Scientists have spotted something unexpected in the cloud decks of Venus, our nearest planetary neighbor. While no one is saying it's aliens just yet, some sort of alien microorganism is on the list of potential explanations for why a chemical that shouldn't be floating around above the planet has been observed there for the first time.
The chemical is phosphine, or PH3, a compound made up of phosphorus attached to three hydrogen atoms. On Earth, certain microbes that thrive in oxygen-free environments, like at a sewage plant, are believed to produce the chemical. The gas is highly toxic to humans and smells like decaying fish.
It was identified in observations of Venus made through telescopes in Hawaii and Chile in 2017 and 2019. Specifically, phosphine was found about 33 to 39 miles (53 to 62 kilometers) above the surface of Venus, a world that is known for being brutally inhospitable, with both extremely hot temperatures and crushing pressures.
Interestingly, however, the altitude where the phosphine was detected is one of the more hospitable areas in the solar system beyond Earth, with temperatures and pressure comparable to the surface of our planet. There's still the problem of the sulfuric acid clouds, however, which would certainly be hostile to much of the life we know and should also destroy phosphine.
"These are conditions not exactly welcoming to life as we know it," says Brendan Burns, an astrobiologist at the University of New South Wales, Australia.
A team led by Jane Greaves from Cardiff University and the University of Cambridge in the UK lays out the discovery in a paper published Monday in Nature Astronomy. They sought to explain the mysterious presence of PH3 in the clouds, considering various atmospheric, chemical and geological processes. Lightning, volcanoes, the solar wind and even meteors were investigated as possible sources, but none fits the observations.
"If no known chemical process can explain PH3 within the upper atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions," the paper reads. "This could be unknown photochemistry or geochemistry, or possibly life."
The scientists go on to "emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry."
Figuring out what exactly is happening in the clouds of Venus may require sending new robotic probes, balloons or other spacecraft to explore and sample them. Meanwhile, the possibilities have many astronomers fired up.
Life on Venus?
"It's tremendously exciting," said David Grinspoon, senior scientist at the Planetary Science Institute. "It's a legitimate candidate biosignature (a piece of data that could indicate the presence of life) on another planet. Potentially the best we've found yet."
Grinspoon is a widely respected expert on Venus who wasn't involved in the new discovery, but first wrote about the possibility of a cloud biosphere on Venus in 1997 and has been pushing the idea ever since. He points out that phosphine had been singled out as an ideal biosignature on rocky planets before this latest discovery.
"It's a molecule that should not be there by ordinary atmospheric chemical processes and should have a very short lifetime, which means if it's there, there's an active source. And then the question becomes what is that source? And there's no obvious non-biological source."
Astronomer Stephen R. Kane at the University of California-Riverside, who is also not involved in the work, points out that some research suggests Venus was habitable in the distant past, perhaps over a billion years ago. He suggests that any "biology in the atmosphere could be the last surviving members of a prior Venusian biosphere."
But Kane says there is reason to be skeptical that "life" in the clouds is the best explanation.
"As noted in the paper, the biological interpretation is being suggested because we cannot currently model a geological solution. The chemistry of possible geological and biological signatures is vast and it is an ongoing effort to fully explore that parameter space. That means there are undoubtedly geological explanations that exist that have not yet been realized."
There's also the problem of how anything, even tiny microbes can make a lifestyle out of floating around in the sky indefinitely, generation after generation. Staying at a Goldilocks altitude above the extreme heat below and the cold, unforgiving transition to space above as a microbe likely floating in liquid droplets would seem very unlikely.
Greaves is also co-author of a paper, published last month, whichwhen they find themselves dried out at lower, hotter altitudes, only to reanimate and continue their life cycle when atmospheric processes lift them higher to be rehydrated again at the habitable zone above Venus.
Kevin McGouldrick, a researcher at the University of Colorado Boulder who specializes in the clouds of Venus, says "it is one more bit of evidence that shows that we don't know as much as we thought we did about the Venus atmosphere."
He told me he sees the discovery as "less earth-shaking" but ultimately helpful in advancing the search for biosignatures and the unknown biology that might be behind them.
"These scientists have found solid evidence for the existence of a molecule that was not expected to be present. Unless the observations are in error, this means that our expectations were wrong. And if our expectations were wrong, then it represents a possibility for growth in knowledge."
What about Mars?
We've found ourselves in a very similar situation before. When NASA landed the , it carried an experiment that could detect chemical reactions in the soil. The experiment, known as the LR experiment, came back positive -- it showed signs the red planet did contain life.
But in the years since, planetary scientists have concluded the discovery was most likely an error. Mars doesn't have extant life on its surface, but it may have in the past. The problem, then, was that NASA put the alien cart before the alien horse. We didn't understand, completely, the geochemical processes occurring on Mars' surface. When we detected some funky chemistry, there was a groundswell of excitement, but we may have jumped the gun.
Although the lead scientists on the LR experiment still believe they did detect life in 1976, definitive proof has not been forthcoming -- and it's now 40 years since the announcement was made.
So our search for life outside of Earth continues.to search for signs in the Martian soil. For now, it seems we can add the clouds of Venus to the list of potentially habitable nooks in the dark forest of the cosmos.
Much work remains to elucidate the true nature of phosphine in its upper atmosphere. Not only will biologists be intrigued, but chemists and geologists will hope to learn more about the chemical, too. The one point everyone can agree on is this new compound, something that might resemble a microbial alien fart, demands a closer look.
"We have a responsibility to investigate further and determine what the true source of the phosphine is," Kane says, pointing to potential missions NASA is developing that could send orbiters, landers or atmospheric probes to our tempestuous neighbor. "It is through these kinds of missions that we will be able to fully answer this question of possible life in the Venusian clouds."