In September, an international team announced that they had detected phosphine gas (PH3) in the atmosphere of Venus, based on data from the Atacama millimeter-submillimeter array (ALMA) in Chile and the James Clerk Maxwell Telescope (JCMT) in Hawaii . The news met with some skepticism and controversy as phosphine is seen as a possible indicator of life (also known as a biosignature).
Shortly thereafter, a number of articles were published that challenged the observations and conclusions. One team even went so far as to say that there was no phosphine at all in the atmosphere of Venus. Fortunately, after re-analyzing the ALMA data, the team responsible for the original discovery came to the conclusion that there is indeed phosphine in the cloud cover of Venus – just not as much as originally assumed.
In the original study, published in the September 14 issue of Nature Astronomy, the team presented results from ALMA and JCMT that indicated the presence of PH3 around the cloud deck of Venus. On Earth, phosphine is part of the phosphorus biochemical cycle and is likely the result of phosphate reduction in decaying organic material. No chemical and photochemical pathways are known for their formation on Venus.
This artistic impression shows Venus. Astronomers at MIT, Cardiff University, and elsewhere may have observed signs of life in Venus' atmosphere. Credits: ESO / M. Kornmesser & NASA / JPL / Caltech
The only inorganic (also known as abiotic) mechanism for producing phosphine is the high temperatures and pressures that are common in the atmosphere of gas giants. In fact, phosphine has been detected in Jupiter's atmosphere, where it is formed by planet-sized convective storms that generate enormous amounts of energy. The only other explanation was bacteria floating in the cloud deck of Venus.
The analysis and interpretation of the ALMA and JCMT datasets was also questioned in a study led by NASA Goddard researchers and published in an article on natural astronomy, "Matters Arising" (October 26, 2020). Here the research team pointed out that the spectral data interpreted as phosphine (PH3) are actually too close to sulfur dioxide (SO2), which is common in the Venusian atmosphere.
According to another study by the University of Leiden (November 17, 2020, Astronomy & Astrophysics), the spectral data obtained from ALMA could be explained by the presence of compounds other than phosphine gas. From this they concluded that there is “no statistically significant evidence of phosphine” in the Venusian atmosphere and that the previous results are actually “false”.
Jane Greaves, who led the discovery team (and is an astronomer at Cardiff University in the UK) claims they were motivated to re-evaluate their original conclusions because the original ALMA data contained a “false signal” that would have affected their results can. When the corrected ALMA data was released on November 16, Greaves and her colleagues performed a new analysis and published it on arXiv prior to peer review.
Artist's impression of the surface of Venus. Photo credit: Greg Prichard
This is the team's first public response to criticism made following their original findings. Their revised results were also presented at a meeting of the Venus Exploration Analysis Group (VEXAG), a NASA community forum, held on November 17th. While they have since stated that their results are "preliminary," they remain confident that Phophene is present in Venus' atmosphere.
According to Greaves and her colleagues, the ALMA data showed a spectral signature that SO2 can only be explained by the compound phosphene. This is further enhanced by the JCMT spectra, which indicate the chemical fingerprints of phosphine. Based on the new ALMA data, the team estimates that phosphine levels average around 1 ppb – about one-seventh of their previous estimate.
They indicate that these levels are likely to reach a maximum of 5 parts per billion (ppm) and will vary over time and by location. If so, this situation is similar to what scientists observed on Mars, where methane levels rise and wave over the course of a Martian year and vary from place to place. In addition to the criticism, the documents were also inspired by the team's original paper, which was presented on November 17th at VEXAG.
Inspired by the possibility, the biochemist Rakesh Mogul of the California State Polytechnic University in Pomona and his colleagues again checked the data from NASA's Pioneer Venus mission. In 1978 this mission investigates the cloud layer of Venus with a probe that fell into the atmosphere. After re-analyzing the data, Mogul and his colleagues found evidence of phosphorus.
Artist's impression of the Pioneer Venus Orbiter. Photo credit: NASA
This could indicate phosphine or some other phosphorus compound, although Mogul and his team believe phosphine is the most likely candidate. Separately, several scientists at VEXAG argued that a modest 1ppm phosphine level was not due to processes like volcanism or lightning strikes. It was also recently announced that the amino acid glycine had been discovered in Venus' atmosphere, another potential biomarker.
For obvious reasons, it would be very appealing to find evidence of phosphine on Venus. In the past, scientists have speculated that life could exist in the planet's cloud deck, where temperatures are stable enough for extremophiles to survive. If this connection is confirmed in Venus' atmosphere, it would indicate that Venus is able to support extreme life forms in niche habitats.
In any case, these results require further investigation and have led to renewed proposals for missions to Venus, possibly in the form of a balloon or an airship. In the meantime, Greaves and other researchers hope to spend more time with earth-based telescopes (including ALMA) to confirm the presence of phosphine. Whether or not that connection exists there, Venus is still a bunch of puzzles just waiting to be solved!
Further reading: Natur, arXiv