An image of Venus taken by the Akatsuki Ultraviolent Imager. Photo: DARTS archive + Meli thev/Wikimedia Commons, CC BY-SA 4.0.
In September this year, a group of astronomers reported via a scientific paper that they had found a lot more phosphine in the atmosphere of Venus than there ought to be: about 20 parts per billion above an altitude of 53 km. The claim was surprising because planetary scientists didn’t expect this toxic gas to have any sources nearby, and certainly not in the quantities the astronomers had reported. The announcement was rendered more glamorous by another of the astronomers’ claims in their paper: that microbes produce phosphine on Earth, so Venus’s ‘excess’ phosphine could be being put out by microbial life on the planet.
To be sure, the group was circumspect in making this claim, and seemed to intend it as one of many possible explanations for the abundance of phosphine.
Sid Perkins wrote for Science, “Phosphorus-containing minerals, one possible raw ingredient for phosphine, aren’t likely to waft up to high altitude from the planet’s surface. Lightning and sunlight-driven chemical reactions also wouldn’t produce enough of the gas. Volcanoes on Earth spew very small amounts of phosphine, but there would need to be about 200-times as much volcanic activity on Venus to account for the levels seen there.” So, the group wrote in their paper, published on September 14 in Nature: “PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.”
Nonetheless, just the possibility of life on Venus catapulted the paper and its publication into the limelight and prompted many news publications to sensationalise the claim.
But a few weeks later, other independent groups of astronomers have started to question the first group’s data, specifically the claim that Venus’s atmosphere contains abundant phosphine in the first place. Their contestations are all pegged on the same or similar issues: whether the original group’s instruments and analytical methods were in order and if they really support the conclusion that Venus’s atmosphere contains more phosphine than can be accounted for by known sources.
The original group didn’t arrive at their now-controversial results overnight. Instead, the group leader, Jane S. Greaves, and her colleagues had been studying Venus’s atmosphere with the James Clerk Maxwell Telescope (JCMT), Hawaii, in 2017 when they spotted a spectral line corresponding to phosphine in the data. When radiation of certain frequencies is beamed at a molecule, the molecule may absorb parts of it, leaving shadows in the radiation that reaches a detector. These shadows are called spectral lines. To confirm its finding, the group used the more powerful ALMA telescope in Chile, where they found the same signal but fainter.
One of the many criticisms that has emerged now is that the group didn’t have enough data – even though they had used two different telescopes – to conclude that the Venusian atmosphere has more phosphine than it should. For example, phosphine has multiple spectral lines, not just one, and independent experts have said the group should have looked for all these lines before announcing their results. The group itself has admitted that it was still looking for such confirmation but that its efforts had been delayed by the ongoing pandemic.
Another line of criticism is that the phosphine might be present in much lower quantities. One group of astronomers led by Therese Encrenaz – and which included two astronomers from the original group as well, including Greaves – reanalysed data they had collected of Venus since 2012. They found that they could place an upper limit on the quantity of phosphine that was four-times lower than what Greaves at al had reported. Yet another group, led by Geronimo Villanueva, reanalysed the same ALMA and JCMT data and found that the signal that had been interpreted as phosphine could have come from sulphur dioxide, which Venus’s atmosphere is known to contain in copious quantities and one of whose spectral lines nearly coincides with the one of phosphine.
None of this is simplified by the fact that the data to be analysed is very noisy to begin with. It is not at all easy to ascertain the presence of a specific molecule in a whole different planet’s atmosphere using ground-based telescopes. Add to this the noise produced by the instruments themselves as well as by other natural sources – like Earth’s atmosphere. Astronomers then take such a dataset and keep subtracting noise until a signal they are looking for shows itself. On the way, they need to be careful to not subtract something they ought to include or include something they ought to subtract.
Indeed, a third line of criticism has been directed at the group’s data-analysis methods. As Nadia Drake wrote for National Geographic on October 23, “To pull the phosphine signal out of a messy data set, the team subtracted the background noise using a high-order polynomial, which means more variables were used to model the data than is typical. In addition, the team modelled the background noise by looking at portions of the spectrum outside the area where they expected to find a signal from phosphine…” Taken together, these methods could have resulted in a “false signal” where there was actually nothing.
By way of comparison, India’s NITI Aayog was guilty of something similar in April this year. On April 24, NITI Aayog member V.K. Paul had presented a study that projected that the number of daily new COVID-19 cases in India would drop to zero after May 16. To arrive at this unbelievable conclusion, the study had modelled the rising case-load at the time using an equation that had more variables to model the data than is typical. And this equation plunged to zero on the y-axis corresponding to the x-axis position of May 16 (see below).
Greaves et al’s JCMT and ALMA datasets corresponded to radiation in the millimetre wavelengths, which overlaps with the radio-wave part of the spectrum. Encrenaz et al, who found phosphine could only be present in much lower quantities, had collected data from the infrared part of the spectrum. Radio- and infrared waves have different origins and penetrative characteristics, so it remains important to explain why the two datasets differ, and where Greaves et al might have gone off the mark.
As Clara Sousa-Silva, one of the members of the original group who was also part of Encrenaz’s group, told National Geographic, “I believe Encrenaz’s work, and so there’s no phosphine – there. It’s just, where is this there? What is the altitude that we’re looking at? And does this mean that we’re probing deep enough, and there’s no phosphine because it was never there? Does it mean there’s no phosphine because it’s variable? Or does it mean we didn’t probe as deep as we thought?” She added that they would look for ‘cleaner’ data in future to put the phosphine question to rest. For now, the claim remains in limbo – neither right nor wrong, but certainly not right.
In fact, Sousa-Silva and other members of the group have welcomed criticism of their work as well as the follow-up analyses that further constrain their finding and its implications. She called the other astronomers’ response “normal”, adding that “this is what science looks like. … I’m so relieved that people are finally looking at this data and it’s not just us.”
It wasn’t all amicable, however. Earlier this week, the preprint paper Villanueva et al had uploaded to the arXiv repository attracted criticism after the authors wrote in the paper’s abstract that the Greaves et al paper should be retracted.
A retraction refers to a journal removing a paper from its record, and from the canonised scientific literature per se, typically for claims that are no longer tenable or because its experimental methods have been found to be invalid. While there is some awareness that retractions are par for the course and shouldn’t be considered bad, a considerable amount of stigma continues to be attached to it – so much so that many scientists think retractions can damage their careers. Either way, demanding a retraction is considered to be strong or harsh language.
But on October 29, Geronimo Villanueva uploaded a statement on Twitter apologising for the language, explaining the reasons for the original choice of words and editing the paper’s abstract accordingly on arXiv.