Extraordinary claims require extraordinary evidence

Carl Sagan, 1980

[This is a quick-reaction piece that is being polished and updated constantly. Last updated: October 1st]

Through the day today there have constant reports in social media and elsewhere in the Internet, leaking news of the discovery of evidence of microbial life in the atmosphere of Venus, as evidenced by the discovery of phosphine gas in the atmosphere. This was confirmed by a press conference organised by the Royal Astronomical Society and the publication of the study in a prestigious scientific journal. Already, conspiracy theorists are at work suggesting that some cover-up of the discovery of life on Venus is underway, with scientists trying to explain-away the presence of phosphine gas as a natural process.

If you have not seen the news, here are two versions of it. First, the publication in Nature Astronomy:

https://www.nature.com/articles/s41550-020-1174-4

(You may need a subscription to be able to read the article). And, alternatively, the BBC’s excellent summary by one of the very best scientific journalists out there:

https://www.bbc.com/news/science-environment-54133538

Ah, Venus! Planet of love. Eternally covered in dense, impenetrable cloud, we can never see its surface. Over my lifetime ideas have changed of what the surface is like have veered from ocean world to volcanic hell. Over the decades, an army of authors have populated the planet with everything from intelligent amphibians to lethal bacteria to mobile vegetation. Percival Lowell drew canals in the atmosphere. Edgar Rice Burrows pictured crystal spires on the planet, but the Soviet Venera probes revealed a hellish surface, lit by a dim red light from the hidden Sun, with a temperature of 430⁰C, a surface pressure ninety times that of the Earth’s atmosphere. In that, poisonous atmosphere of Carbon Dioxide a gentle breeze is blowing that carries the same force as waves in an earthly gale.

We know now that the clouds are of concentrated sulphuric acid, not water. Venus has an acid rain problem the likes of which we can scarcely imagine.

As Patrick Moore put it:

Any astronaut who went out unprotected on the surface of Venus would be roasted, squashed, poisoned and asphyxiated, rapidly and terminally.

How did Venus get this way? We are all familiar with the Greenhouse Effect: Earth is living the consequences as rising levels of Carbon Dioxide retain more heat and temperatures rise. Venus though suffered a Runaway Greenhouse Effect. Initially, Venus must have had about the same amount of water as the Earth: temperatures rose and baked carbonate out of the surface, this in turn increased the Greenhouse Effect by filling the atmosphere with Carbon Dioxide, raising the temperature still further. As the early Venus got hotter and hotter, its oceans evaporated, leading to a Wet Greenhouse Effect, in which the water vapour reinforced still further the heating of the surface to its current temperature. Finally, solar ultraviolet light broke down the water and the hydrogen was lost to space. No one seriously contemplated that any kind of life could survive.

A few decades ago, astronomers looked at how planets and their satellites could be terraformed and made more Earth-like. Curiously, it was thought that this would be easier with Venus than with Mars. The theory was absurdly simple: dump mega-quantities of blue-green algae in the upper atmosphere; these would break down the Carbon Dioxide, locking-up the carbon, releasing oxygen and letting the heat out into space. After a few centuries, the temperature would drop to a more reasonable level.

Could it be though Venus actually does have a highly active ecosystem in its atmosphere? If it were to, it would have huge philosophical implications for our place in the Universe and any future human efforts to terraform Venus would inevitably destroy it

The quote that opens this posting is known as “The Sagan Standard” because the astronomer, Carl Sagan, popularised it in his 1980 programme, Cosmos. It has, though, been used in multiple different phrasings since Thomas Jefferson’s first and more verbose usage, in 1808[1]. What is the basis of this extraordinary claim and what is the extraordinary proof?

The basis is the discovery of the gas phosphine (PH₃) in the atmosphere of Venus. Phosphine is extremely nasty stuff. It is a nerve gas that can be absorbed through the skin and acts as a respiratory poison: paraphrased, when exposed to phosphine, your first reaction is that it smells unpleasantly of rotting fish and your second is that you cannot breathe.

Phosphine is known to exist in the atmosphere of Saturn, where it was first detected, and of Jupiter. One of the popular theories of the Great Red Spot is that its red colour is due to phosphine gas decomposing into red phosphorous within the storm system. Part of its interest is that it is a builder molecule that can combine to form more complicated, organic compounds containing phosphorous. Phosphorous, itself, is also a very basic part of life on Earth – our bones and teeth are formed of Calcium Phosphate.

