Maybe It’s Aliens, or Maybe It’s Volcanoes

Scientists are still trying to figure out what could be brewing on the planet next door.

A computer-generated image of Maat Mons, a Venusian volcano
A computer-generated image of Maat Mons, a Venusian volcano (NASA)

Oh, Venus. What’s going on with you?

I am referring, of course, to Venus the planet, second from the sun, right next door to Earth. The planet with a furnace-like surface and clouds made of sulfuric acid, the one that shows up in our night sky as a golden jewel, and that helped prove the theory that the sun, not Earth, was at the center of the solar system. Although Venus has captivated observers for centuries, the planet remains a bit of a mystery, its particularities hidden. There’s still so much scientists want to know about our planetary neighbor. Especially now. Talk to us, Venus!

The most recent, restless search for new insight began last September, when scientists announced that they had discovered evidence of a gas called phosphine floating in Venus’s clouds. It was quite the surprise: Based on what’s known about phosphine, the gas isn’t supposed to survive for very long in the planet’s atmosphere. On Earth, phosphine has been found in the intestines of animals, us included, and in swampy communities of microorganisms. The group of scientists concluded that something must be replenishing Venus’s supply—perhaps a mysterious Venusian life-form.

Venus instantly became one of the most exciting targets in the search for life beyond Earth. But the phosphine detection was a flicker of evidence, not definitive proof, and the flashy discovery transformed into a slow-burning debate as other scientists joined in. Some researchers said their own analyses turned up empty for phosphine. Others found hints of phosphine buried in decades-old observations. Still others argued that the signal corresponded to an entirely different gas. The original team held on to its conclusions, even after an error in telescope data forced the researchers to revise down the amount of phosphine they had detected. New scientific papers have regularly cropped up since, swinging the discussion in one direction or the other.

The latest entry suggests a rather explosive explanation: If there’s phosphine in Venus’s clouds, it’s probably not because of aliens, but because of volcanoes.

A pair of researchers say that very explosive volcanic eruptions could spew phosphorous compounds from deep within the planet’s mantle into the sky, where they could react with the sulfuric acid and other elements to form phosphine. For phosphine to exist at the altitudes at which the gas was detected requires an event comparable to one of the most destructive eruptions in Earth’s recorded history—the 1883 eruption on Krakatoa, which sank most of the Indonesian island into the sea. No need to consider cloud-hopping microorganisms in this scenario. “The amount of volcanic material that you really need for the phosphine hypothesis is quite small compared to what we think that the active volcanoes on Venus could produce,” Ngoc Truong, a planetary geologist at Cornell University and one of the authors of the new paper, told me.

As with most phosphine-related papers, not everyone is convinced. The researchers behind the original detection don’t think the volcano explanation works, and had ruled it out back in September, when they first went public with the phosphine discovery. “We do not think that deep-mantle plume volcanism can produce sufficient amounts of phosphine to explain the observations,” Janusz Petkowski, an astrobiologist at MIT, told me.

The volcano hypothesis is particularly complicated because, well, we don’t actually know whether Venus has any active volcanoes. It certainly did at some point; the planet’s surface is a tapestry of a volcanic past, its surface bunched into peaks in some places and smoothed by lava in others. Nor do we know the exact chemical composition of Venus. Some close-up observations of the planet have previously suggested that the planet’s volcanoes are still at work, but scientists can’t say for sure without further research.

Team Alien and Team Volcano diverge on a number of points. Petkowski says that we don’t know whether those phosphorous compounds in Venus’s mantle remain unchanged on their way out from beneath the surface; perhaps the eruption alters them in such a way that, when they reach the clouds, they can’t transmogrify into phosphine. Even if these chemicals reach the sky, Petkowski and his colleagues say, it’s actually not clear whether their interactions with the sulfuric acid in Venus’s clouds would produce phosphine. But Truong says laboratory studies of chemical processes have shown that such phosphine production could be possible, and that the other researchers eliminated the volcano option because they believe that the phenomenon requires far more volcanic activity than is actually necessary.

Other researchers I spoke with said that while they’re not entirely convinced by the volcano scenario, they welcome the contribution to the phosphine debate. It seems prudent to continue investigating the possibility of nonbiological origins when the biological origin would upend our understanding of life in the universe. As I’ve written before, the work of planetary science—like all sciences—is supposed to be a bit of a tussle, full of maybes and maybe nots, and this one is still unfolding. Some disagreements come down to differences in the data that researchers use and the way they interpret them, and some to the fact that our understanding of Venus is incomplete, says David Grinspoon, an astrobiologist at the Planetary Science Institute, who has studied the possibility of life on Venus. “Our estimates of the rate of volcanic outgassing rates, and the likely composition of volcanic gases, are just educated guesswork at this time,” Grinspoon told me.

Our picture of Venus is expected to sharpen soon enough. Last month, NASA selected two new spacecraft to travel to Venus for the space agency’s first missions to the planet in more than 30 years. Only one spacecraft—a Japanese probe—is in orbit around Venus now, and scientists believe the planet is long overdue for more attention. One of the NASA missions, which is scheduled to depart at the end of the decade, could prove once and for all whether volcanoes are erupting right now, or at least have in the recent past.

Would that discovery strengthen the volcano hypothesis over the alien one? Possibly. But the discovery of volcanoes rumbling across Venus could bolster the case for habitability in another way, says Rakesh Mogul, a biochemistry professor at Cal Poly Pomona who found potential evidence of phosphine in archival data from 1978, from NASA’s most recent Venus mission. On Earth, volcanoes helped set the stage for life to emerge, ejecting the materials that helped it take hold and flourish. Atmospheric phosphine, Mogul told me, “could be used as fuel for potential microorganisms, either now or in the past.” But, of course, that’s just another hypothesis.