What started as a stunning announcement that the chemical phosphine — a known byproduct of life — had been found in the clouds of Venus and could signal the presence of some lifeform has now been strongly critiqued by a number of groups of scientists. As a result, there is growing doubt that the finding, published in the journal Nature Astronomy in September, is accurate.
The latest critique, also submitted to Nature Astronomy but available in brief before publication, is led by NASA’s planetary scientist Geronimo Villaneuva and others at the Goddard Space Flight Center. They reanalyzed the data used to reach the conclusion that phosphine was present and concluded that the signal was misinterpreted as phosphine and most likely came instead from sulphur dioxide, which Venus’s atmosphere is known to contain in large amounts.
The title of their paper is “No phosphine in the atmosphere of Venus.”
Another paper led by Ignas Snellen from the Leiden Observatory came to a similar conclusion, but finding fault elsewhere. She and her team analyzed the data used in the initial research to see if cleaning up the noise with a 12-variable mathematic formula, as was used in the paper, could lead to incorrect results.
According to Snellan, using this formula actually gave the original team — false results and they found “no statistical evidence for phosphine in the atmosphere of Venus.”
While this critical research does not on its own disprove that phosphine exists in Venus’ atmosphere, it clearly raises doubts about original team’s conclusions.
That original team was lead by Jane S. Greaves, a visiting scientist at the University of Cambridge when when she worked on the phosphine finding. She herself has also has been unable to replicate the level of phosphine found by her team, and was a co-author on a paper that described that. It is now almost impossible to collect new data because of the coronavirus pandemic.
This intense scrutiny continues as staff at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, discovered a separate, unspecified issue in the data that were used to detect the phosphine. “There are some issues with interpretation that we are looking at,” says Dave Clements, an astrophysicist at Imperial College London and co-author of the original study.
Adding to the reanalyses and doubts, several other critical papers are in the pipeline.
The highly significant initial claim that an apparently biogenic molecules had been found in the atmosphere of Venus was certain to be challenged and to be done so vigorously. These reanalyses are a crucial aspect of the scientific process and were made more pressing because Venus — with its baking temperatures, high surface pressures and huge amounts of carbon dioxide and sulfuric acid in the atmosphere — is hardly a strong candidate to harbor life.
Greaves and colleagues had earlier published a paper reporting that phosphine on Venus did not appear to have possible abiotic sources. But while phosphine (one phosphorus and three hydrogen atoms) is definitely a potential biosignature — produced on Earth by bacteria living in sewage, swamps, marshlands, rice fields, and animal intestines—it can be also produced in some industrial processes, and it has been detected on gaseous planets such Saturn and Jupiter where life is not deemed able to survive. The initial paper did explicitly not identify the phosphine as living, but of a potential byproduct of life.
In a late October article in the MIT Technology Review — which is wholly owned by the university but editorially independent of it — Neel V. Patel wrote that: “The truth is, the story of Venus’s putative phosphine is not a simple case of a sensational finding being shot down upon further scrutiny. In fact, the rush of follow-up research is welcomed; science is doing its thing. This is especially true when it comes to the search for extraterrestrial life—after all, extraordinary claims require extraordinary evidence” (a frequently-invoked say of Carl Sagan.)
“Another problem that plagues the phosphine findings is data processing. The two other preprints were written by teams that tried to reprocess the original data used by Greaves and her team, suspecting that the original analysis was flawed. It’s often a challenge to pull signals out of the massive amounts of noise found in telescopic data. Researchers in the original study used a technique called polynomial fitting, which is supposed to remove background noise around the spectral region where phosphine signals should pop up. But as National Geographic reports, the way they went about it might actually have introduced false phosphine signals.”
The initial phosphine discovery now in question involved some serendipity and some long-debated theories about how life might exist in the clouds of Venus. The phosphine findings were based on reading from the powerful James Clerk Maxwell Telescope (JCMT) in Hawaii as well as ALMA.
“This was an experiment made out of pure curiosity, really – taking advantage of JCMT’s powerful technology, and thinking about future instruments,” said Greaves, who is now based at Cardiff University, when her initial paper was published. “I thought we’d just be able to rule out extreme scenarios, like the clouds being stuffed full of organisms. When we got the first hints of phosphine in Venus’ spectrum, it was a shock!”
Since it had been recently proposed in an Astrobiology article, by lead author Clara Sousa-Silva of MIT, that any phosphine (PH3) detected in a rocky planet’s atmosphere is a promising sign of life, Greaves focused on that chemical.
Fortunately, conditions were good at ALMA for follow-up observations while Venus was at a suitable angle to Earth. Processing the data was challenging, however, as ALMA isn’t usually looking for subtle effects in bright objects like Venus.
“In the end, we found that both observatories had seen the same thing – faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below,” said Greaves at the time.
Using existing models of the Venusian atmosphere to interpret the data, the researchers found that phosphine was present but scarce – only about twenty molecules in every billion. The astronomers then ran calculations to see if the phosphine could come from natural processes on Venus.
They caution then that some information was lacking – in fact, the only other study of phosphorus on Venus came from one lander experiment, carried by the Soviet Vega 2 mission in 1985. But they made the case that the phosphine could well have a biological origin because no abiotic Venusian method could be found.
All this and more has now been called into question.
Our initial Many Worlds story about phosphine and Venus was written by Elizabeth Tasker and can be found here.
We will also return to this story and look at one of the central challenges of the search for life beyond Earth: How to determine what chemicals are potential biosignature and what are not. This is an extremely complex field and, as the phosphine study shows, is very challenging now and will be in the decades to come.