Whatever happened to Life on Venus?

[This is a transcript of the video embedded below.]


A few months ago, the headlines screamed that scientists had found signs of life on Venus. But it didn’t take long for other scientists to raise objections. So, just exactly what did they find on Venus? Did they actually find it? And what does it all mean? That’s what we will talk about today.

The discovery that made headlines a few months ago was that an international group of researchers said they’d found traces of a molecule called phosphine in the atmosphere of Venus.

Phosphine is a molecule made of one phosphorus and three hydrogen atoms. On planets like Jupiter and Saturn, pressure and temperature are so high that phosphine can form by coincidental chemical reactions, and indeed phosphine has been observed in the atmosphere of these two planets. On planets like Venus, however, the pressure isn’t remotely large enough to produce phosphine this way.

And the only other known processes to create phosphine are biological. On Earth, for example, which in size and distance to the Sun isn’t all that different to Venus, the only natural production processes for phosphine are certain types of microbes. Lest you think this means that phosphine is somehow “good for life”, I should add that the microbes in question live without oxygen. Indeed, phosphine is toxic for forms of life that use oxygen, which is most of life on earth. In fact, phosphine is used in the agricultural industry to kill rodents and insects.

So, the production of phosphine on Venus at fairly low atmospheric pressure seems to require life in some sense, which is why the claim that there’s phosphine on Venus is BIG. It could mean there’s microbial life on Venus. And just in case microbial life doesn’t excite you all that much, this would be super-interesting because it would give us a clue to what the chances are that life evolves on other planets in general.

So, just exactly what did they find?

The suspicion that phosphine might be present on Venus isn’t entirely new. The researchers first saw something that could be phosphine in two-thousand and seventeen in data from the James Clerk Maxwell Telescope, which is a radio telescope in Hawaii. However, this signal was not particularly good, so they didn’t publish it. Instead they waited for more data from the ALMA telescope in Chile. Then they published a combined analysis of the data from both telescopes in Nature Astronomy.

Here’s what they did. One can look for evidence of molecules by exploiting that each molecule reacts to light at different wave-lengths. To some wave-lengths, a molecule may not react at all, but others it may absorb because they cause the molecule to vibrate or rotate around itself. It’s like each molecule has very specific resonance frequencies, like if you’re in an airplane and the engine’s being turned up and then, at a certain pitch the whole plane shakes? That’s a resonance. For the plane it happens at certain wavelengths of sound. For molecules it happens at certain wave-lengths of light.

So, if light passes through a gas, like the atmosphere of Venus, then just how much light at each wave-length passes through depends on what molecules are in the gas. Each molecule has a very specific signature, and that makes the identification possible.

At least in principle. In practice… it’s difficult. That’s because different molecules can have very similar absorption lines.

For example, the phosphine absorption line which all the debate is about has a frequency of two-hundred sixty-six point nine four four Gigahertz. But sulfur dioxide has an absorption line at two-hundred 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 met. The astrophysicists estimated the contribution from Sulphur dioxide from other lines which this molecule should also produce.

They found that these other lines were almost invisible. So they concluded that the absorption in the frequency range of interest had to be mostly due to phosphine and they estimated the amount with about seven to twenty parts per billion, so that’s seven to twenty molecules of phosphine per billion molecules of anything.

It’s this discovery which made the big headlines. The results they got for the phosphine amount from the two different telescopes are a little different, and such an inconsistency is somewhat of a red flag. But then, these measurements were made some years apart and the atmosphere of Venus could have undergone changes in that period, so it’s not necessarily a problem.

Unfortunately, after publishing their analysis, the team learned that the data from ALMA had not been processed correctly. It was not their fault, but it meant they had to redo their analysis. With the corrected data, the amount of phosphine they claimed to see fell to something between 1 and 4 parts per billion. Less, but still there.

Of course such an important finding attracted a lot of attention, and it didn’t take long for other researchers to have a close look at the analysis. It was not only that finding phosphine was surprising, not finding sulphur dioxide was not normal either; it had been detected many times in the atmosphere of Venus in amounts about 10 times higher than what the phosphine-discovery study claimed it was.

Already in October last year, a paper came out that argued there’s no signal at all in the data, and that said the original study used an overly complicated twelve parameter fit that fooled them into seeing something where there was nothing. This criticism has since been published in a peer reviewed journal. And by the end of January another team put out two papers in which they pointed out several other problems with the original analysis.

First they used a model of the atmosphere of Venus and calculated that the alleged phosphine absorption comes from altitudes higher than eighty kilometers. Problem is, at such a high altitude, phosphine is incredibly unstable because ultraviolet light from the sun breaks it apart quickly. They estimated it would have a lifetime of under one second! This means for phosphine to be present on Venus in the observed amounts, it would’ve to be produced at a rate higher than the production of oxygen by photosynthesis on Earth. You’d need a lot of bacteria to get that done.

Second, they claim that the ALMA telescope should not have been able to see the signal at all, or at least a much smaller signal, because of an effect called line dilution. Line dilution can occur if one has a telescope with many separate dishes like ALMA. A signal that’s smeared out over many of the dishes, like the signal from the atmosphere of Venus, can then be affected by interference effects.

According to estimates in the new paper, line dilution should suppress the signal in the ALMA telescope by about a factor 10-20, in which case it would not be visible at all. And indeed they claim that no signal is entirely consistent with the data from the second telescope. This criticism, too, has now passed peer review.

What does it mean?

Well, the authors of the original study might reply to this criticism, and so it will probably take some time until the dust settles. But even if the criticism is correct, this would not mean there’s no phosphine on Venus. As they say, absence of evidence is not evidence of absence. If the criticism is correct, then the observations, exactly because they probe only high altitudes where phosphine is unstable, can neither exclude, nor confirm, the presence of phosphine on Venus. And so, the summary is, as so often in science: More work is needed.