A few months ago, headlines were shouting that scientists had found signs of life on Venus. It wasn’t long, however, before other scientists objected. So, exactly what they found on Venus? Did you actually find it? And what does it all mean? That’s what we’re going to talk about today.
The discovery that hit the headlines a few months ago was that an international group of researchers said they had found traces of a molecule called phosphine in the atmosphere of Venus.
Phosphine is a molecule made up of one phosphorus and three hydrogen atoms. On planets like Jupiter and Saturn, the pressure and temperature are so high that phosphine can form through random chemical reactions, and indeed phosphine has been observed in the atmosphere of these two planets. However, on planets like Venus, the pressure is not enough to produce phosphine in this way.
And the only other known methods of producing phosphine are biological. For example, on Earth, which is not that different from Venus in size or distance from the Sun, the only natural production processes for phosphine are certain types of microbes. Lest you believe that phosphine is somehow “good for life”, I should add that the microbes in question live without oxygen. In fact, phosphine is toxic to life forms that consume oxygen, which makes up most of life on earth. In fact, phosphine is used in agriculture to kill rodents and insects.
So the production of phosphine on Venus at relatively low atmospheric pressure seems to take life in a sense, which is why the claim that there is phosphine on Venus is BIG. It could mean that there is microbial life on Venus. And just in case microbial life doesn’t excite you too much, this would be very interesting as it would give us an indication of how likely it is for life to evolve on other planets in general.
So, exactly what did you find?
The suspicion that phosphine might be present on Venus is not entirely new. The researchers first saw what could be phosphate in two thousand and seventeen in data from the James Clerk Maxwell Telescope, a radio telescope in Hawaii. However, this signal wasn’t very good so they didn’t release it. Instead, they waited for more data from the ALMA telescope in Chile. They then published a combined analysis of the data from both telescopes in natural astronomy.
Here is what they did. One can look for evidence of molecules by taking advantage of the fact that each molecule reacts to light at different wavelengths. At some wavelengths a molecule may not react at all, at others it may absorb because the molecule vibrates or spins on itself. It is as if every molecule has very specific resonance frequencies, for example when you are in an airplane and the engine is turned on and then at a certain pitch the whole airplane wobbles? That’s a response. For the plane, this happens at certain sound wavelengths. In the case of molecules, this happens at certain wavelengths of light.
So when light, like the atmosphere of Venus, flows through a gas, how much light is allowed to pass through at each wavelength depends on which molecules are in the gas. Each molecule has a very specific signature, and that is what enables identification.
At least in principle. In practice … it’s difficult. This is because different molecules can have very similar absorption lines.
For example, the phosphine absorption line that we are talking about throughout the debate has a frequency of two hundred and sixty six point nine four four gigahertz. But sulfur dioxide has an absorption line at two hundred and sixty six point nine four three gigahertz, and sulfur dioxide is really common in the atmosphere of Venus. That makes it quite a challenge to find traces of phosphine.
But challenges are there to be mastered. The astrophysicists estimated the contribution of sulfur dioxide from other lines that this molecule should also produce.
They found that these other lines were almost invisible. They concluded that most of the absorption in the frequency range of interest must be due to phosphine, and estimated the amount to be about seven to twenty parts per billion, that is, seven to twenty molecules of phosphine for every billion molecules of anything.
It is this discovery that made the big headlines. The results they got for the amount of phosphine from the two different telescopes are slightly different, and such an inconsistency is more of a red flag. But then these measurements were taken every few years and the atmosphere of Venus may have changed during this time, so this is not necessarily a problem.
Unfortunately, after the analysis was published, the team discovered that ALMA was not processing the data correctly. It wasn’t their fault, but it meant they had to repeat their analysis. With the data corrected, the amount of phosphine they supposedly saw dropped to 1 to 4 parts per billion. Less, but still there.
Of course, such an important finding attracted a lot of attention, and it wasn’t long before other researchers looked closely at the analysis. Not only was it surprising to find phosphine, it wasn’t normal not to find sulfur dioxide either. It had been detected many times in Venus’ atmosphere in amounts about ten times what the phosphine discovery study claimed.
Back in October last year, an article appeared claiming the data contained no signal at all, and in which the original study used an overly complicated adjustment of twelve parameters that led them to see something where nothing was. This criticism has since been published in a peer-reviewed journal. In late January, another team published two articles pointing out some other issues with the original analysis.
First, they used a model of Venus’ atmosphere and calculated that the alleged absorption of phosphine came from altitudes over eighty kilometers. The problem is that at such high altitude, phosphine is incredibly unstable because ultraviolet sunlight breaks it apart quickly. They estimated it would last less than a second! This means that phosphine must be present on Venus in the amounts observed and must be produced at a faster rate than the oxygen production through photosynthesis on Earth. You would need a ton of bacteria to do that.
Second, they claim that the ALMA telescope could not have seen the signal at all, or at least a much smaller signal, due to an effect known as line thinning. Line thinning can occur when one has a telescope with many separate shells like ALMA. A signal that is smeared over many dishes, such as the signal from the atmosphere of Venus, can then be affected by interference effects.
According to estimates in the new publication, the line thinning should suppress the signal in the ALMA telescope by about a factor of 10-20. In this case it would not be visible at all. Indeed, they claim that no signal completely matches the data from the second telescope. This criticism has now also passed the assessment by specialist colleagues.
What does that mean?
Well, the authors of the original study could respond to this criticism, and it will likely take some time for the dust to settle. But even if the criticism is correct, it would not mean that there is no phosphine on Venus. As they say, the absence of evidence is not evidence of the absence. If the criticism is correct, precisely because they only examine high altitudes where phosphine is unstable, the observations cannot exclude or confirm the presence of phosphine on Venus. And so the summary is, as is so often the case in science: More work is required.