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.

See also...

Reconstructing cloud cover for ancient Earth, with a view to spotting a new one.

Four the past four billion years or so the continents have ambled back and forth across the face of the Earth, sometimes forming up to form vast supercontinents such as Pangea or Rodinia, at other times going on their own ways. For much of the past five hundred million years these continents have been covered by vegetation.

Both of these have had profound effects upon the climate. The position of the continents effects the flow of the oceans, and therefore the atmosphere; for example the Pleistocene ice ages are believed to have started after a land bridge developed between North and South America, preventing the flow of water between the Atlantic and the Pacific, and diverting the Global Ocean Conveyer-belt through the waters surrounding Antarctica. Vegetation effects cloud cover by pumping water into the atmosphere via transpiration, the process by which plants suck up water from the ground via their roots then let it evaporate from their leaves, driving their circulatory systems.

This month Esther Sanromá and Enric Palle of the Instituto de Astrofísica de Canarias published a paper on the online arXiv database at Cornell University Library detailing the results of an attempt to accurately model the cloud cover of the Earth at various points in its history. The theory behind this is fairly simple; a model is created in which the surface of the Earth is divided up into a large number of cells, each of which has a weather patter, which can be influenced by conditions acting on the cell (heat received from the sun, the nature of the ground cover bellow etc.) as well as the weather in neighboring cells. The maps used were based upon those from Ron Blakey of Northern Arizona University's Department of Geology's Global Paleogeography Website.

Quite accurate models of the modern climate can be made in this fashion, but for ancient Earths the task was going to be harder. Accurate models require modeling vegetative ground cover to understand the effect this has on the climate; thus tropical rain forests, high latitude boreal forests and grasslands have to be treated differently. Unfortunately we don't have a good enough understanding of the vegetative cover over the geological timescale to make this possible. Instead Sanromá and Palle were forced to use a far simpler model of ground cover; either vegetated or desert with desert further divided by latitude.

This had clear limitations, but it was thought that if a reasonably accurate model of the modern Earth could be built up in this way, then it would be worth proceeding with the models of ancient climates. The modern Earth has a photometric variability of 3.29% and a mean albedo of 0.315 (that is to say on average 31.5% of the light that falls onto the Earth is reflected back into space, but that this average can vary by up to 3.29%). The computer simulation was able to create a model with cloud cover of 4.16% and an albedo of 0.325; not an exact match for our climate, but reasonably close.

Having thus calibrated the simulation Sanromá and Palle moved on the model the climate of the Late Cretaceous, 90 million years ago. At this time the ancient continent of Pangea had completely broken up, and the continents had yet to start to collide again, so that they remained as separate entities scattered around the globe. In addition the Late Cretaceous had a much warmer climate than today, so that it lacked ice caps. This lead to the creation of vast inland seas on most of the continents, further breaking up the Earth's land cover. Armed with this information Sanromá and Palle came up with a model which gave the Late Cretaceous Earth a photometric variability of 4.27% and an albedo of 0.331 - not greatly different from that of the modern Earth.

The Earth in the Late Cretaceous.

Next Sanromá and Palle built a model of the Earth in the Late Triassic, 230 million years ago. At this time the world's land masses were joined into a single supercontinent, Pangea, that reached almost from pole to pole. In the Late Triassic this supercontinent was starting to break up, the plates in the east had separated giving Pangea a 'C' shape, and forming a knew ocean, the Tethys. The Triassic had a hot, dry, climate with no glaciation at either pole. Sanromá and Palle's model of the Triassic gave a photometric variability of 5.02% and an albedo of 0.327; still comparable to that of today.

The Earth in the Late Triassic.

After the Triassic Sanromá and Palle moved on to the Mississippian (Early Carboniferous), 340 million years ago. During this period the continent of Pangea was coming together, though the continents were still largely separate. The climate was warmer that today, with inland seas on many continents, but there was still glaciation at the South Pole. The period is noted for extensive forests that covered much of the land masses. The model that Sanromá and Palle constructed of the period has a photometric variability of 4.46 and an albedo of 0.329; again not greatly dissimilar to today.

The Earth in the Mississippian.

Finally Sanromá and Palle constructed a model of the Late Cambrian, 500 million years ago. During the Cambrian a global supercontinent, Pannotia, had started to break up, with three island continents, Laurentia, Baltica and Siberia (roughly analogous to North America, Europe and Asia) and a residual Supercontinent, Gondwana, mad up of the remaining continental plates. The Cambrian had a warm climate, but with some glaciation at the poles. Most importantly, the Cambrian was before the evolution of vascular plants. It is thought that algae, fungi and lichens colonized the land some time before vascular plants, though it is unclear how early. For the sake of the model Sanromá and Palle assumed the Earth's land masses to be lacking vegetation of any sort during the Late Cambrian; this may not be completely accurate, but it is clear that at some point the Earth's landmasses did lack vegetation, so this model has some use. This model produced an albedo of 0.351, not greatly different to that of later periods, but a photometric variability of 12.2% which is distinctive.

The Earth during the Late Cambrian.

It is predicted that within the next few years we will have the technology to detect Earth-sized planets orbiting other stars, and therefore potentially to detect other planets with the capability to support life. However an Earth-like planet supporting life is not necessarily a familiar place; the Earth had unicellular life (bacteria, algae etc.) in its oceans for billions of years before the emergence of multicellular forms such as animals and plants, and after these emerged it took time for them to colonize the land. Sanromá and Palle's models suggest that it would be possible for a telescope to tell the difference between an Earth-like planet with plant cover and one without, on the basis of its photometric variability.

See also The Kepler-18 planetary system, Just how big is Iota Draconis b? The End of the Cretaceous and Exoplantes on Sciency Thoughts YouTube.