperatures, nutrients, and even a bit of water dissolved inside droplets of sulfuric acid.

Now, a team led by Sara Seager, an astrophysicist at the Massachusetts Institute of Technology (MIT), proposes a hypothetical life cycle for how microbes might survive in Venus’ atmosphere. The researchers claim they are the first to hypothesize a specific mechanism by which organisms could persist in the venusian haze and cloud layers, rather than being rained down and destroyed by the fiery surface conditions below. The team outlines how venusian microbes could cycle through the different layers of the atmosphere, surviving the most extreme conditions by transitioning into a dormant state. The new research was published August 13 in Astrobiology.

Microbes in the Clouds of Venus
Cloud-borne microbes are not unprecedented in the solar system. “On our own Earth, we also have what we call an aerial biosphere,” Seager tells Astronomy. “[Microbes] from Earth get upswept and they float around. They get inside droplets and the wind carries them across entire continents and oceans.” Seager explains that, over time, the water droplets housing microbes in Earth’s atmosphere condense, growing larger and heavier. And eventually, they fall down to the surface. But falling from Venus’ clouds would almost certainly be lethal to any microbes, as the planet’s blistering surface temperatures reach more than 860 degrees Fahrenheit (460 Celsius). “So, the question we posed was, ‘Well, how does it live permanently in the clouds?” says Seager.

“Venus is permanently shrouded in clouds. But the middle and lower cloud layers host temperatures and pressures that are surprisingly hospitable to life. But in Venus’ lower haze layer, the conditions take a turn for the worse.”

Co-author Janusz Petkowski, a research fellow at MIT, explains to Astronomy that the team’s hypothesis is that venusian life could exist in a metabolically active state — growing and reproducing — in sulfuric acid cloud droplets within the lower cloud layer. Over a period of a few months, the droplets themselves would grow in mass just like water droplets on Earth. Eventually, they would be large enough to ‘rain out’ of the lower clouds. And that means they would fall into Venus’s lower haze layer. This haze layer, which stretches from 20 to 30 miles (33 to 48 km) above the surface, is much hotter than the clouds above it, with temperatures ranging from about 170 to 368 F (77 to 187 C).

“The proposed life cycle for microbes surviving in the acid clouds of Venus is seen in this illustration. (1) Dehydrated microbes survive in a vegetative state in Venus’ lower haze layer. (2) The spores are lifted by updrafts into the habitable cloud layer. (3) Once encapsulated by liquid, the spores become metabolically active. (4) These microbes divide, and the droplets grow through coagulation. (5) The droplets grow large enough that they sink through the atmosphere, where they begin to evaporate due to higher temperatures, prompting microbes to transform into spores that float in the lower haze layer.”

Seager’s team suggests that if the sulfuric acid droplets fall toward this hyper-heated haze layer, they would evaporate. Without a droplet protecting the hypothetical microbes, they would then go through a metabolic transformation and enter a protective dormant state to survive the extreme heat and dehydration.

These dormant microbes, which the researchers refer to as ‘spores,’ could then persist, suspended in Venus’ lower haze layer. But they don’t stay there forever. Eventually, Venus’ atmospheric updrafts would blow the spores back up into the planet’s temperate cloud layers, where they would be rehydrated, reviving them to an active state.

Searching for Microbes in the Clouds of Venus
Like an aerial biosphere, the proposal that venusian microbes could enter a dormant state to survive harsh conditions has precedent on Earth. Tiny, segmented creatures called tardigrades have become famous for using this strategy to survive freezing, irradiation, and even the vacuum of space. And while this life-in-the-clouds-of-Venus scenario remains hypothetical, “there’s a robust case to be made for thinking about [life on] the planets within our solar system…because we haven’t actually explored them as thoroughly as we’d like,” Sukrit Ranjan, co-author on the paper and postdoctoral fellow at MIT, tells Astronomy. “The possibility of life on Venus is a testable hypothesis. And, so, it’s worth thinking about whether it’s worth investing the resources to carry out that test.”

Dirk Schulze-Makuch, a professor at Technical University Berlin who was not involved in the research but studies habitability on Venus, says that one possible approach to looking for life in venusian clouds would be a fly-over mission, where a spacecraft travels through the lower cloud layers and sucks up samples. These could then be evaluated remotely to see if they contain any intriguing organic molecules. “If we find organics on site, then … [we] say, okay, we actually have to look at this,” Schulze-Makuch tells Astronomy.

If organics were discovered, researchers would then have a case for a sample return mission, wherein venusian cloud samples would be transported back to Earth for analysis with more sophisticated instruments. Such a mission would be costly, but it could be appealing if scientists become more convinced that life could exist on Venus. Pondering how extraterrestrial organisms might function in an unearthly environment is vital to the search for life on other planets. But in their paper, Seager’s team offers a sobering counterpoint to the hypothesis of venusian life.

While acid-loving extremophiles, such as certain archaea species, are often propped up as examples of how life could survive on Venus, the team cautions that Venus’ cloud layer is far more acidic than any environment found on Earth. Plus, DNA, RNA, sugars, and proteins that are necessary for life on Earth are all destroyed by sulfuric acid. So, if floating microbes exist on Venus — whether active or dormant — they are almost guaranteed to be remarkably alien.”

Possible sign of life on Venus stirs up heated debate
by Nadia Drake  /  September 14, 2020

“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 landfillsmarshlands, 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.

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 compoundsstay 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?”

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.”

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.”



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