Scientists have reanalyzed their data and are still seeing a phosphine signal on Venus. Just less

In September, an international team announced that based on data obtained from the Atacama Millimeter-submillimeter Array (ALMA) in Chile and the James Clerk Maxwell Telescope (JCMT) in Hawaii, they had discovered phosphine gas (PH₃) in the atmosphere of Venus. The news was met with its fair share of skepticism and controversy as phosphine is considered a possible indication of life (aka. A biological signature).

Soon after, a series of articles were published that questioned the observations and conclusions, with a team going so far as to say that there was no “phosphine” in Venus’ atmosphere at all. Fortunately, after reanalyzing the ALMA data, the team responsible for the original discovery concluded that there is indeed phosphine in the cloud tops of Venus, but not as much as they initially thought.

In the original study, published in the Sept. 14 issue of Nature Astronomy, the team presented ALMA and JCMT results indicating the presence of PH₃ around the cloud bridge of Venus. On Earth, phosphine is part of the biochemical cycle of phosphorus and is likely the result of the reduction of phosphate in decaying organic matter. On Venus, no chemical and photochemical pathways are known for its creation.

The only non-organic (also known as abiotic) mechanism for phosphine production involves high temperatures and pressures, which are common in the atmospheres of gas giants. In fact, phosphine has been detected in Jupiter’s atmosphere, where it forms as a result of planet-sized convective storms that generate enormous amounts of energy. The only other explanation was that the bacteria floated in the cloud bridge of Venus.

“Spurious”

In one study, led by NASA researchers Goddard and appeared in a Nature Astronomy “Matters Arising” article (October 26, 2020), also questioned the analysis and interpretation of the ALMA and JCMT datasets. Here, the research team indicated that the spectral data interpreted as phosphine (PH₃) was actually too close to sulfur dioxide (SO₂), which is common in the atmosphere of Venus.

According to another study conducted by the University of Leiden (November 17, 2020, Astronomy and astrophysics), the spectral data obtained by ALMA could be explained by the presence of compounds other than phosphine gas. From this, they concluded that there was “no statistically significant detection of phosphine” in the atmosphere of Venus and that the previous results were, in fact, “spurious”.

Jane Greaves, who led the discovery team (and is an astronomer at Cardiff University, UK), says she was motivated to re-examine their original conclusions because the original ALMA data contained a “spurious signal” who could have rejected their results. When the corrected ALMA data was released on November 16, Greaves and his colleagues performed a new analysis and published it prior to the peer review on arXiv.

This is the team’s first public response to the criticisms made in the wake of their original findings. Their revised findings were also presented at a meeting of the Venus Exploration Analysis Group (VEXAG), a NASA community forum, which took place on November 17. Although they have since indicated that their results are “tentative,” they remain confident about the presence of phosphene in Venus’ atmosphere.

So … Less?

According to Greaves and his colleagues, ALMA data demonstrated a spectral signature that cannot be explained by SO₂ anything other than the phosphene compound. This, they claim, is further reinforced by the JCMT spectra that indicated the chemical footprints of the phosphine. Based on the new ALMA data, the team estimates that phosphine levels are on average about 1 ppb, about one-seventh of their previous estimate.

These levels, they indicate, likely peak at 5 parts per billion (ppm) and vary over time and by location. If true, this situation is similar to what scientists have observed on Mars, where methane levels rise and fluctuate over the course of a Martian year and vary from place to place. In addition to the criticisms, the supporting evidence was also inspired by the team’s original document, also presented at VEXAG on November 17.

Inspired by the possibility, biochemist Rakesh Mogul of California State Polytechnic University in Pomona and his colleagues reviewed data from NASA’s Pioneer Venus mission. In 1978, this mission studies the cloud layer of Venus using a probe that has fallen into the atmosphere. Based on their reanalysis of the data, Mogul and his colleagues found evidence of phosphorus.

This could be evidence of phosphine or some other phosphorus compound, although Mogul and his team believe phosphine is the more likely candidate. Regardless, several scientists have argued to VEXAG that even a modest level of 1 ppm of phosphine cannot be attributed to processes such as volcanism or lightning. There was also the recent announcement of the discovery of the amino acid glycine in the atmosphere of Venus, another potential biomarker.

What’s next?

For obvious reasons, finding evidence of phosphine on Venus would be very tempting. In the past, scientists have speculated that life could exist in the planet’s cloud bridge, where temperatures are stable enough for extremophiles to survive. If this compound were confirmed in Venus’ atmosphere, it would indicate that Venus is capable of supporting extreme life forms in niche habitats.

In any case, these findings require further investigation and have led to renewed proposals for missions to Venus, possibly in the form of a balloon or airship. Meanwhile, Greaves and other researchers hope to have more time with ground-based telescopes (including ALMA) to confirm the presence of phosphine. Whether this compound exists or not, Venus is still a bundle of mysteries just waiting to be solved!

Further reading: Nature, arXiv