Life on Venus: Scientists hunt for signals

Introduction

Thousands of volatile molecular species are utilized by living organisms that potentially contribute to the biosphere and its related atmospheric spectrum. Astrobiologists are very interested in the possibility of finding the biomarker phosphine in Venus's atmosphere. Many scientists are convinced that the Venusian atmosphere is too hostile for life, but others argue that life could have adapted to the harsh conditions in the Venusian clouds after the surface became uninhabitable, thanks to natural selection; This is all based on the purported detection of a convincing biomarker and the speculation that early Venus was doubtlessly habitable.

Some of the volatiles would collect in a planet's atmosphere, becoming observable at a distance; these are referred to as biosignature gases. Scientists are conducting experiments to identify and detect biosignature gases. In the case of Venus, the biosignature gas is phosphine (PH3). When considering Earth, PH3 is associated with anaerobic habitats; it could be a biosignature gas on anoxic exoplanets. Based on astronomical observations, we know that phosphine is a spectroscopically active gas present in the atmospheres of stars (namely carbon stars) and the gas giants Jupiter and Saturn. Phosphorus is thought to be present only in PH3 in the deep atmosphere layers of T dwarfs and giant planets, where temperatures are high enough to promote the creation of PH3 thermodynamically. Phosphine is abundant in Jupiter and Saturn; an overabundance of phosphine results from the difference in convective timescale and chemical equilibrium timescales. No phosphine detection has been observed in the case of ice giants like Uranus and Neptune.

The presence of phosphine in Venus's atmosphere (about 55km above the surface) was confirmed using the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter/submillimeter Array in Chile. The millimeter wavelength light absorption seen in the radio data is consistent with an atmospheric phosphine concentration of 20 parts per billion. The atmosphere of Venus is harsh and highly acidic; therefore, the gas should be broken down, so there should be a mechanism for the replenishment of the phosphine gas; this replenishment might be because of a biological process or some unknown chemical process still unknown. Researchers have hypothesized that some airborne bacteria may be able to thrive in the phosphine-rich region of the atmosphere, far from the crushing pressures and burning temperatures of the planet's surface.

The Venusian clouds and their environmental conditions

In the early history of the Solar System, Venus was in its habitable zone, and oceans presumably existed on its surface, much as they did on Earth and Mars. If life did not arise independently on Venus in conditions somewhat comparable to Earth at the time, it could have been brought from either Earth or Mars. It is unknown how long the Venusian surface remained habitable while the Sun's light intensified, but more recent research suggests that Venusian waters may have survived as late as 715 million years ago. It has been hypothesized that microorganisms could have adapted to the harsh environmental conditions in the Venusian atmosphere through natural selection as the planet's last habitat.

The lower cloud deck was hypothesized to be the potential microbial habitat (temperature between 40-90 oC, pressure 1 bar) because its environmental conditions are consistent with the habitat for thermophilic microorganisms on Earth. In comparison, the cloud deck of Venus are more extensive, providing a more continuous and stable environment containing aerosols that last for several months. Chemical compounds are heterogeneously distributed in the Venusian atmosphere, and there is thermodynamic disequilibrium because oxygenated species and reducing species coexist. Methane has also been detected. Even though these are favorable conditions, there are some unfavorable parameters, like water activity (water availability from a microbial perspective). It can range from 0 to 1, where 1 is pure water. Determining the water activity for Venus is challenging because there is no direct way to measure it. There is the presence of sulphuric acids (concentration between 75%-90%) which can lower the water activity to as low as 0.1 (unfavorable for any domain of life).

However, estimated water activities in the Venusian atmosphere are so small that it is difficult to see any Earth life coming close to dominating them. Different biochemistry may be required to overcome the limitation imposed by water activity. It was noted that the amount of sulfuric acid in the Venusian aerosols, which were thought to house life, is based on indirect measurements and computer simulations and that recent findings appear to show the presence of chemical components other than sulfuric acid. In any case, sulfuric acid concentrations may be lower, and water activities may be higher than expected at the relevant scale and location. Scientists have devised some biological solutions to the challenges associated with the putative Venusian environment. Schulze- Makuch hypothesized that microbial cells might be protected from the sulfuric acid environment by elemental sulphur. The Venera 13, 14 and Vega 1 and 2 descent probes have measured the amount of the elements using X-ray fluorescence measurements, and it was found that there is the presence of phosphorous, chlorine and iron. Therefore, the fundamental requirements for life (C, N, P) have been met in the case of the lower clouds of Venus, and therefore these can be attractive sites for biological entities even though the availability of hydrogen might be a problem.

