Millions of space nerds reacted with glee on Monday to a study showing that Venus’s atmosphere contains phosphine, a chemical byproduct of biological life. But no one would have been more thrilled or less surprised by the discovery of the late great Carl Sagan, who said this day could come more than 50 years ago.
Now best remembered as the presenter of the most viewed PBS series ever Cosmos, the author of the book behind the film Contactand the guy who put gold records of terrestrial music on NASA’s Voyager missions, Sagan began studying our two closest planets. He became an astronomer after being inspired as a child by Edgar Rice Burroughs̵
But how Cosmos fans know, Sagan’s sci-fi hopes have never beaten his ruthless science. He shot down one of the first “proofs” of life on Mars. He predicted that the surface of Venus would be incredibly hot even before NASA’s first Venus probe in 1962, which he worked on, confirmed this. And he was the first scientist to see Venus’s hellish landscape as the result of a runaway greenhouse effect – one he knew could point the way to the future of Earth’s climate change.
So it was even more surprising when Sagan wrote an article proposing that one day we may still find microbial life above our sister planet. “If small amounts of minerals are lifted from the clouds from the surface, it is not at all difficult to imagine an indigenous biology in the clouds of Venus,” he wrote in Nature in 1967, two years before NASA landed on the moon. “While the conditions on the surface of Venus make the hypothesis of life there implausible, the clouds of Venus are a completely different story.”
As Sagan pointed out, a carbon-rich atmosphere was no obstacle. Up to the 50-km (31-mile) layer atop the clouds of Venus, conditions are actually hospitable and almost Earth-like. Organisms could thrive in the upper parts in the same way that bacteria thrive around the superheated, CO2-rich vents in Yellowstone. Add sunlight and water vapor to CO2, he said, and you have the recipe for that brick of life, photosynthesis.
“Sagan’s work on Venus was formative, although few today remember its impact,” said Darby Dyar, president of NASA’s Venus Exploration Advisory Group. “His idea was prescient, and it still makes sense today: between the infernal conditions of the surface of today’s Venus and the near-void of outer space there must be a temperate region where life could continue.”
Just 11 years after Sagan made his prediction, another Venus probe discovered methane in the atmosphere, which could be considered a predictor of the presence of organic material. Scientists like Sagan were cautious about the discovery; no one could prove that methane meant life beyond a reasonable doubt. (We also found it on Mars in 2018 and haven’t explained it yet.) However, no one has ever come up with a reasonable alternative to why methane might be around on Venus.
Sagan died in 1996, in the midst of a long period of criminal drought for NASA’s exploration of Venus. But his idea survived. In 2013, we discovered huge amounts of live microbes in the clouds above Earth. More than 300 varieties, to the surprise of the scientists who collected them, microbes actually are Less dense at lower altitudes. In 2016, NASA models showed that Venus once had oceans, for at least 2 billion years. This confirmed a theory by planetary expert David Grinspoon, which suggests that microbial life migrated to the clouds when conditions became too harsh for life on the surface a billion years ago.
Call them the original climate refugees.
The science didn’t stop, even when we only used ground-based telescopes to do it. We found evidence of active volcanoes on the surface, which would “lift minerals” into the atmosphere just as Sagan suggested. In 2018, another study of Venus’s atmosphere revealed mysterious “dark spots” that scientists speculated might be evidence of microbial life – large amounts of it. How much? We would need more studies to find out. “I came to that paper out of frustration,” co-author Sanjay Limaye told me last year. “We have not looked for organisms [on Venus]. Why not?”
Why not really. As I wrote earlier this year, Venus was unfairly sidelined for Mars in NASA’s budget priorities. Even though Venus is closer and more like Earth, Mars had a surface we could stand on, which was an easier sell for our 20th century “space colonization” mentality.
But the more we look at Venus, the more we need to rethink what exploration is like.
Silently, inside and outside NASA, a “Venus community” grew up that wanted to explore its clouds and started asking for budget cuts. Its most exciting moment so far came in 2015, when NASA unveiled a conceptual mission called HAVOC – a Zeppelin, basically, that didn’t need to be filled with helium or hydrogen. Only normal old Earth air would float on Venus’s dense atmosphere. Tear off the balloon tissue and the high pressure could actually prevent air from escaping weeks.
As might be expected, the Venus community was abuzz with excitement over the discovery of phosphine on Monday. Not least because the NASA administrator had just tweeted the magic words: it’s time to prioritize Venus.
Life on Venus? The discovery of phosphine, a byproduct of anaerobic biology, is the most significant development so far in building the case for life off Earth. About 10 years ago NASA discovered microbial life at 120,000 feet in the earth’s upper atmosphere. It’s time to prioritize Venus. https://t.co/hm8TOEQ9es
– Jim Bridenstine (@JimBridenstine) September 14, 2020
There is, of course, plenty of caution. Phosphine is also found in the vast bubbling gas giants of Jupiter and Saturn. But to explain why it would be present on a rocky planet as small as Venus if it weren’t for life, scientists say, you’d have to propose a geological process we don’t yet know.
“The exciting discovery of phosphine in Venus’ atmosphere only reinforces the growing body of evidence that Venus is a probable, perhaps most likely, other place in our solar system where life may have existed now or in the past,” he says. Dyar from NASA. “Venus holds the keys to our understanding of the evolution of rocky planets as homes for life.
“This discovery may be the first of many to come as NASA and other countries renew a Venus exploration program.”
Currently ESA, the Russian space agency and NASA all have plans for Venus spacecraft in the pipeline that could arrive in this decade; the phosphine announcement could move the launch dates upwards. If and when the next probes find more evidence of life above the most mysterious planet in the solar system, we will take a step forward to confirm Carl Sagan’s legacy as a visionary genius.