Microbial life in Venutian clouds?

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Nandkumar M Kamat

On September 14, Jane Greaves, Institute of Astronomy, University of Cambridge, Cambridge, UK and 19 others published a paper in Nature Astronomy with the title ‘Phosphine gas in the cloud decks of Venus’. The team includes a NRI postdoctoral fellow astrophysicist Sukrit Ranjan from Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA.

Since then there is a sense of disbelief among the astronomers because production of this chemical phosphine is difficult on Venus by normally known processes and only anaerobic bacteria can do it. In summary the claim of the team is that their measurements of trace gases in planetary atmospheres help in exploration of chemical conditions
different to those on Earth.

Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. The paper reports “the apparent presence of phosphine gas in Venus’s atmosphere, where any phosphorus should be in oxidised forms. Single-line millimetre-waveband spectral detections from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric phosphine at more than 20 parts per billion abundance is inferred. The team claims that the presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic, or meteoritic delivery.

Explaining how this chemical could be synthesised the team believes that phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life. In conclusion the paper says: “Even if confirmed, we emphasise that the detection of phosphine is not robust evidence for life, only for anomalous and unexplained chemistry. There are substantial conceptual problems for the idea of life in Venus’s clouds—the environment is extremely dehydrating as well as hyperacidic. However, we have ruled out many chemical routes to PH3, with the most likely ones falling short by four to eight orders of magnitude. To further discriminate between unknown photochemical and/or geological processes as the source of Venusian phosphine, or to determine whether there is life in the clouds of Venus, substantial modelling and experimentation will be important. Ultimately, a solution could come from revisiting Venus for in situ measurements or aerosol return”.

So the team has left it to more powerful telescopes and spacecrafts to Venus to conform what is going on in the clouds. Phosphine is shown to be produced by biochemical processes by the microorganisms on Earth, so some astrobiologists believe that similar process may exist in hyperacidic environment in Venutian atmosphere.

Jenkis and others in 2000 had proposed a microbial basis for bioreductive generation of phosphine which could account at least in part for the presence of this toxic gas in natural anaerobic environments and in sewage and landfill gases. They had claimed that monoseptic cultures of certain mixed acid fermenters (Escherichia coli, Salmonella gallinarum, and Salmonella arizonae) and solvent fermentors (Clostridium sporogenes, Clostridium acetobutyricum and Clostridium cochliarium) also generated phosphine.

In 2014, Pasek and colleagues had shown by their work on Florida water samples that no other common reducing agent can produce phosphine at the concentration at which it is observed in nature, as reduction from phosphate is energetically unfeasible unless phosphite or hypophosphite are present in the environment. Having known the processes which produce phosphine on Earth and its significance for understanding the discovery of phosphine in clouds of Venus, astronomer Seth Shostak who works at SETI institute commented: “This discovery was unexpected and a potential game changer. The presence of air-borne phosphine is a little like scat stumbled upon in the desert: a signal that life is in the neighbourhood. But if, indeed, living organisms are floating in the dense air of Venus, it would enormously strengthen the argument that life isn’t a cosmic miracle. If past is prologue, the excitement following the discovery of this malodorous gas will give way to an explanation that doesn’t depend on the presence of life. After all, nature is exceedingly adept at cooking up conditions and compounds that humans haven’t foreseen. If that happens, what is now an exciting discovery will become one that’s merely interesting.”

Nobody on Earth can miss the importance of this discovery because there are a lot of similarities between Earth and Venus. Scientists believe that a runaway greenhouse effect produced the present thick, dense and hyperacidic atmosphere of Venus. But they also know that microbes on Earth have been found in extreme conditions. However the very high temperature on the surface of Venus, a rocky planet rules out any possibility of microbial life at least on the surface so the only possibility is to search for the biosignatures in the atmosphere. The composition and conditions of Venutian atmosphere are conducive to colonisation by hyperextremophilic bacteria. If some of these are producing phosphine then we could visualise vast floating rafts of these aerial microbial colonies or a dense bioaerosol of these cells involved in this unique reaction.

It is just a matter of time when the clouds of Venus get sampled and astronomers get the actual proof of any strange biochemistry happening there. The confidence with which the above cited paper claims the possibility of life in Venutian atmosphere shows that future missions to this bright planet would be very exciting. ISRO (Indian Space Research Organisation) is also planning to launch a spacecraft to explore Venus so India can also join the efforts.