Today, researchers announce that they have observed a chemical in Venus’ atmosphere that has no right to be there. The chemical, phosphine (a phosphorus atom linked to three hydrogens), would be unstable under the conditions found in Venus̵
This is leading to much speculation about the equally unlikely prospect of life somehow surviving in Venus’s upper atmosphere. But much of this work requires input from people not involved in the initial study, which today’s publication is likely to require. While there are certainly reasons to think that phosphine is present on Venus, its detection required some rather complex computer analysis. And there are sure to be some creative chemists who will want to rethink our closest neighbor’s possible chemistry.
What is phosphine?
Phosphorus is one line below nitrogen on the periodic table. And just as nitrogen can combine with three hydrogen atoms to form the familiar ammonia, phosphorus can bond with three hydrogens to form phosphine. Under Earth-like conditions, phosphine is a gas, but not a pleasant one: it is extremely toxic and has a tendency to burn spontaneously in the presence of oxygen. And that next feature is why we don’t see much of it today; it is simply unstable in the presence of oxygen.
We make some for our own uses. And some microbes that live in oxygen-free environments also produce it, although we have not identified either the biochemical process that does it or the enzymes involved. However, any phosphine that manages to escape into the atmosphere quickly collides with oxygen and is destroyed.
This is not to say that it does not exist on other planets. Gas giants like Jupiter have it. But they also have an abundance of hydrogen in their atmosphere and no oxygen, allowing chemicals like phosphine, methane, and ammonia to survive in the atmosphere. And the intense heat and pressure closest to a gas giant’s core provide the conditions under which phosphine can form spontaneously.
So we have a clear division between gas giants, with hydrogen-rich atmospheres where phosphine can form, and rocky planets, where the oxidizing environment should ensure its destruction. For this reason, people have suggested that phosphine may be a biosignature that we can detect in the atmospheres of rocky planets: we know that it is produced by life on Earth and is unlikely to be present on these planets unless it is constantly replaced. And this is how some researchers have pointed a telescope at the atmosphere of Venus.
Looking for signs
Specifically, the researchers turned to the 15-meter James Clerk Maxwell Telescope in Hawaii. The JCMT is capable of displaying wavelengths around one millimeter, which is interesting for the Venusian atmosphere. The warm lower atmosphere of Venus produces an abundance of radiation in this area of the spectrum. And the phosphine absorbs at a specific wavelength in the area. So, if phosphine is present in the upper atmosphere, its presence should create a space at a specific point in the flow of radiation produced by the lower atmosphere of Venus.
In principle, this is an extremely simple observation. In reality, however, it’s a bit of a nightmare, just because the levels are so low. Here on Earth, where we know that phosphine is produced, the steady state level in the atmosphere is on the order of one part per trillion because it is destroyed so quickly. Venus also moves relative to the Earth, which means that the position of any signal must be adjusted to take into account the Doppler shift. Finally, any signal would also be complicated by what researchers call “ripples” or instances where parts of the spectrum have undergone reflection somewhere between Venus and the telescope.
This required extensive computer processing of the telescope data. But apparently to the surprise of the scientists, this analysis appeared to show the presence of phosphine. (In their paper, the researchers write: “The target was a benchmark for future developments, but unexpectedly, our initial observations suggested a detectable amount of Venusian PH.3 was present. “) Then they asked someone else to repeat the analysis independently. The signal was still there. The researchers also confirmed that their approach was capable of detecting water with deuterium, an isotope of hydrogen, which we know to be present in the atmosphere of Venus. They also ruled out the possibility that they had mistakenly identified a sulfur dioxide absorption line nearby.
With the obvious problems ruled out, they took the time on a second telescope. That second telescope was the Atacama Large Millimeter Array, or ALMA. It has much better resolving power, allowing researchers to treat Venus as more than a point light source. This confirmed that the phosphine signal was still present and most intense in mid-latitudes while apparently absent from the poles and the equator. This means that it is present in sites where there is more atmospheric circulation from top to bottom.
The researchers finally concluded that phosphine is present at levels in the area of 20 parts per billion.
How the hell did you get there?
Assuming the analysis holds up, the big question becomes how the phosphine got there. The researchers estimated how quickly it would be destroyed by the conditions of the Venusian atmosphere and used it to calculate the amount of phosphine needed to maintain levels of 20 parts per billion. And then they went looking for some sort of chemical reaction that could produce so much.
And, well, there isn’t a host of good options. Under the conditions that prevail in the atmosphere, both phosphorus and hydrogen will typically be oxidized, and there isn’t much of either around. Although solar radiation could potentially release some of the hydrogen present, it would do so very slowly and thermodynamics would indicate that it is more likely to react with something other than phosphorus. Likewise, reaction pathways based on probable Venus volcanism would fail to produce enough phosphine for factors of around one million.
All of this leads the researchers to a somewhat frustrating conclusion: “If no known chemical process can explain PH3 within the upper atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions.” Obviously, however, one of the implausible ones that must be considered is the whole reason that people were looking for phosphine in the first place, namely that it could be made by living things.
But there is no lack of implausibility related to life on Venus. Nothing we would recognize as life could survive on a fiercely hot planetary surface that is bathed in supercritical carbon dioxide. The temperature in the upper atmosphere, where the phosphine signature originates, is much more moderate. But it would require some form of life to perpetually circulate in the upper atmosphere and somehow survive contact with the planet’s sulfuric acid clouds.
So we are left in an awkward place. One of the researchers who conducted this work said: “It took us about 18 months to convince us that there was a signal.” You can expect the rest of the field to take some time to try to convince itself as well, likely aiming a whole bunch of additional telescopes at Venus. Meanwhile, chemists will try to think of further reaction pathways that could work under Venus-like conditions.
There is a reasonable possibility that we will report the results of these efforts before long, indicating that nothing unusual is happening on the second planet from the Sun. that we need to do more to explore Venus. Some plans have circulated involving airships that could spend long periods moving in the upper atmosphere of Venus. If these results hold up, airships seem to be the perfect way to understand what this chemical is producing.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1174-4 (About DOI).