Enlarge /. The spectral signature of phosphine superimposes an image of Venus.
Today researchers announce that they have observed a chemical in the atmosphere of Venus that has no right to be there. The chemical phosphine (an atom of phosphorus attached to three hydrogen atoms) would be unstable under the conditions in Venus' atmosphere, and there is no obvious way for the planet's chemistry to produce much of it.
This leads to much speculation about the equally improbable prospect of life in the upper atmosphere of Venus. Much of this work, however, requires input from people not involved in the first study, which is likely to prompt today's publication. While there are definitely reasons to believe that phosphine is present on Venus, detecting it required some rather complicated computer analysis. And there are definitely some creative chemists out there who want to rethink our closest neighbor's possible chemistry.
What is phosphine?
Phosphorus is one line below nitrogen in the periodic table. And just as nitrogen can combine with three hydrogen atoms to form the well-known ammonia, phosphorus can combine with three hydrogen atoms to form phosphine. In Earth-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 this later feature is why we don't see much of it today. It's just unstable in the presence of oxygen.
We make some of this for our own use. And some microbes that live in oxygen-free environments also produce it, although we haven't identified the biochemical process that does it, nor the enzymes involved. However, any phosphine that escapes into the atmosphere quickly drains into oxygen and is destroyed.
That doesn't mean it doesn't exist on other planets. Gas giants like Jupiter have it. But they also have a lot of hydrogen in their atmosphere and no 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 provide conditions in which phosphine can spontaneously form.
So we have a clear distinction between gas giants with hydrogen-rich atmospheres where phosphine can form and rocky planets where the oxidizing environment should ensure that it is destroyed. This is why people have suggested that phosphine could be a biosignature that we can detect in the atmosphere of rocky planets: we know that it is produced by life on Earth and is likely not to be present on these planets if not all the time is replaced. Some researchers pointed a telescope at the atmosphere of Venus.
Looking for signs
In particular, the researchers turned to the 15-meter James Clerk Maxwell Telescope in Hawaii. The JCMT can image in wavelengths around one millimeter, which is interesting for the Venusian atmosphere. The hot lower atmosphere of Venus generates an abundance of radiation in this region of the spectrum. And phosphine absorbs at a certain wavelength in that area. So, if phosphine is present in the upper atmosphere, its presence at any given point should create a void in the flood of radiation created by Venus' lower atmosphere.
In principle, this is an extremely simple observation. In reality, however, it's a nightmare just because the levels are so low. Here on Earth, where we know phosphine is made, the steady-state level in the atmosphere is on the order of a part per trillion because it is destroyed so quickly. Venus also moves relative to Earth, which means the position of signals must be adjusted to account for the Doppler shift. Finally, any signal would also be complicated by what researchers call "waves," or cases where parts of the spectrum were reflected somewhere between Venus and the telescope.
This required extensive computer processing of the telescope data. But to the surprise of the scientists, this analysis appeared to show the presence of phosphine. (In their paper, the researchers write, "The goal was a measure of future developments, but unexpectedly our first observations suggested that a detectable amount of Venusian PH3 was present.") They had someone else independently repeat the analysis. The signal was still there. The researchers also confirmed that their approach was able to detect water containing deuterium, an isotope of hydrogen that we know is present in Venus’s atmosphere. They also ruled out the possibility that they misidentified a sulfur dioxide absorption line nearby.
Since the obvious problems were eliminated, they had time for a second telescope. That second telescope was the Atacama Large Millimeter Array (ALMA). It has much better resolving power, so researchers can treat Venus as more than one point light source. This confirmed that the phosphine signal was still present and most intense in the mid-latitudes, while it was apparently absent at the poles and the equator. This means that it is present in places where there is more atmospheric circulation from top to bottom.
The researchers eventually concluded that phosphine is present in amounts in the range of 20 parts per billion.
How in the world did that get there?
Assuming the analysis holds, the big question is how did phosphine get there? The researchers estimated how quickly it would be destroyed by conditions in the Venusian atmosphere, and calculated how much phosphine would have to be produced to maintain the 20 parts per billion. And then they looked for a chemical reaction that could produce so much.
And there aren't many good options. Under the conditions that exist in the atmosphere, both phosphorus and hydrogen are typically oxidized, and there is not much of either. While solar radiation could potentially release some of the hydrogen there, it would do so very slowly, and thermodynamics would suggest that it is more likely to react with something other than phosphorus. Similarly, pathways based on probable volcanism of Venus would not produce enough phosphine by factors of about a million.
All of this leads the researchers to a somewhat frustrating conclusion: "If no known chemical process can explain PH3 in the upper atmosphere of Venus, it must be made by a process that has not previously been considered plausible for Venusian conditions." Obviously, however, one of the implausible factors to consider is the whole reason humans looked for phosphine in the first place, namely that it could be produced by living things.
But there is no shortage of implausibility in life on Venus. Nothing we would recognize as life would possibly survive on a wildly hot planetary surface submerged in supercritical carbon dioxide. The temperature in the upper atmosphere, where the phosphine signature arises, is much more moderate. But it would require some form of life that is constantly circulating in the upper atmosphere and somehow survives contact with the planet's sulfuric acid clouds.
So we're staying in an uncomfortable place. One of the researchers who led this work said, "It took us about 18 months to convince us that there was a signal." You can assume that the rest of the field will now spend some time convincing themselves, likely by aiming a whole bunch of additional telescopes at Venus. In the meantime, chemists will try to find additional pathways that could work under Venus-like conditions.
There is a reasonable chance that we will report shortly on the results of these efforts, indicating that nothing unusual is happening on the second planet from the sun. But if that doesn't happen, it will give a big boost to the steady chorus of voices that have argued that we need to do more to explore Venus. There were some plans for airships that could be in the upper atmosphere of Venus for extended periods of time. If these findings hold, airships seem like the perfect means of discovering what is producing this chemical.
Nature Astronomy, 2020. DOI: 10.1038 / s41550-020-1174-4 (Via DOIs).