Earth, as far as we know, is the only planet in the entirety of the known universe that harbors life. But given the sheer size and scale of the universe around us, many believe that it is nye-on inevitable that some form of life has arisen in other places. But where?
Perhaps the most obvious places to look might be one or other of the various planets and moons that exist in our neighborhood, the Solar System. While many might point to the moons of Triton or Ceres, Io, Ganymede, or Titan, there are other less obvious places that might also be worth exploring for signs of life.
Venus, for example. While an inhospitable hell hole to any life that evolved on Earth, life may have "found a way" to rise and exist even there. But, before we get into the nitty-gritty of what are the current theories of lifeforms on the planet, let's see what exactly potential life there would have to deal with.
What is the environment of Venus?
The planet Venus is the second planet from the Sun and, as it turns out, is the Earth's closest neighbor (other than the Moon, of course). One of the so-called "inner four" planets, Venus is a terrestrial, rocky planet that is often called "Earth's Twin" because the two are similar in size and structure.
However, despite this, Venus is radically different from our home planet in many ways. In fact, "toxic twin" may be more accurate.
The first most notable difference is Venus' atmosphere, which is much thicker than Earth's and incredibly toxic to life.
Carbon dioxide levels on Venus are much higher than on Earth, comprising in excess of 95%, and the planet is perpetually covered in a thick, yellow-colored cloud made up primarily of sulfuric acid. This combination of 543 gases and the planet's closer proximity to the Sun has caused what has commonly been termed a "runaway greenhouse effect."
These thick clouds start at an altitude of between 28 and 43 miles (45 to 70km). If you could actually survive the atmosphere without a helmet, it would also reek of rotten eggs.
Because of this greenhouse effect, which traps heat, Venus is the hottest planet in the Solar System, despite Mercury being much closer to the Sun. According to NASA, Venus' surface temperature is somewhere in the region of 900 degrees Fahrenheit (475 degrees Celsius).
That is very hot, hot enough in fact to melt lead. The surface of the planet is also desolate, consisting of volcanic rock blasted by high temperatures and pressures, jagged mountains, and mile upon mile of volcanoes. Many of these are believed to still be active today.
Unlike Earth, the surface of Venus as it exists today is also relatively young. Estimates range from between 1 billion to as young as 150 million years. Geologically speaking this is very young and scientists are currently stumped as to why the surface of Venus is so much younger than the planet itself.
But wait, it doesn't end there. Air pressure on the planet is literally crushing. Compared to Earth, the atmospheric pressure at the surface is around 92 times what we experience on the surface of Earth. This is similar to the pressure found about 3,000 feet (900 m) below the surface of the ocean.
Venus also has other peculiarities relative to Earth, like the fact that it spins in the opposite direction of Earth (and of many other planets in the Solar System for that matter). This means that on Venus the Sun "rises" in the west and sets in the east.
Another difference with Earth is that Venus has relatively longer days and much shorter years. Venus rotates very slowly about its axis with one day lasting, roughly, 243 standard Earth days. However, being much closer to the Sun than Earth, its year's a much shorter, roughly 225 Earth days in fact.
This means that a Venusian day is actually longer than its year! Amazing.
As you can see, Venus is not exactly the most hospitable place to live - at least for life as we know it. However, this hasn't stopped some scientists from thinking up imaginative ways that life could actually evolve and thrive on Venus.
What kind of life could live on Venus?
As we've previously mentioned, Venus isn't exactly the best place for life to exist. But, experts on the subject have theorized potential scenarios in which some form of life could conceivably eke out a living.
These organisms would be weird in the extreme. In fact, they might even make Earth-based extremophiles blush (if they could).
For example, and as we'll expand on later, one of the prime locations on Venus might be in the higher atmosphere of the planet. Here, temperatures are much cooler, and the pressure much lower. For life as we know it, these kinds of conditions would be more conducive to life.
At around 30 miles up (about 50 kilometers), temperatures range from 86 to 158 Fahrenheit (30 to 70 Celsius), which is low enough to accommodate some types of earth-like microbial life, such as “extremophiles”. Atmospheric pressure at this height is also similar to Earth’s surface.
The idea that life may have once existed on Venus gained a boost by the detection of a substance called phosphine in the planet's atmosphere. Phosphine is a colorless, flammable, toxic gas that has the very characteristic smell of rotting fish. For life on Earth, high concentrations of phosphine are highly toxic for animals' respiratory systems but lower concentrations can be found in small amounts in Earth's atmosphere.
Other than being synthesized in a laboratory, phosphine is formed on Earth from decaying organic matter. This means that naturally occurring phosphine on Earth is primarily produced by biological activities.
If this is also the case on Venus, it could be an indication that some form of life may exist there. Scientists have also postulated that Venus could have been more habitable for life in its distant past.
Some have gone as far as to suggest that the planet may have once had liquid water on its surface before its runaway greenhouse effect kicked into high gear, turning all the surface water into vapor, which then leaked slowly into space.