So, why the fuss? The way that phosphine gas forms on the Earth and on Jupiter is fundamentally different. On or, more accurately, in Jupiter, it is formed by reactions at exceptionally high temperatures and pressures that do not exist inside the Earth or Venus. On Earth, phosphine gas is a so-called trace gas, an extremely rare by-product of industrial processes, of swamps and of the gut bacteria of some animals such as penguins. Venus has no industry, no swamps and no penguins, so should have no phosphine… unless it can be produced in some way that we do not understand.

The amount of phosphine is tiny – 20 parts per billion, meaning that just one molecule in every fifty million in the Venerian (or Cytherian) atmosphere is phosphine. That is a tiny amount but, in such a dense atmosphere, turns out to be an immense quantity of gas.

The mass of the atmosphere of Venus is 4.8×10²⁰kg.

That means that there is about 10¹²kg of phosphine in the atmosphere: 1000 million tonnes.

In contrast, the mass of phosphine in the Earth’s atmosphere is around 100 000 tonnes.

Could the detection of phosphine gas in the atmosphere of Venus be an error? Well, the detection is at a statistical level of about 15 sigma. To give you an idea of what this means, a one sigma detection means that about one third of the time a given result occurs just by random chance (about the same as the chance of tossing two coins and have both come up “heads”). Two sigma means just one chance in 22 that the result is obtained by random chance (similar to tossing a coin and getting 5 “heads” in a row). Six sigma means that there is about one chance in 500 million that the result is a statistical fluke (if something has a six sigma chance of being correct it is a close to an absolute certainty as most people are ever going to need. The fifteen sigma detection means that, if the statistics are accurately stated, there is just about a 1 in 1 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 chance (I hope that this is the correct number of zeroes) that it is just a statistical fluke result.

What is even more interesting is that the amount of phosphine is not constant over the planet: there is less at the equator, more at temperate latitudes and none at the poles. That makes it even less likely that there is some subtle error of interpretation. Something in the atmosphere of Venus concentrates the Phosphine gas in a quite restricted region.

There are many ways that phosphine gas could be produced naturally – volcanic gases, lightning and meteorite bombardment are just some of them – but, all of these fall about a factor of ten thousand short of producing enough phosphine to explain the quantities observed in the high atmosphere of Venus.

So, here is the leap of logic that has set the Internet buzzing: if there is no way that you can explain the presence of this gas by natural processes, there must be a real possibility that it is produced by something living, presumably bacteria, in the atmosphere of the planet. As Sherlock Holmes would have put it:

If you eliminate the impossible, whatever remains, however improbable, must be the truth.

So, what else do we know? Well, the phosphine is shown as an absorption in the spectrum of Venus, superimposed on microwave radiation emitted from a layer of the atmosphere at an altitude of 53-61km. In other words, the phosphine is in a higher and cooler layer of the atmosphere than the clouds at 53-61km, but we do not know how high. It is assumed though to be not much greater altitude as, at an altitude of 80km, a phosphine molecule is, on average, destroyed by reactions with other atmospheric components in just half an hour: in other words, no sooner does the gas venture this high up than it is destroyed. At 55km altitude on Venus, the atmospheric pressure is about half that on the surface of the Earth and the temperature around 27⁰C, so it is certainly survivable, apart from the Carbon Dioxide and the concentrated sulphuric acid clouds

The result is certainly impressive and in need of explanation. However, the life of a putative bacterium floating at, for example, just 70km in the Venusian atmosphere, at the altitude of the cloud-tops on  the day side of Venus would not be a happy one. There, the temperature is as low as -43⁰C and the atmospheric pressure about one thirtieth of that at the Earth’s surface. It would need to stay in a very narrow range of atmosphere in the clouds. If it started to sink too far, it would soon be roasted. It would need some inert protecting layer to stop it being dissolved by the acid all around it (meaning that there would be problems both to absorb nutrients from the atmosphere and to eliminate waste gases such as phosphone through this protective armour plating). Deeper in the clouds it would be in increasingly dim light, making photosynthesis (if Venerian bacteria use photosynthesis) increasingly inefficient as form of producing energy. So, the “survivable”, if still hostile, layer of the atmosphere is a very thin one: a little higher and any microbes would freeze; a little lower and they would roast.

The sweet spot – the layer of the atmosphere at which, concentrated sulphuric acid excepted, a bacterium could live comfortably – may be only about 5km thick around the 60-65km level.

