Extraordinary claims require extraordinary evidence
Carl Sagan, 1980
[This is a quick-reaction piece that is being polished and updated constantly. Last updated: September 18th]
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:
(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:
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. 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 if 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 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. 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 against there being large amounts of Martian methane and, just possibly, there is none at all. 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, September 18th:
Scientists have pointed out that the Bepi-Colombo probe, currently en route to Mercury, will make two close fly-bys of Venus on October 15th 2020 and August 10th 2021. Bepi has a spectrometer, MERTIS, that could be used to observe phospine in the atmosphere of Venus. At the moment, it is not known if MERTIS has the required sensitivity to make a detection, but it is a splendid opportunity for scentists to attempt to confirm the discovery from a spacecraft that could eliminate some of the uncertainties that observing from within the Earth’s atmosphere, even from a site as good as Mauna Kea, can cause.
 “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.