November 2020

The Nation Reviewed

The breath of Venus

By Benjamin Pope
Does the detection of phosphine gas point to life among Venus’s clouds?

Driven by climate change, this year’s wildfires in California have been the worst on record, choking San Francisco like the cities of Australia were last Christmas. Fires advanced to within 150 metres of the historic Mount Wilson Observatory, north of Los Angeles, bringing back painful memories of the destruction of the Australian National University’s Mount Stromlo Observatory in the 2003 Canberra bushfires. But now, perhaps for the first time, the astronomical community and the objects of its study are bound together on two worlds by the unbreathable heat of climate change.

On September 14, a team led by Jane Greaves of Cardiff University announced it had discovered traces of phosphine in the cloud decks of Venus. This toxic gas, PH3, is lethal in large doses, and in smaller amounts has the odour of garlic or rotting fish, but to scientists it has the whiff of glory and discovery. On Earth, phosphine is only produced by microbes in unsavoury oxygen-free environments: biologically in sewage or guano, or in industrial processes. Phosphine was entirely unexpected from any geological chemistry on Venus – and if the hotly contested result stands up to scrutiny, this could be the first hint of alien life. Such life must have survived the aeons since the planet’s surface became uninhabitable, clinging to existence high in the cooler clouds.

The detection of phosphine was made from Earth using two telescopes – the James Clerk Maxwell Telescope in Hawai’i and the Atacama Large Millimeter Array in Chile – that looked at the frequencies of light absorbed by different molecules in Venus’s atmosphere. Every atom or molecule has a spectral fingerprint, absorbing and emitting light at a specific set of frequencies that identify it uniquely. With independent data from both telescopes, one of the frequencies of phosphine showed up unexpectedly.

Greaves turned to Clara Sousa-Silva at Massachusetts Institute of Technology to help her understand this new detection. Oxygen shows evidence of life on Earth, as it persists in our atmosphere only because it is replenished by plants and algae. Having completed a PhD on phosphine at University College London, Sousa-Silva published a study finding that the highly reactive gas could be an even more convincing sign of life than oxygen – but had expected that we might discover this on extrasolar planets rather than on nearby Venus. Her MIT group responded to Greaves’ new detection by putting together a 100-page paper exhaustively listing possibilities for producing phosphine, finding no explanation for the estimated millions of tonnes a year on Venus in terms of known chemistry, except possibly for life.

Hidden beneath the planet’s thick and perpetual sheath of cloud, the surface of Venus – with a gravity just less than Earth’s, and receiving twice the amount of sunlight – remained a mystery until the dawn of the Space Age. This blank slate was filled in by the imaginations of both serious scientists and science-fiction writers. The Nobel Laureate chemist Svante Arrhenius thought Venus might be a swampy world, and Edgar Rice Burroughs in his novels described a tropical planet with humanoids and megafauna.

These views of Venus, like Burroughs’ princess-rescuing novels, aged poorly. Radio observations in 1958 showed that the surface is hot enough to melt lead; Carl Sagan explained this as being the result of an intense greenhouse effect. The planet’s atmosphere is mostly carbon dioxide, its surface a desert, and its clouds are sulphuric acid. When the Soviet spacecraft Venera 7 landed on Venus in 1970, the first successful landing on another planet, it was destroyed by the harsh conditions in minutes. Since the last Soviet lander, Vega 2 in 1985, no space probe has landed there. The search for life on other worlds has since mostly been directed elsewhere: to Mars, to the icy moons Enceladus and Europa, and to distant solar systems.

But the dreams of Arrhenius and Burroughs might actually have been true in the past. Venus, Earth and Mars were born with similar compositions but developed very differently. Venus was perhaps a habitable world for most of its history, but this was destroyed by the greenhouse effect in a runaway process. Its thick CO2 atmosphere warmed the planet; this boiled the oceans, and the water vapour (also a greenhouse gas) accelerated the transformation of Venus into a hellscape hotter than even the worst-case scenarios of global warming on Earth. Any life there today, it seems, must be climate refugees in the cooler clouds.

