Scientists stunned the world last year by claiming to have discovered traces of phosphine in the Venusian clouds. New research suggests this gas—which, excitingly, is produced by microbes—was not actually responsible for the signal they detected. Instead, it was likely sulfur dioxide, a not-so-thrilling chemical.
Extraordinary research published in Nature last September is being challenged by a paper set to be published in The Astrophysical Journal, a preprint of which is currently available at the arXiv. This is not the first paper to critique the apparent discovery of phosphine on Venus, and it’s probably not going to be the last.
That phosphine might be present on Venus was a revelation that blew our minds, and that’s because living organisms are one of the only known sources of the stinky gas. The team responsible for the apparent discovery, led by astronomer Jane Greaves from Cardiff University, found the evidence in spectral signals collected by two radio dishes: the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA). Spectral lines at certain wavelengths indicate the presence of specific chemicals, and in this case they implied the presence of phosphine in the Venusian cloud layer.
The authors of the Nature study were not claiming that life exists on Venus. Rather, they were asking the scientific community to explain their rather bizarre observation. Indeed, it was an exceptional claim, as it implied that Venus—one of the most inhospitable planets in the solar system—might actually be habitable, with microscopic organisms floating through the clouds.
Alas, this doesn’t appear to be the case.
“Instead of phosphine in the clouds of Venus, the data are consistent with an alternative hypothesis: They were detecting sulfur dioxide,” Victoria Meadows, a co-author of the new study and an astronomy professor at the University of Washington, explained in a statement. “Sulfur dioxide is the third-most-common chemical compound in Venus’ atmosphere, and it is not considered a sign of life.”
Meadows, along with researchers from NASA, the Georgia Institute of Technology, and the University of California, Riverside, reached this conclusion by modeling conditions inside the Venusian atmosphere, which they did to re-interpret the radio data gathered by the original team.
“This is what’s known as a radiative transfer model, and it incorporates data from several decades’ worth of observations of Venus from multiple sources, including observatories here on Earth and spacecraft missions like Venus Express,” explained Andrew Lincowski, a researcher with the UW Department of Astronomy and the lead author of the paper, in the statement.
Equipped with the model, the researchers simulated spectral lines produced by phosphine and sulphur at multiple atmospheric altitudes on Venus, as well as how those signatures were received by ALMA and JCMT. Results showed that the shape of the signal, detected at 266.94 gigahertz, likely came from the Venusian mesosphere—an extreme height where sulphur dioxide can exist but phosphine cannot owing to the harsh conditions there, according to research. In fact, so extreme is this environment that phosphine wouldn’t last for more than a few seconds.
As the authors argue, the original researchers understated the amount of sulphur dioxide in the Venusian atmosphere and instead attributed the 266.94 gigahertz signal to phosphine (both phosphine and sulphur dioxide absorb radio waves around this frequency). This happened, according to the researchers, due to an “undesirable side-effect” known as spectral line dilution, study co-author and NASA JPL scientist Alex Akins explained in the statement.
“They inferred a low detection of sulfur dioxide because of [an] artificially weak signal from ALMA,” added Lincowski. “But our modeling suggests that the line-diluted ALMA data would have still been consistent with typical or even large amounts of Venus sulfur dioxide, which could fully explain the observed JCMT signal.”
This new result could prove devastating for the Nature paper, and it’ll be interesting to hear how the authors respond to this latest critique. That said, some scientists believe the writing is already on the wall, or more accurately, the trash bin.
“Already quickly after publication of the original work, we and others have put strong doubts on their analysis,” wrote Ignas Snellen, a professor at Leiden University, in an email. “Now, I personally think that this is the final nail in the coffin of the phosphine hypothesis. Of course, one can never prove that Venus is completely phosphine-free, but at least there is now no remaining evidence to suggest otherwise. I am sure that others will keep on looking though.”
Back in December, Snellen and his colleagues challenged the Nature study, arguing that the method used by the Greaves team resulted in a “spurious” high signal-to-noise ratio and that “no statistical evidence” exists for phosphine on Venus.
The apparent absence of phosphine on Venus, and thus the absence of any hints of microbial life, is far less interesting than the opposite, but that’s how it goes sometimes. Science makes no claims or promises about the interestingness of all things, and we, as defenders of the scientific method, must come to accept our unfolding universe as we find it.