Venus is seen in this photograph taken by the Galileo spacecraft in February 1990. Photo: NASA/JPL via Reuters.
A tantalising clue to the possible presence of microbial life on Venus has sent a frisson of excitement across the scientific world. An international team of researchers has detected traces of phosphine (PH3) – a chemical compound comprising one atom of phosphorus and three of hydrogen – considered to be a chemical signature of life, in the higher reaches of Venus’s atmosphere.
Phosphine is a nauseating gas that is lethal to most life forms on Earth, and is only associated with anaerobic ecosystems where it is produced by bacteria in the absence of free or bound oxygen. This makes it a potential biosignature even in anoxic atmospheres, like that of Venus, where there is no free oxygen, only bound oxygen.
In the mid-1980s, the Soviet Vega probes had spotted a phosphorus-containing chemical in Venus’s atmosphere, but the spacecraft’s instruments were not able to make precise measurements. Subsequent probes from orbiting spacecraft studied the exotic chemistry of Venus’s atmosphere at different heights, helping astronomers build better computer models of it.
The latest discovery of phosphine on Venus was actually made in 2017 using the James Clerk Maxwell Telescope in Hawaii, but the researchers took three years to verify their findings with the Atacama Large Millimeter/submillimeter Array in Chile before publishing it on September 14.
Elsewhere in the Solar System, phosphine is produced in the hydrogen-rich atmospheres of Jupiter and Saturn. Deep down in the atmospheric layers of these gas giants, phosphine is formed at high temperatures and pressures before convection storms push it to higher atmospheric levels. This said, phosphine is short-lived in atmospheres that are rich in oxygen compounds, such as those of Earth and Venus. This makes its discovery in Venus’s atmosphere more surprising.
Venus and Earth are often called twins because of their similar size, mass, density and gravity. Yet the two could not be more different. Although Venus is not the planet closest to the Sun, it is the hottest thanks to its dense atmosphere that traps heat in the same way the greenhouse effect warms Earth. So it is inconceivable that the harsh surface of Venus – where even lead would melt – could harbour life as we know it. The same goes for the Venusian atmosphere: it is the heaviest in the Solar System and is laden with carbon dioxide and sulphuric acid clouds.
What is possible, however, is that the higher atmospheric layers of Venus are cool enough for, say, single-celled organisms to survive inside sulphuric acid droplets. According to the latest findings, the sheer quantity of phosphine detected in the Venusian clouds – 20 parts per billion – can’t be explained by any known ‘non-life’ sources.
This echoes Earth’s story. Until around 2.4 billion years ago, Earth’s atmosphere had very little oxygen but computer models show life thrived in that anoxic world. Phosphates, one of the key building blocks of life, were probably delivered to the infant Earth from outer space by meteorites or comets. Astronomers now know that phosphates are indeed produced in interstellar space by chemical reactions that are started by phosphine.
However, it is too soon to describe the latest discovery as definitive proof of life on the second planet from the Sun – or even suggestive proof. A number of factors could have led to these findings, starting from the most obvious: a false signal from the observing telescopes or errors in the way the data was processed.
Of course, to be fair to the researchers, they do not claim life has been detected. They only say their discovery “strongly suggests the possibility of some form of biology” that generated the phosphine. To pass astrobiological muster, the discovery needs to be verified by several rounds of observation and data-crunching so that every possibility of error is ruled out.
It is not implausible, for example, that an alien chemistry – such as some strange geologic or light-induced chemical reactions – is at work on Venus to produce phosphine in the absence of life, and we simply don’t know about it. After all, natural rocks and minerals on planets are known to release trace amounts of phosphine when they dissolve in mineral acid, when triggered by lightning or electrical discharges.
And Venus’s skies are laced with lightning bolts as sulphuric acid rains down on the planet’s surface. Iron-rich compounds on Venus containing trace amounts of phosphorus could also react with the highly acidic atmosphere to generate an ersatz biosignature.
It is also not far-fetched either to think that an unknown molecule in the Venusian atmosphere, whose chemical signature closely resembles that of phosphine, mimicked it on the electromagnetic spectrum. In this case, data-processing could have mistakenly identified this signal at the same frequency and wavelength as that of phosphine. It is not unusual for Earth’s atmosphere to absorb many of the spectral features of molecules when studied through terrestrial telescopes.
Unambiguous identification of the biosignature depends on analysing several signals of the same molecule on as many frequencies and wavelengths as possible, perhaps using space-telescopes as well. Researchers must ponder all these possibilities if they are to validate what they have found, as with all scientific discoveries.