At around 50 kilometres above the surface of Venus, temperatures and pressures are not too far from Earth’s surface standard, and, sulphuric acid aside, the clouds there might offer a welcoming environment. Earth’s clouds are full of bacteria, viruses and fungal spores. Sara Seager of the MIT team hypothesised a cloud ecosystem on Venus: microbes in liquid droplets that stay aloft for months, reproducing as fast as they can; when they sink down towards the scorching depths, they dry out and linger until winds can lift them back to their cooler habitat.

Such life would hardly be alien to us. Indeed, it could have an Earthly origin: the descendants of invasive species, hitching a ride to Venus on rocks blasted off our own planet. But that life evolved on Venus independently is equally possible. That’s an unsettling possibility for some. Oxford philosopher Nick Bostrom hopes the search for extraterrestrial life finds nothing. Discovering even a microbe on Venus or Mars independent of Earth origin would tell us that life must be common in the universe. But if so, why haven’t we had contact with other intelligent life? Bostrom argues that this could only be because civilisations wipe themselves out, by accident or with weapons of mass destruction:

If it is true that almost all intelligent species go extinct before they master the technology for space colonization, then we must expect that our own species, too, will go extinct before reaching technological maturity.

But Bostrom may not have his fears realised just yet. And the news is being treated differently to similar events in the past. When Bill Clinton announced fossil microbes in a meteorite from Mars in 1996, it was soon debunked. The detection of phosphine on Venus, and its association with life, has already proven controversial. The MIT team is appropriately cautious in its choice of words:

… while we cannot rule out life as a source of the phosphine on Venus, the hypothesis that the phosphine is produced by life cannot a priori be favored over the hypothesis of unknown photochemistry or unknown atmospheric chemistry. All seem equally unlikely, and hence all call for further investigation. We note … that the extraordinary claim of life should be the hypothesis of last resort only after all conceivable abiotic alternatives are exhausted.

Speaking to, astronomers John Carpenter, Michael Way and David Catling disputed the detection of phosphine, saying the data could instead show a frequency of boring old sulphur dioxide. That view was shared by a group led by Geronimo Villanueva, at NASA’s Goddard Space Flight Center, and they have also questioned the algorithms used to analyse follow-up observations as well as the theoretical chemical models used to infer the quantity of phosphine. Similarly, Ignas Snellen and his team from Leiden Observatory have challenged the data analysis methods, arguing the detection is not statistically significant. The phosphine frequency might simply be noise. And Ngoc Truong and Jonathan Lunine from Cornell University suggested in a scientific paper just 10 days after the announcement that volcanic activity could be sufficient to explain the phosphine.

The only way to be sure will be by sending a space probe to Venus’s atmosphere. The Japanese spacecraft Akatsuki is currently in orbit around the planet but lacks the instruments required to detect phosphine. There is a suite of national missions to Venus already planned, which now have special impetus to follow up the phosphine detection: two American projects competing for NASA funding, and other missions from the European Space Agency, Russia, and India.

Australian astronomer Stephen Kane, based at the University of California, Riverside, is a key science member of the American DAVINCI+ team. He explains that, by diving into the atmosphere, its probe will provide direct measurements of the atmospheric chemistry. Like Sousa-Silva, he normally studies more distant planets, but notes that “we will not be able to travel to those planets to validate the results of postulated biosignature detections. Venus is the closest planet to the Earth, and so presents an opportunity to test a biosignature claim from in-situ data.”

Aside from the big players, New Zealand–American company Rocket Lab is collaborating with members of the MIT team to build a quick-turnaround private mission to Venus, possibly launching as early as 2023. But Lucianne Walkowicz of the Adler Planetarium in Chicago wrote in Slate of her concern that a private mission, potentially lacking the rigorous safety of a public space agency, could contaminate Venus with Earth microbes, rendering the question of life on Venus permanently unanswerable. It would be unimaginably reckless to risk introducing pests from Earth and having them march like cane toads across another planet.

Caution is essential, both in interpreting the data and planning a follow-up mission. If it truly exists, the Venusian cloud ecosystem is a vestige of a world ruined by natural climate change, its story one of tragic loss and remarkable adaptation. We may be seeing our own reflection.

Benjamin Pope

Benjamin Pope is a NASA Sagan Fellow at New York University.

Cover of The Monthly, November 2020

November 2020

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