High in the toxic atmosphere of the planet Venus, astronomers on Earth have discovered signs of what might be life.
If the discovery is confirmed by additional telescope observations and future space missions, it could turn the gaze of scientists toward one of the brightest objects in the night sky. Venus, named after the Roman goddess of beauty, roasts at temperatures of hundreds of degrees and is cloaked by clouds that contain droplets of corrosive sulfuric acid. Few have focused on the rocky planet as a habitat for something living.
Instead, for decades, scientists have sought signs of life elsewhere, usually peering outward to Mars and more recently at Europa, Enceladus and other icy moons of the giant planets.
The astronomers, who reported the finding Monday in a pair of papers, have not collected specimens of Venusian microbes, nor have they snapped any pictures of them. But with powerful telescopes, they have detected a chemical - phosphine - in the thick Venus atmosphere. After much analysis, the scientists assert that something now alive is the only explanation for the chemical's source.
Some researchers question this hypothesis, and they suggest instead that the gas could result from unexplained atmospheric or geologic processes on a planet that remains mysterious. But the finding will also encourage some planetary scientists to ask whether humanity has overlooked a planet that may have once been more Earthlike than any other world in our solar system.
"This is an astonishing and 'out of the blue' finding," said Sara Seager, a planetary scientist at the Massachusetts Institute of Technology and an author of the papers (one published in Nature Astronomy and another submitted to the journal Astrobiology). "It will definitely fuel more research into the possibilities for life in Venus' atmosphere."
"We know that it is an extraordinary discovery," said Clara Sousa-Silva, a molecular astrophysicist at Harvard University whose research has focused on phosphine and is another of the authors. "We may not know just how extraordinary without going back to Venus."
Sarah Stewart Johnson, a planetary scientist and head of the Johnson Biosignatures Lab at Georgetown University who was not involved in the work, said, "There's been a lot of buzz about phosphine as a biosignature gas for exoplanets recently," referring to the search for life on worlds that orbit other stars. "How cool to find it on Venus."
She added: "Venus has been ignored by NASA for so long. It's really a shame."
David Grinspoon of the Planetary Science Institute in Tucson, Arizona, who was not part of the work but has long promoted the possibility of life in Venus' clouds, said, "That is pretty damn exciting!"
The work needs to be followed up, he said, "but this could be the first observation we've made which reveals an alien biosphere and, what do you know, it's on the closest planet to home in the entire cosmos."
Venus is one of the most beautiful objects in Earth's sky. But at a closer glance, the less lovely it becomes.
Often called Earth's twin, Venus is roughly the same mass as Earth. Many scientists think that Venus was once covered in water and possessed an atmosphere where life as we know it could have flourished.
In earlier days of the solar system, Earth was not so hospitable to the likes of us. There was life here then, even an entire biosphere that did not survive in the oxygen-rich environment that later developed. And much as Earth over time became a home for jellyfish, ferns, dinosaurs and Homo sapiens, Venus was transformed by something into a hell.
Today, the second planet from the sun has an atmosphere stifled by carbon dioxide gas and surface temperatures that average more than 800 degrees Fahrenheit. The dense atmosphere of Venus exerts a pressure of more than 1,300 pounds per square inch on anything at the surface. That is more than 90 times the 14.7 pounds per square inch at sea level on Earth, or the equivalent to being 3,000 feet underwater in the ocean.
It is hardly a place that makes visiting or research easy, although that doesn't mean people haven't tried. Space programs have tried dozens of robotic missions to Venus, many of them in the Soviet Union's Venera series. But the planet eats metal, within minutes melting down and crushing spacecraft that have landed there. Of all those attempts, only two managed to directly capture images of the planet's surface.
Whereas frigid Mars is currently ringed by orbiters and prowled by NASA rovers, Venus is being studied by only one probe, the lonely Japanese spacecraft Akatsuki. Future missions to the planet are still mere concepts.
Although the surface of Venus is like a blast furnace, a cloud layer just 31 miles below the top of its atmosphere may reach temperatures as low as 86 degrees Fahrenheit and has a pressure similar to that at ground level on Earth. Many planetary scientists, including Carl Sagan and Harold Morowitz, who proposed the idea 53 years ago, have hypothesized life may exist there.
Jane Greaves, an astronomer at Cardiff University in Wales, set out in June 2017 to test that hypothesis using the James Clerk Maxwell Telescope in Hawaii, looking for signs of various molecules on Venus. Different species of molecules will absorb radio waves coming through the clouds at different characteristic wavelengths. One of the chemicals was phosphine. She did not expect to find it.
"I got intrigued by the idea of looking for phosphine, because phosphorus might be a bit of a sort of go-no-go for life," Greaves said.
Chemists compare phosphine to a pyramid - one atom of phosphorus topping a base of three hydrogen atoms. NASA spacecraft Cassini detected it in the atmospheres of Jupiter and Saturn. In that setting, Sousa-Silva said, life is not necessary to form phosphine. The immense heat and pressures can jam the phosphorous and hydrogen atoms together to form the molecule.