For me, the biggest issue is the still unexplained one of how can you transport a billion tonnes of phosphorous into the upper atmosphere of Venus? It has to come from somewhere for our hypothetical bacteria to be able to metabolise it into phosphine gas. What is the source of so much phosphorous, especially given that the Earth has so little in comparison in its atmosphere although the composition of the bulk of the two planets is so similar and phosphorous is quite common in the surface rocks?

For now, most scientists are fascinated by the result, but sceptical of the possibility of a high altitude floating ecosystem in the Cytherian atmosphere. Some recall the announcement of the “discovery” of fossil bacteria in the Martian meteorite ALH84001: the announcement, in 1996, was so sensational that President Bill Clinton gave a speech about it. Now, few scientists believe that ALH84001 shows any evidence of fossils. Similarly, the detection of methane in the atmosphere of Mars has produced so many claims and counter-claims that it is hard to know what to believe and the weight of evidence is swinging heavily against there being large amounts of Martian methane and, just possibly, there is no methane at all, which has implications for both any residual internal activity and any biological activity. There is a lot of caution about the implications of the new results for Venus because there is the possibility that there is a subtle misinterpretation or misunderstanding about the phosphine and its interpretation, even if we do not yet know what it might be.

Update, October 1st:

A lot of things are going-on in the background, even if this story is no longer filling the pages of the popular press:

A huge paper (more than 100 pages) of analysis of phosphorous chemistry in the atmosphere of Venus, in support of the original article, is about to be published. This contains a huge amount of analysis of possible scenarios for phosphine production in the atmosphere of Venus to support the claim that there is no obvious abiological way to produce such large quantities of the gas.

Venus Express scientists are going back over their own data obtained over nine years at Venus to see if they can find confirmatory evidence of phosphine in their data.

A study of the evolution of the orbit of Venus by scientists at the University of California at Riverside (https://iopscience.iop.org/article/10.3847/PSJ/abae63) suggests that the orbit of Venus was greatly influenced by Jupiter as that planet’s distance from the Sun changed in the early history of the solar system. The study suggests that Venus’s orbit, currently the most circular in the solar system, could have been strongly elliptical in the distant past. Depending on how close Jupiter was to the Sun in the early solar system, Venus could have had an orbital eccentricity as high as 0.31, meaning that it would have been a lot closer to the Sun than now at perihelion, but a lot more distant at aphelion. As a planet moves faster at perihelion than at aphelion, the “cool” part of the orbit would have lasted much longer than the hot part. With the Sun less luminous than it is now, it is plausible that the climate could have been suitable for the development of life. However, this scenario also means that water loss on Venus would have been even faster – if only by a small amount – than previously supposed, as the moist greenhouse effect in the atmosphere would have been accelerated.

Two weeks after publication, no group has yet produced a plausible scenario for the production of the observed quantities of phosphine gas, although there are some potential suggestions, similarly, there are no obvious atmospheric contaminants that could have given a false positive.

Update: December 12th

Phosphine on Venus, going… going… gone…?

As more details of the observations and their reduction emerge, scepticism about the results is increasing considerably to the point that now, most scientists seem to believe that the detection was an honest mistake.

So, ultimately, the reason why no one can produce a plausible scenario to create phosphine in the upper atmosphere of Venus is because there is none, but none is needed. Phosphine simply does not exist in such quantities.

One reason to be extremely cautious is that only a single spectral line was detected. In molecular astronomy this is always a warning sign. To avoid the danger that other, unknown, or unexpected molecules give you a spectral line that imitates the molecule that interest you, normally one would search for several lines in the spectrum. You would observe them knowing which should be the strongest and how big the others will be compared to it. If you expect to see three lines with a known range of strengths and see all three and each has the intensity that you expect, that is solid proof that you have not be tricked by some instrumental or atmospheric issue, or by an unexpected alternative molecule.

A second is that details of the processing of the phosphine observations have been examined carefully and one particular issue has been flagged. That is that an extremely aggressive procedure was used to smooth and to reduce structure in the background of the spectrum. Technically, a twelfth-order polynomial was used to fit this background. High-order polynomials are a false friend and can create false structures in the spectrum as well as remove them. There is a strong suspicion that the detection of phosphine was due to an artefact created by this procedure that was designed to reveal the line but, in the end, may simply have imitated it.

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[1]  “A thousand phenomena present themselves daily which we cannot explain, but where facts are suggested, bearing no analogy with the laws of nature as yet known to us, their verity needs proofs proportioned to their difficulty.