Wednesday 10 April 2024

Assessing the habitability of the clouds of Venus.

The surface of Venus has long been considered to be unsuitable for life. The temperature here averages about 427°C, which is not only far to hot for liquid water to exist, but which would also cause the near instantaneous inactivation of any enzymes, the denaturing of any proteins, and the thermal decomposition of almost all biological organic molecules. This has not, however, prevented all speculation about the possibility of life existing on Venus, with some scientists suggesting that it might be possible for life to exist within the planet's cloud layer. One proposed theoretical dweller in the clouds of Venus is the spherical hydrogen gasbag isopyenic organism, which would keep its position within the cloud layer by means of a bag-like body containing the lighter-than-air gas hydrogen, although calculations suggest that such an organism would require a minimum diameter of 4 cm to stay aloft on Venus, which seems improbable.

The detection of phosphene gas in the clouds of Venus, which on Earth would  be considered a clear indicator of microbial activity, has led to renewed consideration of the possibility of life in the Venusian clouds, possibly in the form of microbes within fluid droplets, a habitat which would protect them from dehydration, otherwise a problem for small organisms floating in an atmosphere. However, living within floating droplets would present micro-organisms with another problem, due to the limited lifespan of droplets large enough to sustain such organisms, which would tend to settle out of the cloud layer and evaporate. 

In a paper published on the arXiv database at Cornell University on 8 April 2024, Jennifer Abreu of the Department of Physics and Astronomy at Lehman College at City University of New York, Alyxander Anchordoqui of the John F. Kennedy School in Somerville, Massachusetts, Nyamekye Fosu, Michael Kwakye, Danijela Kyriakakis, and Krystal Reynoso, also of the Department of Physics and Astronomy at Lehman College at City University of New York, and Luis Anchordoqui, again of the Department of Physics and Astronomy at Lehman College, and of the Department of Physics at the Graduate Center at City University of New York, and of the Department of Astrophysics at the American Museum of Natural History, present a potential scenario for the survival of microbes within the Venusian cloud layer. 

The cloud layer completely encircles Venus, giving it a planetary albedo of about 0.8 (i.e. causing it to reflect about 80% of the light it receives back into space). The base of the cloud layer is about 47 km above the surface, at which altitude the atmosphere has a temperature of about 100°C. The top of the cloud layer reaches about 74 km above the surface at the equator, decreasing to about 65 km at the poles. This cloud layer can further be divided into three zones, an upper layer, above 56.5 km, a middle layer between 50.5 km and 56. 5 km, and a lower layer less than 50.5 km above the ground. Droplets within the cloud layers can be divided into three types, based upon their size. Type 1 droplets have a diameter of about 0.2 µm, type 2 droplets have a diameter of 1-2 mm, and type 3 droplets have a diameter of about 4 mm. Types 1 and 2 are found in all three cloud layers, whereas type 3 droplets are only found in the middle and lower layers.

Within the cloud layer, the availability of carbon dioxide, sulphuric acid compounds, and ultraviolet light would provide any microbes with sources of food and energy. Optimum conditions for survival would probably be found about 50 km above the surface, where the temperature fluctuates between about 60°C and about 100°C, and the pressure is about one atmosphere.

On Earth, long lived aerosols, acidic clouds, and atmospheric temperatures consistently above the boiling point of water are not present. However, life, in the form of Bacteria, Pollen, and Algae, has been found in the atmosphere as far as 15 km above sealevel, and Bacteria have been found growing in droplets of moisture gathered from super-cooled clouds above the Alps. It has been proposed that microbes have been able to reach such heights via evaporation, storms, eruptions, or meteor impacts, all processes likely to have been found on an early Venus, where the surface is likely to have been more conducive to life. Unlike their Earthly counterparts, the clouds of Venus are not transient, but are a constant, global phenomenon, where individual aerosol particles can be sustained for a long period of time, rather than for a maximum of a few days, potentially providing a stable home for Venusian life.