But on smaller, rocky planets like Earth and Venus, the researchers say, there is not enough energy to produce copious amounts of phosphine in the same way. There is one thing, however, that appears to be very good at producing it: anaerobic life, or microbial organisms that don't require or use oxygen.
On such worlds, "as far as we can tell, only life can make phosphine," Sousa-Silva said. She has long studied the gas, on the theory that finding it being emitted from rocky planets that orbit distant stars could be proof that life exists elsewhere in the Milky Way.
Here on Earth, phosphine is found in our intestines, in the feces of badgers and penguins, and in some deep sea worms, as well as other biological environments associated with anaerobic organisms. It is also extremely poisonous. Militaries have employed it for chemical warfare, and it is used as a fumigant on farms. On the TV show "Breaking Bad," the main character, Walter White, makes it to kill two rivals.
But scientists have yet to explain how Earth microbes make it.
"There's not a lot of understanding of where it's coming from, how it forms, things like that," said Matthew Pasek, a geoscientist at the University of South Florida in Tampa. "We've seen it associated with where microbes are at, but we have not seen a microbe do it, which is a subtle difference, but an important one."
Sousa-Silva was surprised when Greaves said that she had detected phosphine.
"That moment plays in my mind a lot, because I took a few minutes to consider what was happening," she said.
If there really was phosphine on Venus, she believed there could be no other obvious explanation than anaerobic life.
"What we find circumstantially also makes complete sense with what we know thermodynamically," she said.
The team needed a more powerful telescope, and the scientists next used the Atacama Large Millimeter Array, in Chile, in March 2019.
This time, they found, all signs pointed to phosphine, and a lot of it, ranging from 5 to 20 parts per billion. Although those numbers might seem small, that's thousands of times more than what is in Earth's atmosphere.
Sousa-Silva, Greaves and their colleagues had planned to complete additional telescope observations earlier this year. But the coronavirus pandemic and Venus' limited time above the horizon interfered with their ability to gather more evidence, leaving many questions unanswered.
"The finding itself is astonishing," said Paul Byrne, a planetary scientist at North Carolina State University in Raleigh who was not involved in the research. He said that although he was "skeptical of it being life, I don't have a better explanation for what it is."
The team spent a year re-creating the Venusian environment in computer simulations to test different explanations for the phosphine's source and abundance.
"The light is constantly breaking the phosphine down, so you have to continuously replenish it," said William Bains, a biochemist at MIT and one of the co-authors of the papers.
Volcanic activity and lightning on Venus would not be sufficient to add more of this constantly disappearing phosphine, according to the researchers' models. But living things could emit enough of the gas.
"What we've done is rule out all other sources of phosphine other than life," Bains said.
Other planetary scientists counter that a nonbiological origin cannot be ruled out.
"Despite prior speculation (mostly by the same authors), this can hardly be taken as a biosignature," Gerald Joyce, a biologist at the Salk Institute in California who has experimented with creating life in the lab, said in an email. In their own paper, he noted, the researchers wrote that "the detection of phosphine is not robust evidence for life, only for anomalous and unexplained chemistry."
A similar note of caution was voiced by James Kasting, a geoscientist and expert on planetary habitability at Pennsylvania State University, who said, "The model atmospheric composition that they show is, at best, incomplete."
The finding also follows a history of detections of gases on other worlds that can be byproducts of life. But these gases, such as burps of methane or oxygen on Mars, can also be produced by chemical reactions that do not involve life at all. So far, such signals have been intriguing, but they are not convincing proof of aliens.
While few doubt whether this phosphine is there, what kind of life in the clouds of Venus would it take to actually make the gas?
Such living things would have had to evolve to survive in a high-acid environment, perhaps with protective outer layers similar to microscopic organisms in Earth's most extreme environments.
In a paper published in August, Seager and her colleagues suggested that microbes borne aloft on air currents called gravity waves could live, metabolize and reproduce inside droplets of sulfuric acid and water. And given the amount of gas being produced, the population of these microbes would be ample.
As to how these microbes got there, the best guess, she said, is that they originated on the surface when Venus had oceans as late as 700 million years ago, but they were forced into the skies when the planet dried up.
And nobody knows whether the microbes, if real, are based on DNA like us or something entirely different.
"When looking for life elsewhere, it's so hard to not be Earth-centric," Sousa-Silva said. "Because we only have that one data point."
Before their imaginations run away, the researchers want to gather more telescope data and see their models tested and challenged. Robotic space missions to Venus could also advance the search.
India's space agency has proposed a mission, in the coming years, as has a private rocket company, Rocket Lab.
And NASA, which has declined to fund a number of Venus missions in recent decades, announced in February that it would consider a pair of proposed spacecraft among four finalists competing for a round of funding.
"For the last two decades, we keep making new discoveries that collectively imply a significant increase of the likelihood to find life elsewhere," said Thomas Zurbuchen, head of NASA's science directorate, who helps select missions to explore the solar system. "Many scientists would not have guessed that Venus would be a significant part of this discussion. But, just like an increasing number of planetary bodies, Venus is proving to be an exciting place of discovery."
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