Last months’ sensational news that microbes might be living in Venus’ atmosphere is now being called into doubt, as two new studies by separate teams of scientists say there is no clear signal of phosphine (PH3) high in the clouds of our “twin” planet.
On 14th September, a team of researchers headed by Jane Greaves at Cardiff University made what is arguably one of the biggest announcements in science – that signs of life had been detected on another world. And not one thousands of light years away, but one right next door to us.
Their reasoning was the detection of a biogas called phosphine – PH3 – a compound that on our planet could only be produced artificially in a lab, or by bacteria and microbes that don't require oxygen to thrive. For Venus, it almost certainly meant one thing..
Their analysis was based on two different detections; one using the Atacama Large Millimeter/submillimeter Array in Chile and another using the James Clerk Maxwell Telescope – the largest single-dish telescope that operates in the far-infrared to microwave wavelengths of the electromagnetic spectrum.
As ground-breaking as these results were, the team did not explicitly specify that microbes were behind the signature and added with caution that the chemical signature could also be due to reactions from unknown photochemistry or geochemistry on the planet.
Still, the result looked very positive that tiny Venusian lifeforms could be hiding out in a pocket of temperate gas 53 – 62 kilometres above the surface that was potentially habitable.
What followed was a flurry of papers from researchers around the world discussing topics such as how much biomass was required to produce phosphine, to the potential pathways of formation of PH3, to how life, if it was there, could have persisted aloft in the cloud tops for hundreds of millions to billions of years.
While these studies looked to expand upon the work published by Greaves and colleagues, others assessed the statistical reliability of the phosphine detection in the data collected by the two telescope facilities.
One such team, based in the Netherlands and headed by Ignas Snellen re-examined the ALMA data, while a second collaboration headed by Geronimo Villanueva has taken another look at the JCMT results.
Unfortunately for those hoping to find signs of life on our neighbouring planet, neither study agrees with the work conducted by Greaves and colleagues.
Snellen and team, who used the same data downloaded the same way from ALMA, argue that the method used by Greaves and colleagues to extract the signal is not one that is conventionally used when examining data collected by an array of telescopes.
Greaves and colleagues used a statistical approach called a polynomial to best fit the data, but say the Dutch team, the level of fit was too high.
If a lower polynomial fit was used, as is conventional with observations from an array, then the PH3 signal gets lost in the data. There is just too much ‘noise’ (spurious signals) and not enough phosphine signal to distinguish between the two.
That doesn’t mean the PH3 signal isn’t there they say, just it is not as clear as it should be to claim a detection.
“GRB20 [Greaves and colleagues] provide several arguments to support the validity of their identification of the PH3 feature, including comparison to the JCMT data and a test at offset frequencies. Our analysis, however, shows that at least a handful of spurious features can be obtained with their method, and therefore conclude that the presented analysis does not provide a solid basis to infer the presence of PH3 in the Venus atmosphere,” write the authors in their paper submitted to the arxiv database.
While this paper has yet to be peer reviewed by other scientists, Villanueva and colleagues re-analysis of the JCMT data follows a similar prognosis.
Villanueva and team, who have submitted their findings to Nature Astronomy "Matters Arising” states that another molecule, SO2, could be the source of the signal instead.
After absorbing starlight, SO2 re-emits radiation at a frequency of 266.943329 GHz. Phosphine on the other hand emits a signal at 266.944513 GHz.
These two lines in the spectrographic data appear too close together to be distinguished separately say Villanueva and team, adding that they also disagree with how high above the surface Greaves and colleagues suggest PH3 has been found.
The chemical models produced by Greaves and colleagues indicate PH3 would be at altitudes between 53 – 62 kilometres above the surface. However, say Villanueva and team, the abundances at this altitude need to be much higher than that.
“For any PH3 signature to be produced in either ALMA or JCMT spectra, PH3 needs to present at altitudes above 70 kilometres, in stark disagreement with the G2020 [Greaves and colleagues] photochemical network,” Villanueva and team write.
There is also another twist to the story. Since Greaves and colleagues published their results, the ALMA observatory have also updated the way data is processed at the telescope – a practice called data reduction.
Before a scientific analysis can be made from data collected by a telescope, vast amounts of post processing is required. Reducing data from a single dish observation is hard enough, but when you have over 60 telescopes acting as one, it makes sense to automate many of the processes.
Different parameters obviously produce different results, so now that ALMA has updated its processing package, there is a chance Greaves and team’s initial results will be different.
Villanueva and team say they have already updated some of the methods used to analyse the data and that ultimately, their analysis of the data using several different approaches “reveals no signature of PH3.”
As compelling as these two new studies are, it is doubtful that the last word on the subject has been given and Greaves and colleagues have yet to issue a formal response on the matter. It is anticipated that they will publish one soon.
Even when they do, many agree that our best chances of finding out for certain whether microbes exist in Venus' atmosphere or not, is by waiting for an interplanetary mission such as the one proposed by Rocket Labs, who aim to cruise through the planet's skies and sample its constituents directly.