Abreu et al. suggest that life may be present in the lower haze layers of the Venusian atmosphere as desiccated spores, ready to become activated when updrafts carry them into the habitable layer within the clouds. After reaching the habitable middle and lower cloud layers, the spores would act as condensation nuclei around which water droplets would form, allowing them to germinate and become metabolically active. Metabolically active microbes could grow by division (as with Prokaryotes on Earth) within the droplets, which would themselves grow further by coagulation. Eventually the droplets would grow so large that they would gravitationally settle out of the cloud layer into the hotter layers beneath, causing their liquid content to evaporate, and the micro-organisms to desiccate and re-enter the spore state. The desiccated spores would then be small and light enough to resist further downwards movement, remaining in the haze layer until a new updraft carries them back into the clouds and the process begins again.

Hypothetical life cycle of the Venusian microorganisms. Top panel: Cloud cover on Venus is permanent and continuous, with the middle and lower cloud layers at temperatures that are suitable for life. Bottom panel: Proposed life cycle for Venusian aerial microbial life. Abreu et al. (2024).

All living organisms have to be able to replicate, and in Bacteria, used as the model for Abreu et al.'s proposed Venusian microorganisms, this is achieved by binary fission; i.e. the division of one Bacterium into two. Thus, a population of Bacteria can double in size in a single generation, and the time which it takes for a population of Bacteria to double can be taken as a generation time. For many common Bacteria the generation time is less than an hour, for example in Escherichia coli it is typically about 20 minutes under ideal conditions (which assume an aerobic, nutrient-rich environment at an optimum temperature), which the marine Bacterium Vibrio natriegens can have a generation time as low as 7-10 minutes. 

If a Bacterium has a generation time of 20 minutes, then it would go through three generations in an hour (so that one Bacterium will become sixteen) and 36 in 12 hours (so that one Bacterium will become about 70 000 000 000, assuming a suitable source of nutrients).

Abreu et al. further calculate that a droplet with a diameter of about 0.1 µm would be effectively neutrally buoyant in Venus's cloud layer, and that a droplet would need to reach about 100 µm in diameter before it began to fall consistently at a velocity of 1 m per second, within a cloud layer that can be up to 27 km thick, enabling plenty of time for reproductive fission.

Unlike Earth, Venus lacks an intrinsic magnetic field which prevents cosmic radiation from reaching the atmosphere. This means that molecules in the upper atmosphere are ionized by ultraviolet radiation, forming an ionosphere, with interactions between this ionosphere and the solar winds producing a magnetic field within the outer atmosphere, capable of slowing down particles with energies of up to a few hundred kiloelectron volts and diverted them around the planet. More energetic particles are able to penetrate this weak magnetic field. Particles with energies of greater than one giga electron volt will provoke cascade reactions in the atmosphere, with nuclei scattered by interaction with the initial particle interacting to cause secondary, tertiary, and subsequent generations of particles, losing energy with each reaction. 

The terrestrial biosphere on Earth benefits from about 1033 g/cm² of protection against the harmful effects of cosmic rays, compared to about 200 g/cm² of shielding available to any Venusian organisms at the top of the cloud layer. However, this protection will have risen to about 1000 g/cm² in the middle cloud layer, comparable to conditions on Earth. Furthermore, cosmic rays are generally considered to be less harmful to Prokaryotic organisms than to multicellular organisms (an interaction with a charged particle may well kill an individual Bacterial cell - but this has no effect on any other cell, and the high rates of reproduction mean that lost Bacteria are replaced quickly, whereas a multicellular organism with many cells needs most of them to survive until the whole organism can reproduce).

Based upon these calculations, Abreu et al. calculate that micro-organisms with similar properties to those found on Earth could potentially exist within the clouds of Venus, reproducing exponentially within droplets in the clouds during the lifetime of those droplets.

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