What happened to the life on Venus headlines?
In September of 2020, a group of UK Astronomers submitted a paper called “Phosphine gas in the cloud decks of Venus,”[1] Which caused quite a buzz in the news, for supposedly finding evidence of life.
Since its first publishing, there has been some discussion about the validity of this finding, and recently, a team of researchers from the University of Washington published a report scrutinizing this original paper.
So, how did they find life on Venus?
By Detection of “Bio-signatures” in the atmosphere. A biosignature is something that can be detected, and we can tell with certainty would not be there without life being present.
For example, in the early earth, there was little oxygen present [2]. The first single-celled organisms that existed on the planet survived without elemental oxygen (O2). For an alien scientist looking at the earth, there’s no way to know if there’s life or not, unless they actually stepped on the planet and collected samples. When the first prokaryotic photosynthesizers evolved, they gave out Oxygen as a waste product. As Oxygen is a direct result of Photosynthesis occurring, it is something that could not have existed here, without life, and can be considered a biosignature. Hence, if this Alien scientist looked at the earth, and saw Oxygen gas, they could conclude with reasonable certainty that the earth contained life.
However, there are other natural ways to produce oxygen [3], which don’t involve photosynthesis, so it’s not a perfect example of a biosignature.
There are many types of Biosignatures. Such as Technological (Technological signs can’t exist without intelligent life), Chemical (Specific chemicals existing only because of life processes), or Atmospheric gases (like oxygen in the example).
The process of determining if something is a biosignature or not is a complex process. We need to consider every single possible alternative for why the particular thing might exist. And biosignatures might vary based on the planet and its features. Along with this, it should also be easy to detect with certainty. There should not be any room for confusion with anything else.
In the words of the authors of the paper,
An ideal biosignature gas would be unambiguous. Living organisms should be its sole source, and it should have intrinsically strong, precisely characterized spectral transitions unblended with contaminant lines—criteria that are not usually all achievable
For a rocky planet like Venus, Phosphine is considered a good biosignature. Phosphine is found in Gas Giants. It’s formed in its hot core, where there’s high pressure. This Phosphine then comes up to the surface layers of the atmosphere. The natural production of Phosphine typically requires high temperature and pressure, something not available on a rocky planet. As such, if phosphine is found in the atmosphere in a rocky planet, it can be concluded with reasonable certainty that it was because of microbial life.
However, as the paper itself points it out, any such Biosignatures we find are not necessarily conclusive evidence of the existence of life, but only confirm anomalous and unexplained chemistry.
We need to find Phosphine gas. How do we detect gases on other planets then?
In an atom or molecule, electrons exist sitting in energy levels. If we give energy to an electron, the electron can jump to a higher energy level. If an electron jumps from a higher energy level to a lower one, it gives off energy.
Since light is a form of energy, these transactions can occur with light as well. While it took a long time to understand why, Gustav Kirchoff had explained this spectral behavior with help of his Laws of Spectroscopy. [4]
- Any solid body (or liquid or gas under high pressure) when heated will emit a continuous spectrum.
- If light passes through a cool gas, it will absorb some frequencies and give an absorption spectrum.
- If a gas is heated under low pressure, it will give a discontinuous emission spectrum.
Hydrogen Spectrum
All atoms and molecules will have their own emission and absorption spectra called their spectral signature. Phosphine’s spectral signature is a wavelength of 1.123053 nm. And here, we’re measuring an absorption line. So the Stronger the Dip, the more the gas is present.
Image Credit: https://youtu.be/mjbKzxh1NpQ
However, there’s a slight issue. Sulfur Dioxide has a wavelength that’s very close (1.123058 nm). And if the detector is not very sensitive, these both readings might come off combined, which was the case here, with the data taken from the James Clark Maxwell Telescope (JCMT)
So, how do we correct this?
We try to detect Sulphur Dioxide from a different wavelength. As you remember, this is a spectrum, and multiple wavelengths are blocked in an absorption spectrum. Sulfur also blocks a wavelength of 1.102564. So with Data from Atacama Large Millimeter Array (ALMA), they decided to do the same. With this, we can know how much Sulphur DiOxide is present, and then correct for how much Phosphine is present.
The Venusian Cloud Decks are at around 56 kilometers above the surface, and they estimate that Sulphur Dioxide has an abundance of 10 ppb. A previous study, again taking data from ALMA, had shown similar concentration levels, so they believed that their readings were right.
Once they did that, they found that the amount of phosphine we find in the Venusian Cloud Decks is quite high (20 ppb), and unexplained with the known ways of Phosphine production. As a result, they concluded that there’s an unknown chemical/biological/geological process producing Phosphine. Since we saw that there are microbes on earth capable of producing Phosphine, biological means were not totally out of the picture.
What did the University of Washington Team find?
In their paper[5], they explain that the dip we see in the JCMT data can be fully explained with just Sulphur DiOxide gases, with no requirement for Phosphine in the explanation. And by Occam’s Razor, the simplest explanation is probably the right one.
The first issue they found was that the concentration of Sulphur Dioxide was very low. It’s the third most abundant gas on Venus. It shouldn’t be this dilute in the atmosphere. The ALMA readings, which were taken for reference, were tuned to a higher altitude (80 Kms), while the main readings were taken at a height of 56 Kms. As we know, the atmosphere becomes thin with height, and as a result, the concentrations of particles will change as well. Based on this, data modeling suggests that the concentration of Sulphur dioxide would be around 6-10 ppm around a height of 60 km, which shows that the experiment had an error by two orders of magnitude.
Now, if we say that the JMTC data was tuned for a higher altitude. Then the results calculated for Sulphur would be better explained. But that also means that the Phosphine was also detected at a higher altitude. But the conditions required to sustain that concentration at that height, given the short lifespan of the molecule, would be implausible.
The second issue is with the telescope itself. ALMA has a special process of collecting data, which leads to Line Dilution, and it appears as if the gas we’re looking for is much more dilute than it actually is. This means that the reference line for SO2 would be undetectable as it was.
With this, they concluded that the data we saw can be explained better with Venusian Mesospheric SO2, and not the presence of Phosphene.
Conclusion:
Well, there’s likely no life on Venus, with what we know so far. But Perseverance just landed, depending on when you’re reading this post. So that’s some good news. We might just yet be able to find some life, outside earth, with conclusive evidence.
Sources:
[1] Phosphine gas in the cloud decks of Venus | Nature Astronomy
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