A team of scientists from the United Kingdom and the United States recently announced the detection of phosphine, PH₃, in the upper atmosphere of Venus, the second planet in our solar system (“Phosphine Gas in the Cloud Decks of Venus,” by Jane S. Greaves et al. Nature Astronomy [2020]; https://www.nature.com/articles/s41550-020-1174-4). Using millimeter spectroscopy, a common astronomical technique used to detect molecules, the authors presented a case for the presence of the phosphorus-containing molecule in the cloud decks, about fifty-six kilometers above the surface. To confirm its detection, the astronomers verified their ability to detect water and sulfur dioxide (SO₂), the latter of which has a signal (at 267.5375 GHz or 1.121 mm) very close to that of PH₃ (266.9445 GHz or 1.123 mm). Their data analysis indicated that the new signal was definitely not SO₂, and they concluded that phosphine was the only plausible source of the signal. They estimate a concentration of about twenty parts per billion.

Why is this noteworthy? Because on Earth the only natural sources of PH₃ are biological, most likely from the anaerobic decomposition of phosphates in decaying organic (that is, related to life) matter. Phosphine is not stable for very long in an atmosphere that contains oxygen gas, so it would have to be constantly produced at low levels from decay of living organisms. There are no known geological or atmospheric sources for terrestrial phosphine.

Schrödinger’s Phosphine?

Not so fast, say some other scientists. At least two other research groups have submitted manuscripts (available on arxiv.org) contesting the conclusion that the signal was phosphine—or that there was any signal in the first place. Both groups, one American and one European, used the same publicly available dataset as the original researchers but subjected them to different calibration and baseline corrections. These corrections, usually in the form of a mathematical equation that can digitally “erase” part of the detected signal, are necessary because the raw data from experimental light sources—in this case, Venus—is always messy with background noise, especially when the data is more detailed; that is, when the data has a higher resolution. Calibration and baseline corrections work to minimize the background noise and allow researchers to detect a true signal. It’s the same as using noise-reducing headphones to remove the annoying hiss when listening to an audio program.

In both cases, the research groups claim that if a different background correction was applied, the claimed signal from PH₃ disappears—or is more correctly identified as SO₂. In addition, both research groups used a simpler background correction, with one group arguing that the background correction used in the original paper added more detail to the signal, leading the researchers to claim a signal that was not there. One researcher recalled a quote attributed to John von Neumann, a well-known mathematician and computer scientist: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” The original researchers used twelve parameters, and the new critics argue that this was unnecessarily complex and led to an incorrect conclusion.

Now What?

Well, is there or is there not phosphine—and maybe life—on Venus? Right now the answer is we don’t know. The chemistry of Venus’s atmosphere is not very well understood, in part because of the extreme difficulty of sending robotic missions to that planet. Venus has the most inhospitable conditions of any rocky planet in the inner solar system; with surface conditions of approximately 850°F and ninety-two atmospheres, it is even more extreme than Mercury, which is half the distance from the Sun. Of several landers that have been sent to Venus, the longest-lived was the Soviet probe Venera 14, which landed on the surface in 1982 and lasted 127 minutes before losing contact.

Right now we are limited to exploring the nature of our closest planetary neighbor by analyzing the light it reflects or emits, with the Sun sometimes getting in the way. As this research shows, it’s not easy. But to those who argue that this is another example of science not making up its mind (a common trope of the antiscience crowd), the reality is that this is how science works. To paraphrase a well-known Starfleet captain, science is a crucible in which we burn away all irrelevancy until what is left is the truth. What we see here is science happening before our very eyes, and it’s a wonderful show.