Scientists say they've detected a gas in the clouds of Venus that, on Earth, is produced by microbial life.

The researchers have racked their brains trying to understand why this toxic gas, phosphine, is there in such quantities, but they can't think of any geologic or chemical explanation.

The mystery raises the astonishing possibility that Venus, the planet that comes closest to Earth as it whizzes around the sun, might have some kind of life flourishing more than 30 miles up in its yellow, hazy clouds.

Nothing could live on what passes for land on Venus; its smooth volcanic plains are a scorching hellscape hot enough to melt lead, where the temperatures exceed 800 degrees Fahrenheit. High in the clouds, however, the pressures and temperatures and acidity levels would be less intense — though still vile.

The clouds are far more acidic than any environments where microbes make their home on Earth. And instead of water, the clouds on Venus contain droplets of concentrated sulfuric acid; the atmosphere is so bereft of water that it's many times drier than the driest desert on Earth.

All in all, it seems like an unlikely place for life. Nonetheless, the new report in the journal Nature Astronomy has astrobiologists and planetary scientists talking. Two different telescopes, at two different times, looked at Venus and saw the chemical signature that is unique to phosphine. If this gas is really there, Venus has either got some kind of geologic or chemical activity going on that no one understands, or alien life might be living right next door.

In many ways, Venus is similar to Earth. "Before its quite dramatic, runaway greenhouse effect, the surface was pretty habitable," says Clara Sousa-Silva at MIT, who explains that there has long been a theory out there that Venus might have once been inhabited and that life could have retained a stronghold in the clouds. Even Carl Sagan entertained this idea back in the 1960s.

That's why she was so intrigued when Jane Greaves of Cardiff University contacted her. Greaves shared that she and some colleagues had recently found an estimated abundance of 20 parts per billion of phosphine in Venus' clouds. Sousa-Silva had been studying phosphine as a possible biosignature that could indicate the potential for life being present on planets that orbit distant stars.

At first glance, phosphine might seem like a funny molecule to associate with life, given that it's a "highly flammable, extremely toxic, outrageously foul-smelling molecule," says Sousa-Silva. "It's an extremely dangerous molecule that kills in a variety of imaginative ways, all of which are very final and macabre."

"It's used widely as a fumigant and it was used as a chemical warfare agent in the first world war," she says, noting that the colorless gas can burn with a green and blue light.

As for its odor, well, "apparently it smells basically like death," says Sousa-Silva. "It just smells horrific. We once, I think, found a report of someone saying it smelled like the rancid diapers of the spawn of Satan."

Because phosphine interferes with oxygen metabolism, it's toxic for the majority of life on Earth. Still, says Sousa-Silva, "there's plenty of life, mostly in the shadows, that doesn't particularly enjoy oxygen and doesn't rely on oxygen. And these anaerobic ecosystems on Earth happily produce phosphine in reasonably large quantities."

Life that makes phosphine on Earth is found in swamps and sewage plants and the bottoms of lakes, she says, as well as in the intestines of animals — that's why phosphine can be detected in their flatulence.

"This is not life that we would find pleasant," says Sousa-Silva. "Then again, they probably find us disgusting."

Phosphine is so chemically reactive that it breaks down quickly, however, so how was it accumulating in the clouds of Venus? The researchers considered possible sources on the surface of Venus, as well as delivery by meteorites, creation by lightning, or obscure chemical reactions in the atmosphere. Nothing they dreamed up could do the trick.

That left the possibility of life. On Earth, any microbes in the clouds circulate up and down from the surface, but that wouldn't be possible on Venus because the surface is so deadly, says Janusz Petkowski of MIT. Any life in the clouds of Venus, he says, would have to somehow survive in highly concentrated sulfuric acid that's around a billion times worse than any acidic environment on Earth.

That's very difficult to imagine, says Petkowski. "But is it impossible? I would say that it is not impossible."

To find out what's really going on, he says, scientists might end up having to just send a mission to Venus that could sample the cloud chemistry.

Calls for a new look at Venus have already been growing, even before this new discovery. A mission that would send a spherical probe plunging through the atmosphere of Venus down to its surface, for example, is one of the proposals that NASA is currently considering for future solar system exploration.

"Venus is like a giant unknown," says Hilairy Hartnett of Arizona State University. "It's one of the planets that we almost know the least about in our own solar system."

NASA astrobiologist Giada Arney agrees. "If there's life in the Venus clouds, that would be extraordinary, but there's still much we don't understand about the Venus environment," she says. While the research team that produced this new study clearly has done a lot of thinking about what non-living processes might produce phosphine on Venus, "there is much about Venus we still don't understand, or that we understand poorly. It'll take the combined work of the Venus and astrobiology communities to answer this important question fully."

Venus was the first planet ever visited by a spacecraft, when NASA's Mariner 2 flew by in 1962. Before that mission, scientists could only peer at its shroud of clouds and wonder what lay beneath them. Mariner 2 showed that the surface of Venus was an inhospitable furnace, so it couldn't be the kind of primordial jungle that some had imagined.

Conditions on the surface are so extreme that it's hard to send a probe that can survive. In 1982, the Soviet spacecraft Venera 13 lasted only a couple of hours after landing, sending home photos of orange-brown rocks before it succumbed.

Venus wasn't always this way; it used to be much more cozy. New climate models show that Venus could have sustained liquid water on its surface as recently as a billion years ago, says Stephen Kane of the University of California, Riverside.

If the phosphine find represents "the remnants of some past ecosystem, that means it probably would have had to have sustained its presence in the clouds for about a billion years," says Kane, calling that "an extremely difficult problem" to solve.

"We do need to seriously consider that there is a much more natural geological explanation that we just haven't figured out yet," says Kane.

He notes that frequently when scientists talk about finding "biosignatures" that could indicate the possible presence of life, they focus on far-off planets around stars other than our sun. Those planets, notes Kane, are so remote that they're basically unreachable in human lifetimes, but Venus is close at hand.

"This is a test for us because in this case, we can go to Venus," says Kane. "This is really an incredibly important test for the whole concept of biosignatures."

Just the presence of phosphorus in Venus' atmosphere is fascinating, even if it doesn't turn out to be linked to life on Venus, says Hartnett. She points out that on Earth, phosphorus is the backbone of the genetic code and the energy currency of cells, yet scientists know little about how phosphorus is distributed in planets.

"The phosphine detection is exciting," agrees Bethany Ehlmann, a planetary scientist at Caltech. "Of course, you know, the big tantalizing aspect is that it could be life."

But, echoing Carl Sagan, she says, " 'Extraordinary claims require extraordinary evidence.' Put phosphine on Venus in the list of mysteries, big mysteries, in the solar system."

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