A putrid "swamp gas" known to be one of the most foulest smelling vapours on our planet has been detected in the atmosphere of Venus. Why is this a good thing? Because on Earth, the gas can only be produced artificially in a lab, or by bacteria and microbes that don't require oxygen to thrive, meaning that scientists might just have found traces of life high in the clouds above Venus’ hellish surface.
Venus is often remarked to be Earth’s “twin” planet due to the many similarities the two worlds share, such as size, mass, density, composition and gravity.
Venus is also strewn with mountains, valleys, and tens of thousands of volcanoes. It’s highest mountain, Maxwell Montes, is 8.8 kilometres (20,000 feet) high, similar to the height of Mount Everest.
But that’s where many of the similarities end, as although Venus is not the planet closest to the sun, it is the hottest.
The planet’s dense, toxic atmosphere traps heat, leaving surface temperatures to reach a sweltering 471 degrees Celsius (880 degrees Fahrenheit) - hot enough to melt lead.
It is safe to say then, that astronauts are not likely to be setting foot on Venus’ surface anytime soon then.
However, although conditions at the surface of Venus are hostile to life as we know it, 53–62 kilometres above the surface in the planet’s upper cloud deck, the environment is temperate.
These more pleasant conditions, coupled with unusual stripes dubbed "blue absorbers" or "ultraviolet absorbers” seen high above the planet, has prompted some to suggest that hardy microbes could be making their home in the cloud tops of Venus.
Some are even preparing missions to go and explore the Venusian atmosphere to find out.
One other way of detecting life on another planet without leaving the comforts of Earth, is to hunt for biosignatures.
Biosignatures are chemical species that may indicate the presence of life. Oxygen or O2 is a natural starting point for many searches as not only is it crucial for our survival, on Earth, it is a key byproduct of life and is not produced abiotically (I.e from non-biological processes) in large quantities.
But other things besides life make oxygen too, so scientists look for stranger molecules that might not be made as often and only point to limited sources for its production.
One such molecule is phosphine (PH3) and Clara Sousa-Silva, a research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences, has been studying the formation of this smelly, poisonous molecule and its role as a biosignature for the past 11 or so years.
PH3 is found in trace amounts in Earth’s atmosphere and it has also been found in the high observable layers on both Jupiter and Saturn.
Its presence on these two massive gas giants has been put down to high temperatures and crushing pressures where it is violently dredged up by huge, planet-sized convective storms, says Sousa-Silva and colleagues in research published last year.
Venus might be hot but phosphorus in the form of PH3 is only expected to form at temperatures greater than 527 degrees Celsius (800K). It also takes an enormous amount of energy to smash the molecule together - PH3 is made from one phosphorous and three hydrogen atoms and normally they do not come together naturally – and this type of environment is generally only found in extreme environments like those of Jupiter and Saturn.
So, for temperate, rocky worlds like Earth, “it is implausible that PH3 can be produced without biological intervention,” say the team. If phosphine is detected, that planet must be harbouring life of some kind, concludes the team’s research.
Cue what could be the biggest revelation in astrobiology to date, as a research team headed by Jane Greaves at Cardiff University, UK and who also includes Sousa-Silva, has found traces of the pungent molecule on Venus.
Using the James Clerk Maxwell Telescope and the Atacama Large Millimeter/submillimeter Array in 2017 and 2019, respectively, the international team detected a spectral signature that is unique to phosphine.
The team estimate an abundance of 20 parts-per-billion of phosphine in Venus’s clouds.
“The presence of even a few parts per billion of PH3 is completely unexpected for an oxidised atmosphere (where oxygen-containing compounds greatly dominate over hydrogen-containing ones),” write the authors in the their research paper published today in Nature.
Claiming to have found the signatures of life on another planet is a huge, huge call and one that is not taken lightly. Sousa-Silva and colleagues have spent years ruling out the possibility that phosphine could be produced by anything other than life, and Greaves and colleagues have also been diligent in their search for the true nature of the signal.
To cover all bases and rule out any further obvious sources of PH3, the authors investigated different ways the phosphine may have been produced, including lightening, micrometeorites and sources on the surface of the planet. and chemical processes happening within the clouds.
The team also took into consideration ~75 relevant chemical reactions to account for the production of PH3, but say its formation is "not favoured" even under "thousands of conditions encompassing any likely atmosphere, surface or subsurface properties." There is just not enough energy for the reactions to take place by abiotic means say the team.
Although Greaves and colleagues rule out the production of PH3 by various chemical reactions, other researchers have put forward ways to generate the toxic, fish-smelling molecule that is also flammable and explosive at ambient temperatures.
10 years ago, a team of Chinese scientists reported that their tentative experiments show that phosphine gas could be produced by aqueous or acid corrosion from phosphorus-containing impurities in iron. “The concentration of phosphine liberated by aqueous corrosion was of the same order of magnitude (ng/kg) as those detected in natural environment,” say the authors in their research.
Acid could accelerate corrosion and promote the production of phosphine, write the authors and Venus is described as being hyperacidic; its clouds are literally made up of sulphur dioxide and drops of sulphuric acid - the type Geng and colleagues say can liberate phosphine.
Ultimately though, Greaves and team were unable to determine the source of the trace quantities of phosphine found in their study.
Despite what could be one of the biggest discoveries of all time, the authors do urge that the detection of phosphine is not robust evidence for microbial life “and only indicates potentially unknown geological or chemical processes occurring on Venus.”
Further observations and modelling are needed to explore the origin of phosphine in Venus’s atmosphere, say the team. “Ultimately, a solution could come from revisiting Venus for in situ measurements..,” they add.
ESO astronomer and ALMA European Operations Manager Leonardo Testi, who did not participate in the new study, says: “The non-biological production of phosphine on Venus is excluded by our current understanding of phosphine chemistry in rocky planets' atmospheres. Confirming the existence of life on Venus's atmosphere would be a major breakthrough for astrobiology; thus, it is essential to follow-up on this exciting result with theoretical and observational studies to exclude the possibility that phosphine on rocky planets may also have a chemical origin different than on Earth.”