Today, scientists announce that they have observed a chemical in the atmosphere of Venus that has no right to be there. The chemical, phosphine (a phosphorus atom attached to three hydrogen atoms), would be unstable under the conditions found in Venus̵
This leads to a lot of speculation about the equally unlikely prospect of life somehow surviving in Venus’ upper atmosphere. But much of this work requires input from people not involved in the initial study, which publication today is likely to ask for. While there are certainly reasons to believe that phosphine is present on Venus, the detection of it required some pretty involved computer analysis. And there are definitely some creative chemists who will consider the chemical chemistry of our nearest neighbor.
What is phosphine?
Phosphorus is a series under nitrogen in the periodic table. And just as nitrogen can be combined with three hydrogen atoms to form the well-known ammonia, phosphorus can bind with three hydrogens to form phosphine. Under soil-like conditions, phosphine is a gas, but not a pleasant one: it is extremely toxic and tends to burn spontaneously in the presence of oxygen. And that later move is the reason we do not see much of it today; it is simply unstable in the presence of oxygen.
We make some of it for our own use. And some microbes that live in oxygen-free environments also produce it, even though we have neither identified the biochemical process that does it, nor the enzymes involved. Yet any phosphine that manages to escape into the atmosphere quickly runs out of oxygen and is destroyed.
That does not mean 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 not oxygen, so chemicals like phosphine, methane and ammonia can survive in the atmosphere. And the intense heat and pressure closer to the core of a gas giant provides conditions under which phosphine can form spontaneously.
So we have a clear distinction between gas giants with hydrogen-rich atmospheres, where phosphine can be formed, and rocky planets, where the oxidizing environment must ensure that it is destroyed. For this reason, people have suggested that phosphine may be a biosignature 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. Which is how some scientists ended up pointing a telescope at Venus’ atmosphere.
Looking for characters
Specifically, the scientists turned to the 15-meter James Clerk Maxwell Telescope Telescope in Hawaii. JCMT is capable of imaging in wavelengths about a millimeter, which is interesting for Venus’ atmosphere. The warm lower atmosphere of Venus produces an abundance of radiation in this area of the spectrum. And phosphine is absorbed at a certain wavelength in the range. So if phosphine is present in the upper atmosphere, its presence must create a hole in a certain place in the radiation stream produced by Venus’ lower atmosphere.
In principle, this is an extremely simple observation. In reality, though, 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 in the range per Trillions because it is destroyed so quickly. Venus also moves relative to the earth, which means that the location of any signals must be adjusted to take into account Doppler shifts. Finally, any signal will also be complicated by what scientists call “ripples” or cases where parts of the spectrum underwent reflection somewhere between Venus and the telescope.
These required extensive computer processing of the telescope data. But apparently to the researchers’ surprise, this analysis seemed to show the presence of phosphine. (In their paper, the researchers write, “The goal was a benchmark for future development, but unexpectedly, our first observations suggested a detectable amount of Venusian PH3 was present. “) So they got someone else to repeat the analysis independently. The signal was still there. The scientists also confirmed that their approach was able to detect water with deuterium, an isotope of hydrogen, which we know is present in the atmosphere of Venus They also ruled out the possibility that they incorrectly identified a sulfur dioxide absorption line nearby.
With the obvious problems ruled out, they were given time on another telescope. The second telescope was the Atacama Large Millimeter Array or ALMA. It has a much better resolving power that allows scientists to treat Venus as more than a light source. This confirmed that the phosphine signal was still there and most intense at the mid-latitudes, while apparently being absent from the poles and equator. This means that it is present in places where there is more atmospheric circulation from top to bottom.
The researchers ultimately concluded that phosphine is present in levels in the range of 20 parts per Billion.
How on earth did it get there?
Assuming the analysis stops, the big question becomes how phosphine got there. The researchers assessed how quickly it would be destroyed by the conditions in the Venusian atmosphere, and they used it to calculate how much phosphine would have to be produced to maintain the 20 parts per year. Billion levels. And then they searched for a kind of chemical reaction that could produce so much.
And there is not an abundance of good opportunities. Under the conditions prevailing in the atmosphere, both phosphorus and hydrogen will typically be oxidized and there is not much of anyone around. While solar radiation could potentially release some of the hydrogen that is there, it would do so very slowly, and thermodynamics would indicate that it is more likely to react with something other than phosphorus. Similarly, reaction paths based on Venus’ likely volcanism would not be able to produce enough phosphine with factors of around one million.
All this leads the scientists to a somewhat frustrating conclusion: “If no known chemical process can explain PH3 within Venus’ upper atmosphere, it must be produced by a process that was not previously considered plausible for Venusian conditions.” Of course, one of the unlikely things to consider is the whole reason why people initially looked for phosphine, namely that it could be produced by living things.
But there is no shortage of improbability involved in life on Venus. Nothing we could recognize as life would possibly survive on a tremendously hot planetary surface bathed in supercritical carbon dioxide. The temperature in the upper atmosphere, where the phosphine signature originates, is much more moderate. But it would require a form of life that constantly circulates in the upper atmosphere and somehow survives contact with the planet’s sulfuric acid clouds.
So we are left in an awkward place. One of the researchers who led this work said, “It took about 18 months to convince ourselves that there was a signal.” You can expect the rest of the field to now also spend some time convincing themselves, probably by pointing a whole lot of extra telescopes at Venus. Meanwhile, chemists will try to think of additional pathways that may work under Venus-like conditions.
There is a reasonable chance that we will soon report on the results of this effort, indicating that nothing unusual is happening on the other planet from the Sun. But if it does not end up happening, it will give a big push to the constant chorus of voices that have argued that we need to do more to explore Venus. A few planes have floated around involving airships that may spend longer periods moving around Venus’ upper atmosphere. If these results hold up, airships seem to be the perfect means of figuring out what produces this chemical.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1174-4 (About DOIs).