Microbial Life in the Venusian Cloud: Proposed Adaptations

For putative life on Venus, UV irradiation would be an asset since the Venusian cloud layers are enormous and contain elemental sulphur (particularly cycloocta sulphur S8), which has the unique property of adsorbing UV irradiation and then re-radiating it in the visible spectrum of light. According to spectral analyses, the aerosols (called mode 3 particles) in the lower cloud deck of Venus are coated with elemental sulphur. It was proposed that these aerosols could potentially be microorganisms that utilize elemental sulphur for anaerobic photosynthetic reactions. Further, it was proposed that the coating on the cells would contain hydrophilic filaments and elemental sulphur for the uptake of critical liquids.

A life cycle was suggested to address the problem of falling of the microbial cells through the clouds towards the surface, leading to their deaths and permanent removal from the environment. It was conceptualized in the model that the microbial cells would be dried during settling to form desiccated spores that would be returned to the cloud layer because of gravity waves. When these are back in the cloud layer, they would be rehydrated because of condensation, and the cycle would be complete.

Detection of Phosphine: A biosignature gas?

Phosphine was detected in the Venusian atmosphere at a concentration of 20 parts per billion (measured using spectral analysis and with the help of two telescopes in 2017 and 2019). There have been concerns that phosphine detection could be misidentified sulphur dioxide. In principle, there is enough phosphorous in the Venusian atmosphere for the presence of phosphine. Phosphine is a colorless gas, toxic to aerobic life and associated with anaerobic life on Earth. Phosphate can be reduced to phosphite by phosphate-reducing bacteria when coupled with NADH oxidation, and phosphine can be produced because of the combined action of phosphite disproportionating bacteria and phosphate-reducing bacteria. Scientists have examined potential photochemical pathways, lightning as a potential source, underground sources such as volcanic outgassing, and asteroid or cometary impacts, but none have adequately explained the quantity of phosphine observed. After completing their analysis, they determined that the only remaining possibilities were either (A) an unknown geochemical or photochemical mechanism or (B) life in the clouds of Venus. Given Venus's rich chemical endowment and the likelihood of several heterogeneous reactions, it appears reasonable to conclude that potential abiotic geochemical or photochemical routes are likely to exist. Catling proposed, for instance, that PH3 may be produced by routes involving phosphorus trioxide (P4O6) and phosphoric acid (H3PO3). The reaction involved is:

P4O6 + 6H2O → PH3 + 3H3PO4

We do not know much about the chemistry in Venus's lower atmosphere, as it mostly remains an alien planet. The majority of missions to Venus occurred during the Soviet era (1960–1980). Therefore, there is a great deal of doubt about the composition and abundance of atmospheric gases. When considering option B, we have to consider the extremophiles because of the challenging conditions of the Venusian atmosphere. Hyperacidity is stronger than present in any natural environment on Earth; therefore, the survival of any organism is difficult. Some adaptation has to be there for the organisms to survive.

Conclusion

The purported presence of phosphine in Venus's oxidizing atmosphere is surprising, especially considering that the gas has never been found on any other terrestrial planet than Earth. However, this does not demonstrate the existence of life. Numerous mysteries surround our "twin planet," which remains largely foreign to humans. Numerous processes and chemical reactions potentially occurring in the atmosphere and on the surface of Venus remain poorly understood. There are several mysteries and unanswered questions, regardless of whether we consider an unknown chemical pathway or biology as a potential explanation for the observed phenomena.

References:

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2. Sousa-Silva C, Seager S, Ranjan S, Petkowski JJ, Zhan Z, Hu R, Bains W. Phosphine as a Biosignature Gas in Exoplanet Atmospheres. Astrobiology. 2020 Feb;20(2):235-268. doi: 10.1089/ast.2018.1954. Epub 2019 Nov 22. PMID: 31755740.

3. O'Callaghan J. Life on Venus? Scientists hunt for the truth. Nature. 2020 Oct;586(7828):182-183. doi: 10.1038/d41586-020-02785-5. PMID: 33020619.