Biosignature on Venus likely to be mistake, says new study
Back in September last year, astronomers made a splash when they announced the discovery of phosphine in the atmosphere of Venus, which could be a sign of life. But a new study has found that a different gas could explain the signature detected, and this one is far more common for Venus and doesn’t indicate life.
With a similar size, mass and composition as Earth, Venus is sometimes called our sister planet – but don’t let that fool you into thinking it could be a haven for life. Choking under an atmosphere of 96 percent carbon dioxide, the surface has crushing pressures about 92 times higher than Earth’s sea level, and temperatures that soar to 464 °C (867 °F).
That’s enough to cross it off the list of places for humans to visit, but scientists have hypothesized that microbial life could thrive at altitudes between 53 and 62 km (33 and 38.5 miles), where the temperature and pressure are much more hospitable.
In September 2020, researchers reported new evidence that seemed to support that possibility of life. A UK team apparently spotted the spectral signature of phosphine in Venus’ atmosphere – a molecule that’s usually created by bacteria and other microbes, and as such has been listed as a potential biosignature for other planets.
That of course caused quite a stir – could one of our nearest neighbors also be home to life? But as science is supposed to do, it didn’t take long for other researchers to poke holes in the story. Independent reanalysis of the data revealed that the processing techniques used may have created false positives from the background noise, and the phosphine signal wasn’t strong enough to be statistically significant.
In the new study, a team led by the University of Washington re-examined the original radio telescope observations that underpinned the claimed detection of phosphine, and found a more likely culprit.
“Instead of phosphine in the clouds of Venus, the data are consistent with an alternative hypothesis: They were detecting sulfur dioxide,” says Victoria Meadows, co-author of the study. “Sulfur dioxide is the third-most-common chemical compound in Venus’ atmosphere, and it is not considered a sign of life.”
The confusion appears to arise because both phosphine and sulfur dioxide absorb radio waves around the same frequency. In 2017, the original team used the James Clerk Maxwell Telescope (JCMT) to discover a feature in Venus’ radio emissions at a frequency of 266.94 GHz, which could have been attributed to either. So they followed it up in 2019 with ALMA observations, and from those they concluded that the sulfur dioxide levels in Venus’ atmosphere were too low to account for the signal, so they attributed it to phosphine.
For the new study, the UW researchers modeled Venus’ atmosphere, and simulated signals from both phosphine and sulfur dioxide at different altitudes. Then, they modeled how they would appear to the two radio telescopes used, in the configurations that they were in at the time of the original observations.
And sure enough, the model favored sulfur dioxide over phosphine, in two different ways. For one, the emission feature was coming from much higher in the atmosphere than the first team realized – about 80 km (50 miles) above the surface. At that altitude, in a region of the atmosphere called the mesosphere, phosphine would break down much faster.
“Phosphine in the mesosphere is even more fragile than phosphine in Venus’ clouds,” says Meadows. “If the JCMT signal were from phosphine in the mesosphere, then to account for the strength of the signal and the compound’s sub-second lifetime at that altitude, phosphine would have to be delivered to the mesosphere at about 100 times the rate that oxygen is pumped into Earth’s atmosphere by photosynthesis."
The second point is that the researchers likely underestimated how much sulfur dioxide there was, thanks to an unexpected quirk of the telescope.
“The antenna configuration of ALMA at the time of the 2019 observations has an undesirable side effect: The signals from gases that can be found nearly everywhere in Venus’ atmosphere — like sulfur dioxide — give off weaker signals than gases distributed over a smaller scale,” says Alex Akins, co-author of the study.
With both of those points in mind, the team on the new study concludes that the signal the original researchers detected most likely came from sulfur dioxide. It seems to be a case of Occam’s razor – it’s probably the gas we already knew to be plentiful on Venus, rather than one that would upend our entire understanding of atmospheric chemistry and life in the solar system.
The research is due to be published in the Astrophysical Journal.
Source: University of Washington