In September last year, an article announced a surprising finding: evidence that a highly unstable chemical is present in the atmosphere of Venus. Since the chemical is expected to destroy fairly quickly in the Venusian environment, it appears to be a stable source of the chemical that carried it into the planet’s atmosphere. By examining the components of the atmosphere, the researchers concluded that there is no clear way to manufacture it, which creates a mystery.
Since the chemical, phosphine (PH3), already suggested as a possible sign of living things, immediately began to speculate about the possibility that it was evidence of something living in the clouds of Venus.
But reports like these always invite criticism from the wider scientific community. Now, months later, much of the criticism has come in, and the authors have gone back and revised some of their initial analyzes. In general, the best case scenario is that the levels of the chemical are much smaller than originally reported. But many other researchers say it may not be there.
What is it about?
The original report had two important sections. One was a look at the possible chemical pathways that may be active under the conditions that occur in Venus’ atmosphere. It could not come up with ideas on what other than life can make phosphine. There could still be potential problems here, but so far no one has turned up. Instead, criticism of the original analysis focused on the second part of the September article: the evidence that phosphine is in the atmosphere of Venus. It was obtained by using telescopes to look at a point in the electromagnetic spectrum where phosphine absorbs light, which creates a feature of its presence.
Overall, this evidence seemed pretty strong. It was based on data from two telescopes, so it seems unlikely that hardware will be a complication. The researchers processed the data using two independent software pipelines, suggesting that the math behind the analysis would probably also be solid. The biggest complication is the presence of another chemical, sulfur dioxide, which we find in the atmosphere of Venus. Sulfur dioxide has a spectral signature line near the location of the signal created by phosphine.
But the researchers searched for other spectral signatures of sulfur dioxide, and they saw nothing. They therefore concluded that it was scarce or absent at the height where they were searching for phosphine (just above the clouds of the planet).
The critique, and at least one clear issue, focused on different aspects of this spectral analysis. Most of them are still waiting to receive formal peer review; instead, communication takes place on the arXiv peer review server, which enables researchers to share draft documents before publication. These criticisms fall into a number of categories, and we will go through them individually.
A recognized issue
One of the potential issues has already been addressed by the authors of the original article. One of the telescopes they used for their Venus observations is the Atacama Large Millimeter Array, or ALMA. As the name suggests, ALMA is a range of smaller telescopes that work in collaboration. It is clear that the raw data of these telescopes requires extensive processing and calibration before it can be used for any form of analysis. This is usually handled by the ALMA team itself, which then hands over the data to researchers and is eventually available.
In this case, the calibration had problems, and the data was processed before being placed in a public archive. The researchers therefore went back and redone their analysis using the updated ALMA data. While they say the signal is still there, it is not so prominent. Originally, the researchers suggested that phosphine levels were about 20 parts per billion. With the recalibrated data, it drops to somewhere between one and four parts per billion.
The researchers still indicate that the detection is ‘fairly safe’, but the reduced levels make it easier for other noise sources to flood.
Other pipelines
As mentioned above, the researchers developed two different software pipelines to process the data to search for the spectral signal of phosphine. This makes it less likely that the detection was an artifact hidden in the details of the processing. But ‘less likely’ is not the same as ‘impossible’.
Two manuscripts have been placed that use yet other approaches to process the same data and search for spectral signatures. The first of these finds that the method used by the original paper artificially suppresses background noise and thus increases the apparent meaning of any signals. When the researchers do the analysis to deal with the problem, the phosphine signal is still there, but it falls below the standards for statistical significance because there is more noise.
The second document merely attempts a variety of statistical adjustments to the data and finds that most of them do not produce a significant phosphine signal. It therefore comes to the conclusion that there is no significant signal there.
Where are we looking for?
Part of the original argument for the presence of phosphine is that the researchers’ original analysis indicated that they were looking at the cloud tops of Venus. This is important because the composition of the atmosphere of Venus varies according to altitude and affects potential sources of confusing signals and also affects the ability of phosphine to survive in the chemical environment at any time.
But there appeared at least two manuscripts to the arXiv indicating that the data did not come from the cloud trees, but from an area of the upper atmosphere called the mesosphere. The first manuscript merely examines whether the signal may actually be sulfur dioxide. The conclusion is that sulfur dioxide in the mesosphere can produce a signal that is indistinguishable from that seen in the original report. For a good measure, the concept also performs its own calibration of the ALMA data and sees the phosphine signal drop to below one part per billion.
In the second paper, the authors use a system that models what absorption spectra will look like, given different atmospheric concentrations of sulfur dioxide and phosphine. They also find that sulfur dioxide in the mesosphere produces a signal that is indistinguishable from that which the original research attributed to phosphine. And the conditions in the mesosphere would also suppress the other signals of sulfur dioxide that the first report used to argue that it was not present.
Phosphine in the mesosphere can produce a similar signal, but the researchers calculate that the different environment there means that a typical phosphine molecule has a half-life of one second. In order to produce enough phosphine to supply the mesosphere, it must be made at a higher dose than the production of oxygen by all the photosynthetic organisms on Earth. Since this is just a little unlikely, the authors suggest that we just look at sulfur dioxide.
Suppress
Our final entry into the world of phosphine critiques also takes a while to recalibrate the original ALMA data and find that the signal from phosphine decreases dramatically, so this is in line with some of the work done by other teams.
But this also comes into question as to how ALMA’s range of telescopes was set up during these observations. And the researchers calculated that this configuration would limit the appearance of the sulfur dioxide signatures that the researchers used to deduce that this chemical was not there. If sulfur dioxide was there but not detected, it could easily explain the signal the researchers gave to phosphine.
Is that so then?
None of these aspects eliminate the possibility of phosphine occurring at a certain level, although the level will have to be lower than the original research reported. What they achieve collectively is to indicate that there are several possible explanations for the signal that the authors see, and that all have a chemical in Venus’ atmosphere. It should therefore be seen as the primary explanation for what we have observed so far.
Most of the articles very clearly indicate that, despite this, the potential importance of phosphine there means that it is worthwhile to do follow-up observations with hardware that is specifically configured and calibrated to get our clear data on what is in Venus’ atmosphere occurs.
The other thing that makes it all obvious is that the original researchers may be wrong, but that they really want to find out. One of the new documents specifically thanked them for the part of the software with which they calibrated and processed images, which was essential for the re-analysis. And when the original team members became aware of the problems with the ALMA data, they went back and did their analysis again.
It therefore generally looks like a case of science working as it really should. Even if the end result turns out to be the death of an exciting result, seeing the process properly will help to give more confidence in the results that survive a careful re-analysis.