Life on Venus! Actually, No! Actually, Maybe?

Image Courtesy of Jungbin Cha (Jaime)

The story goes that Isaac Newton was contemplating the orbit of the Moon when an apple fell on his head, and—eureka! His theory of universal gravitation was born. Historians say this story about the apple is dubious, but it’s still woven into the mythos of science because it illustrates how an unexpected observation can transform the way scientists understand an idea. A surprising insight, and presto, new science—it sure worked in Newton’s time. 

So now imagine you’re Jane Greaves, a planetary astronomer at Cardiff University in Wales, and after eighteen months of crunching data from observations made in June 2017 on the James Clerk Maxwell Telescope in Hawai‘i, you’ve convinced yourself of the impossible—that you and your team have detected a signature of life emanating from the warm clouds of Earth’s seemingly desolate neighbor Venus. Finding alien life might be as simple as looking next door. The apple has fallen. What comes next?

As indicators of life go, this signal from Venus was about as good as it gets. Greaves and her colleagues had detected a relatively small concentration of the gas phosphine, which is a molecule made of a phosphorus atom bonded to three hydrogen atoms. Astrobiologists call it a “biosignature gas.” With limited exceptions, it is only produced by life. On Earth, anaerobic bacteria that live underwater are the only organisms known to partake in the strange and challenging chemistry required to produce something as peculiar as phosphine. And scientists in labs aren’t much better: there is no known chemical mechanism by which phosphine can be produced without temperatures far more extreme than those found on Earth and Venus. If the phosphine in the atmosphere of Earth can only be created through life, is the same true on Venus?

Encouraged by the results of their first observation, Greaves and her colleagues wasted no time to observe the Venusian atmosphere again with a more advanced telescope in March 2019. This time, they used the Atacama Large Millimeter Array radio telescope (ALMA) in Chile. Like the first telescope, ALMA recorded the intensity of radiation coming from Venus at different frequencies and performed a complex series of calculations to produce data that scientists could interpret, like a fingerprint for the concentrations of various gases. Once again, Greaves and her colleagues calculated a statistically significant concentration of phosphine in the Venusian atmosphere: about twenty molecules per billion located kilometers above the planet’s surface in its thick clouds. And since phosphine breaks down under the conditions of the Venusian atmosphere, the team calculated that a constant influx of new phosphine into the atmosphere would be necessary for them to detect this.

Data in hand, Greaves and her colleagues had quite the paper to write. They had to explain to the scientific community that they had detected something that was either at odds with our modern knowledge of chemistry or evidence of alien life. In September 2020, they published their paper in the journal Nature Astronomy. In a press briefing for the Royal Astronomical Society, Greaves and three of her coauthors presented slides that cautiously broke their news. One slide reads, “We are claiming that we have detected phosphine gas whose existence is a mystery: either new chemistry or possibly life production.” News outlets boasted headlines about the possibility of life on Venus, gently noting the possibility of unknown chemistry. But then in July 2021 came a shock. In the same journal, a different group of researchers responded with their own paper, brutally blunt in its title: “No evidence of phosphine in the atmosphere of Venus from independent analyses.”

“It felt pretty bad, on a personal level,” Greaves said in a recent interview, reflecting on the scientific controversy that followed the response paper. The response paper shrouded their original publication in questions of scientific validity and academic rigor. “There was anxiety, in case we had made a genuine mistake—but the critics had not involved us in any collegial discussion before publishing their critique,” Greaves said.

The response paper opposed the original work on several fronts. First, the authors pointed out that the staff at ALMA had made an obscure but impactful mistake in their own data processing procedure before passing the data on to Greaves and her colleagues for their analyses. This mistake was corrected, as were two other small mistakes that the telescope staff discovered following an investigation, but it was not a fatal blow to Greaves’ paper—the signal was still there after adjusting the data. The bigger problem was that the response paper made the dramatic accusation of data misinterpretation. The signal, these authors claimed, came from a contaminant, not phosphine. 

The distinct frequency of radiation that phosphine absorbs—the missing frequency in radiation from the atmosphere of Venus that alerted Greaves to phosphine’s presence—is close to the frequency associated with sulfur dioxide. Through their own calculations and analyses, the authors of the response paper argued that the signal Greaves picked up on was actually from sulfur dioxide. 

But wait!—Greaves and her colleagues said in a third paper published in the same journal. Greaves’ team had already considered this kind of contamination and ruled it out. “We looked at the criticisms and found (in most cases) the critics hadn’t read the long supplement to our discovery paper, where we had already tested and answered the things they thought we’d done wrong,” Greaves said. The group reiterated their point from the first paper—there is indeed phosphine on Venus—and they recalculated their estimates for the concentration of phosphine based on the now-corrected ALMA data. Even after the revisions, it remains possible that phosphine is being actively generated on Venus by some as-of-yet unknown mechanism. “I’d love it to be life, but other origins would be cool too,” Greaves said of the updated findings.

Greaves noted that the scrutiny surrounding her group’s work extended beyond just the response paper published in 2021. Some astronomers are still skeptical of the presence of phosphine on Venus in the absence of new data. But telescope time is tightly managed, so no one has been able to search for phosphine signals from Venus since the team’s ALMA observation.

Greaves also highlighted the personal side of academic vitriol. “I still encounter problems, as does almost every woman astronomer I speak with,” Greaves said. “In the Venus case, it’s very clear that Anita [M. S. Richards] and I were the data analysts on the paper, and very clear also that people thought we must have made the most basic errors. It’s hard to see how that can be—given a quick search would have shown that we are both senior career stage and former staff at the telescopes we used for Venus—unless it was easy to assume women are incompetent.”

The meaning of the phosphine detected on Venus is beyond what scientists can currently explain. When pushing the boundaries of knowledge, science requires a high degree of caution—after all, it took Newton two decades to publish his work on the theory of universal gravitation. So when answering the trickiest questions, equal parts skepticism and humility are required. We still don’t know what’s going on in the clouds of Venus, but it’s worth sincerely endeavoring to figure it out.