Chemical that shouldn’t be there spotted in Venus’ atmosphere

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Image of a pale circle with irregular lines in front of it.
Enlarge / The spectral signature of phosphine superimposed on an image of Venus.


Today, researchers are announcing that they've observed a chemical in the atmosphere of Venus that has no right to be there. The chemical, phosphine (a phosphorus atom hooked up to three hydrogens), would be unstable in the conditions found in Venus' atmosphere, and there's no obvious way for the planet's chemistry to create much of it.

That's leading to a lot of speculation about the equally unlikely prospect of life somehow surviving in Venus' upper atmosphere. But a lot about this work requires input from people not involved in the initial study, which today's publication is likely to prompt. While there are definitely reasons to think phosphine is present on Venus, its detection required some pretty involved computer analysis. And there are definitely some creative chemists who are going to want to rethink the possible chemistry of our closest neighbor.

What is phosphine?

Phosphorus is one row below nitrogen on the periodic table. And just as nitrogen can combine with three hydrogen atoms to form the familiar ammonia, phosphorus can bind with three hydrogens to form phosphine. Under Earth-like conditions, phosphine is a gas, but not a pleasant one: it's extremely toxic and has a tendency to spontaneously combust in the presence of oxygen. And that later feature is why we don't see much of it today; it's simply unstable in the presence of any oxygen.

We do make some of it for our own uses. And some microbes that live in oxygen-free environments also produce it, although we have neither identified the biochemical process that does so nor the enzymes involved. Still, any phosphine that manages to escape into the atmosphere quickly runs into oxygen and gets destroyed.

That's not to say it doesn't 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 closer to a gas giant's core provide conditions in which phosphine can form spontaneously.

So we have a clear divide between gas giants, with hydrogen-rich atmospheres where phosphine can form, and rocky planets, where the oxidizing environment should ensure it's destroyed. For that reason, people have suggested that phosphine might be a biosignature we can detect in the atmospheres of rocky planets: we know it's produced by life on Earth and is unlikely to be present on these planets unless it's constantly replaced. Which is how some researchers ended up pointing a telescope at Venus' atmosphere.

Looking for signs

Specifically, the researchers turned to the 15-meter James Clerk Maxwell Telescope telescope in Hawaii. The JCMT is able to image in the wavelengths around one millimeter, which is an interesting one for Venus' atmosphere. The hot lower atmosphere of Venus produces an abundance of radiation in this area of the spectrum. And phosphine absorbs at a specific wavelength in the area. So if phosphine is present in the upper atmosphere, its presence should create a gap at a specific location in the flood of radiation produced by Venus' lower atmosphere.

In principle, this is an extremely simple observation. In reality, however, it's a bit of a nightmare, just because levels are so low. Here on Earth, where we know phosphine is made, the steady-state level in the atmosphere is in the area of a part-per-trillion because it's destroyed so quickly. Venus is also moving relative to Earth, meaning the location of any signals need to be adjusted to account for Doppler shifting. Finally, any signal would also be complicated by what researchers call “ripples,” or instances when parts of the spectrum underwent reflection somewhere between Venus and the telescope.

These required extensive computer processing of the telescope data. But seemingly to the scientists' surprise, this analysis appeared to show the presence of phosphine. (In their paper, the researchers write, “The aim was a benchmark for future developments, but unexpectedly, our initial observations suggested a detectable amount of Venusian PH3 was present.”) So they had someone else repeat the analysis independently. The signal was still there. The researchers 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'd misidentified a sulfur dioxide absorption line that's nearby.

With the obvious problems ruled out, they got hold of time on a second telescope. That second telescope was the Atacama Large Millimeter Array, or ALMA. It has a much better resolving power, allowing the researchers to treat Venus as more than a point source of light. This confirmed that the phosphine signal was still there and most intense at the midlatitudes while seemingly absent from the poles and equator. This means it's present at sites where there's more top-to-bottom atmospheric circulation.

The researchers ultimately concluded that phosphine is present, at levels in the area of 20 parts-per-billion.

How in the world did that get there?

Assuming that analysis holds up, the big question becomes how phosphine got there. The researchers estimated how quickly it would be destroyed by the conditions in the Venusian atmosphere, and they used that to calculate how much phosphine would need to be produced to maintain the 20 parts-per-billion levels. And then they went searching for some sort of chemical reaction that could produce that much.

And, well, there isn't a plethora of good options. Under the conditions that prevail in the atmosphere, both the phosphorous and hydrogen will typically be oxidized, and there's not much of either around. While solar radiation could potentially liberate some of the hydrogen that is there, it would do so very slowly, and thermodynamics would indicate it's more likely to react with something other than phosphorus. Similarly, reaction pathways based on Venus' likely volcanism would fall short of producing enough phosphine by factors of roughly a million.

All of which 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 implausibles that needs to be considered is the whole reason that people looked for phosphine in the first place, namely that it could be produced by living things.

But there's no shortage of implausibility involved with life on Venus. Nothing we'd recognize as life would possibly survive on a ferociously hot planetary surface that's 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 that perpetually circulates in the upper atmosphere and somehow survives contact with the planet's sulfuric acid clouds.

Unconvinced

So we're left in an awkward place. One of the researchers who led this work said, “It took about 18 months to convince ourselves there was a signal.” You can expect the rest of the field will now spend some time trying to convince itself as well, probably by pointing a whole bunch of additional telescopes at Venus. Meanwhile, chemists are going to be trying to think of additional reaction pathways that could work under Venus-like conditions.

There's a reasonable chance that we'll be reporting back on the results of these efforts before too long, indicating that there's nothing unusual happening on the second planet from the Sun. But if that doesn't end up happening, it will give a big push to the steady chorus of voices that has been arguing that we need to do more to explore Venus. A few plans have floated around involving airships that could spend extended periods moving about Venus' upper atmosphere. Should these results hold up, airships would seem to be the perfect means of figuring out what is producing this chemical.

Nature Astronomy, 2020. DOI: 10.1038/s41550-020-1174-4 (About DOIs).

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