main article image
False-colour image of Venus in ultraviolet, imaged by Atatsuki. (PLANET-C Project Team)

Fact Check: What's Going on With That Phosphine Detection on Venus?

29 JANUARY 2021

Ever since the discovery of a chemical called phosphine on Venus was announced in September last year, the scientific community has been in a tizzy. Scientists have published papers back and forth, trying to debunk or bolster the claim.

 

With two new papers landing this week, some are claiming the nails are being hammered into the phosphine coffin. We suspect, however, that the detection will continue to be scrutinised and discussed for some time to come.

So what's the actual deal? Read on for a brief primer.

Phosphine on Venus? Why does it matter?

The discovery itself is pretty fascinating. Using two different instruments at different times - the James Clerk Maxwell Telescope (JCMT) in 2017 and the Atacama Large Millimeter/submillimeter Array (ALMA) in 2019 - a team led by astrobiologist Jane Greaves of Cardiff University in the UK detected the spectral signature of a chemical called phosphine in the Venusian atmosphere, at 20 parts per billion. The findings were published in Nature Astronomy.

As we reported at the time, here on Earth, phosphine has been found in abundance in anaerobic (low in oxygen) ecosystems. It's found in swamps and sludges, where anaerobic microbes thrive. It's found in intestines and, well, farts. Somehow, anaerobic microorganisms produce phosphine. And the clouds of Venus are anaerobic.

Although Greaves and her team ruled out many possible abiotic Venusian phosphine formation pathways, they were very careful to note that there could be other ways the chemical could appear. For one, here on Earth volcanoes produce phosphine, and we have evidence that Venus is still volcanically active. (A volcanic origin was later found plausible in another preprint.)

Either way, the detection was a fascinating one, but the mention of a microbial origin drove a lot of speculation, and a lot of follow-up scrutiny from other scientists.

 

What happened next?

Well, it all got a bit complicated. First, a team of scientists had a look at historical Venus data, and found that the Pioneer probe could have detected phosphine all the way back in 1978. That paper has not yet been accepted for publication. Another, submitted to the journal Science and also not yet peer reviewed, claimed to have detected the amino acid glycine - a protein building block - on Venus.

Other scientists started looking at the data. Three separate papers - one since published in Astronomy & Astrophysics on the ALMA data, another published in the Monthly Notices of the Royal Astronomical Society on the JCMT data, and the other reanalysing both datasets and still awaiting peer review - found no significant detection of phosphine in the atmosphere of Venus.

Then it turned out there had been an error processing the data from the ALMA observations. Greaves requested that the data be reprocessed; those reprocessed data were made available to the public in November 2020.

Greaves and her team analysed the new data, and found they could still detect phosphine on Venus, but in lower amounts - a global average of 1 to 4 parts per billion, with localised peaks of 5 to 10 parts per billion.

 

Since sulphur dioxide and phosphine both absorb radiation near the 266.94-gigahertz frequency, some suggested that Greaves and her team may have detected sulphur dioxide (also produced by volcanic activity) and not phosphine. In their new paper, Greaves et al. ruled out sulphur dioxide. The spectral absorption line interpreted as the chemical fingerprint of phosphine, they said, was too wide to be sulphur dioxide, and there wasn't enough of it on Venus to produce the observed signal.

A third paper from Greaves and her team followed, defending the robustness of the phosphine signal.

OK, so why is it back in the news now?

Two new papers have dropped, one of which has been published in The Astrophysical Journal Letters, and the other of which has been accepted for publication in The Astrophysical Journal Letters, reanalysing the data. Both papers contribute to the mounting pile against phosphine.

The first paper reanalysed both sets of ALMA data, before and after they were reprocessed. The team found a spectral line at 266.94 gigahertz in the earlier dataset, but no significant signal after the reprocessing. They also found that sulphur dioxide could appear in at least 10 parts per billion and not be detected by ALMA, suggesting it could be more abundant than Greaves and her team thought.

 

The second paper used data from decades of Venus observations to model the conditions in the Venusian atmosphere, and determine how phosphine and sulphur dioxide would behave. They found that the 266.94-gigahertz signal best fit an origin at about 80 kilometres (50 miles) in altitude, above the cloud decks, rather than 50 to 60 kilometres, as proposed by Greaves and her team.

At this altitude, phosphine would not last long at all, so the best explanation would be sulphur dioxide, they concluded.

Is that the end of it? Is the Venus phosphine detection dead?

Not even close! For starters, Greaves and her team will likely respond to the new papers, which will spark more responses, with more simulations and modelling and number-crunching and maybe even experimentation to determine what the possibilities and likelihoods are.

In addition, nothing we have seen so far is conclusive. It's more than likely that the only way we will put the controversy to rest is by taking more detailed observations with more powerful instruments. We may be waiting a while for that. There are several proposed missions to Venus in the pipeline, but it's often a long time between proposal and execution.

However, this is science at its absolute best. There is a 'true' and a 'false' here. Either there is phosphine on Venus, or there is not. Scientists will use their creativity to try and solve the problem, which will lead to refined techniques and analysis tools.

Eventually, we will learn the truth. And whatever that truth is, it will teach us something new about our Universe.