Nightside of Venus captured with the IR2 (infrared) camera on JAXA’s Akatsuki climate orbiter (JAXA).

On September 14 at 3pm GMT, an embargo lifted on a research paper reporting evidence for biological activity on Venus. Speculation about the discovery had been spreading rapidly through social media for several days, proving that scientists are incapable of keeping secrets.

With a surface temperature sufficient to melt lead, Venus is not the usual candidate for extraterrestrial life. However, the reported signature resides not on the surface of the planet, but in its clouds.

Led by Professor Jane Greaves at Cardiff University, the research team report an observation of phosphine; a molecule consisting of one atom of phosphorous and three atoms of hydrogen (PH3). On Earth, the trace amounts of phosphine in the atmosphere all come from either human or microbial activity. But does that make the presence of phosphine irrefutable evidence of life on Venus?

The case for phosphine as a biosignature

Phosphine has been found in the atmospheres of the gas giant planets, Jupiter and Saturn. However, this phosphine forms at the high temperatures and pressures existing deep within the giants’ colossal hydrogen-rich atmospheres. This process is not possible on the terrestrial planets, where the atmospheres are vastly thinner and hydrogen poor.

Instead of hydrogen, Venus’s atmosphere consists predominantly of carbon dioxide with clouds of sulfuric acid. While both ingredients sound abysmal for the prospect of life, the molecules consist of carbon and sulfur bounded to oxygen atoms. The prevalence of oxygen atoms should have resulted in any phosphorous present in the atmosphere to chemically react in a similar fashion to form a phosphate molecule (phosphorous and oxygen), rather than the observed phosphine (phosphorus and hydrogen).

Surface photographs from the former Soviet Union’s Venera 13 spacecraft, which touched down in March 1982. Temperatures on the surface are sufficient to melt lead, while the sulfur in the clouds gives the air its yellow/orange colour (NASA).

Despite considering thousands of possible reactions that might occur within Venus’s atmosphere, Greaves and her team failed to simulate the production of phosphine on Venus through abiotic (non-biological) means. Energetic processes such as lightening, volcanic activity or delivery via meteorites were also ruled out as possible sources, as the quantities they produced should be too low to explain the detection.

Estimates for the lifetime of phosphine also remove the chance that the molecules are leftover from an earlier epoch when the young Venus hosted a more clement environment. While water may once have flowed over a cooler Venus, such conditions evaporated at least a billion years ago. By contrast, phosphine will be broken apart in Venus’s atmosphere in less than 1,000 years. Another process must therefore be renewing the supply today.

The observed quantity of phosphine is tiny, amounting to just 20 ppb (20 parts in every 1000 million parts). This leads to question whether the detection is robust, or due to an instrument or analysis error. To combat this possibility, the team repeated their observations with two different telescopes; the James Clark Maxwell Telescope (JCMT) and the Atacama Large Millimetre/submillimetre Array (ALMA). Both instruments saw evidence of phosphine in the Venusian clouds.

The conclusion is that the phosphine must either be created by unknown chemical or geological processes, or its origin is similar to that on Earth and it is produced by life.

Is life plausible?

Perhaps surprisingly, the notion that life might exist in the clouds of Venus is not a new idea. The concept stretches back to the 1960s in a paper co-authored by Carl Sagan and has been revisited on multiple occasions as recently as the last couple of years.

Cloud cover on Venus is permanent and continuous, with the middle and lower cloud layers have temperatures that are suitable for life (figure 1 in Seager et al. (2020) Astrobiology vol. 21)

These papers all noted that while hellish temperatures and pressures exist on the Venusian surface, conditions above an altitude of about 50 km are far closer in temperature and pressure to that of Earth. Large life-forms would presumably struggle to stay airborne, but smaller micro-organisms might be able to remain suspended within the clouds.

However, cooler temperatures do not resolve all issues for would-be cloud residents. The sulfuric acid and lack of water form an extreme environment and any organism would need to remain afloat to survive. Micro-organisms do exist in the Earth’s atmosphere, but they are convected upwards from the surface which is not an option on Venus where the lower conditions would result in destruction.

There is also the issue of how life began. While some theories support a cooler, wetter, young Venus, other theories suggest the planet was never able to host an ocean. Without water to act as a solvent, even simple life would be unable to get started. Potentially life could travel in from another planet, such as the Earth. However, even if a microbe could survive the journey tucked inside layers of rock, it would be difficult for an Earth-born life form to adapt quickly enough to the strenuous Venusian conditions.

How exciting is this discovery?

There is no doubt that finding life on another planet would be an incredible discovery. It would confirm that the Earth is not unique, and life found on Venus would confirm the versatility of biology adapt to any possible niche.

But this discovery offers more than one important result.

This illustration shows the seven Earth-size planets of TRAPPIST-1, whose atmospheric compositions will be the target of future telescopes looking to identify signs of life (NASA/JPL-Caltech)

Perhaps the most exciting is the potential to test our ability to detect life on an extrasolar planet. These distant worlds are beyond the range of our spacecraft, so their habitability must be judged through identifying molecules observed in their atmospheres. But will our conclusions be accurate? The detection of phosphine in the atmosphere of Venus and the possibility to compare with in-situ data gathered by visiting the planet may be our one chance to test the theory that it is possible to ascertain life exists at a distance.

Another exciting possibility is that the phosphine is not biological in origin at all. While Greaves’ team were meticulous in ruling out alternative ways to produce the molecule, these possibilities were based on conditions that have been tested on Earth.

“The biological interpretation is being suggested because we cannot currently model a geological solution,” explains Stephen Kane, Associate Professor at the University of California Riverside who was not involved in the study. “The chemistry of possible geological and biological signatures is vast and it is an ongoing effort to fully explore that parameter space. That means there are undoubtedly geological explanations that exists that have not yet been realized.”

The environment on Venus is wildly different than that on our planet, and this may lead to chemical reactions or geological processes that have never been previously contemplated. This would point to a diversity in rocky planets that might lead to truly exotic scenarios orbiting other stars.

Venus may have had water oceans in the distant past (NASA).

Finally, the excitment of a potential biological signature on Venus may lend support to future missions to the hot planet. Venus is the only other Earth-sized planet we have the possibility of visiting, and understanding how its evolution differed so dramatically from that of the Earth is a vital piece in the jigsaw to understanding how a habitable environment arises.

“Venus has eveything to do with habitability!” exclaims Stephen Kane, noting that if Venus once had oceans, then it may have developed an ecosystem. If true, the planet is an example of how habitability evolved very differently from that of Earth.

In addition to the Japanese orbiter currently studying the climate of Venus, a number of future mission ideas are being developed by the US, Europe and Russia. With the surface of the planet difficult to reach due to the high temperatures and pressures, one possibility to investigate the phosphine observation could be air-born balloons similar to the Vega program’s balloon operations in the 1980s.

“The only thing that will resolve if there is life or if there is no life,” notes Sara Seagar, Professor at the Massachusetts Institute of Technology and co-author of the research paper led by Greaves. “is going to Venus and making more measurements.”