If true, this could mean that life emerged on the planet but either evolved beyond all recognition today or became completely extinct as conditions became too unbearable over time.
In fact, if the mechanism that delivered water to Earth is common among other planets in or near the habitable zone of the Sun, Venus might well have had liquid water for some time. Estimates do vary but range from between 1 and 3 billion years ago, up to between 600 and 750 million years ago.
This window of time would have offered ample time for microbial life to emerge and speciate. The end of this window is, incidentally, the moment in time that the surface of Venus was completely resurfaced through an as yet unknown mechanism.
Though the main culprit was probably mass planet-wide volcanic activity. Though, this is not widely accepted among planetary scientists.
If life was able to get a foothold on Venus in the past, it is quite likely that what exists there today is radically different from its ancestors. Today, any life that does exist is most likely microbial in nature.
However, until we can actually build and land a probe that can survive and explore for an extended period of time the extreme surface conditions of the planet Venus, we can only really guess at this point.
What do people think life on Venus could look like?
So, are you ready to find out what kind of life might actually exist on Venus? Hold on tight, this is going to get a little wild, so to speak.
Let's take a look.
1. Venus might be like Star Wars' Bespin, but for microbes
One of the most recent hypotheses about potential life on Venus is that it may have evolved to live in the clouds of the planet. This theory is, in part, due to the fact that scientists believe the large amounts of ammonia detected on the planet are, in their view, likely of biological origin.
What's more, such lifeforms would likely be something similar to bacteria. The rationale is that ammonia, at least in the quantities detected, should not really exist on Venus given our current understanding of the chemical processes on the planet. Its existence, the researchers believe, can only really be explained by the presence of some biological process.
However, it is also important to note that we can't be entirely sure that ammonia is present, but it appears to be the case. If true, this would be very interesting, as many xenobiologists have previously believed that the planet is too hot for any lifeforms to actually survive today.
The team in question drew together experts in the field from various institutions around the world, including Cardiff University in Wales, UK, MIT, and Cambridge University. They built a special computer model that was devised to determine if chemical reactions within the atmosphere could, potentially, neutralize the high amounts of sulfuric acid if other chemicals like ammonia are present.
The idea is that if this did occur, then the relative acidity of the clouds could drop significantly. While the resulting pH would still be relatively high, it could, in their view, be tolerable for some types of lifeforms.
Co-author of the study Dr. William Bains, from Cardiff University's School of Physics and Astronomy, said, "we know that life can grow in acid environments on Earth, but nothing as acid as the clouds of Venus were believed to be."
"But if something is making ammonia in the clouds, then that will neutralize some of the droplets, making them potentially more habitable," he added.
This would be an interesting development if true and adds another dynamic to the study of the atmosphere of the planet. Scientists have been very interested in Venus' atmosphere since the 1970s, in particular the levels of ammonia and life's potential to survive there given the high temperatures.
If this new model is correct, microbes similar to bacteria could survive higher up in the atmosphere. This could then open up a potential location to explore when looking for life on the planet. If true, the bold claim that the large amounts of ammonia on the planet are biological in origin would overturn decades of belief that it was likely formed by lightning and volcanic eruptions.
Professor Sara Seager, another co-author of the study, from MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS), explained that "ammonia shouldn't be on Venus."
"It has a hydrogen attached to it, and there's very little hydrogen around. Any gas that doesn't belong in the context of its environment is automatically suspicious for being made by life," he added.
Until we can actually capture these microbes and study them in the future, this will remain speculation for many years to come. We may just get that chance with the upcoming "Venus Life Finder Mission."
2. Life on Venus would likely use UV light for fuel
Other than existing primarily, in the clouds, it has been speculated that any lifeforms on Venus would use UV light for energy. This hypothesis originates from the presence of so-called "unknown UV absorbers" in the atmosphere of Venus.
These "unknown UV absorbers" are dark patches of the Venusian atmosphere that are conspicuous by the absence of emitted UV light from the planet. This theory is not new and was first proposed by the late Carl Sagan, in the 1960s.
More recent studies of Venus also support the presence of these UV-less areas on the surface of Venus. In fact, these UV absorbing patches have been shown to be long-term and to also affect the overall albedo of the planet and, subsequently, the planet's weather.
Whether these dark areas are the consequence of micro-organisms absorbing UV as a fuel source or are caused by the byproducts of life in the upper atmosphere, or for some other reason not connected to life, is not yet clear, but something very interesting is going on nevertheless.
Other supporting evidence for the potential for life is the fact that Venus appears to be volcanically active for about the last 2.5 million years. If true, this could provide a mechanism for supplying any life in the high atmosphere with a potential source of nutrients.
Other tantalizing information comes from an even more recent study in 2021 that suggests that the "unknown UV absorber" bands are very similar to something called "red oil". This is a known substance that consists of a mix of organic materials dissolved in concentrated sulfuric acid.
But could there be another explanation for this phenomenon? On Earth, substances like water vapor, ozone, carbon dioxide, and oxygen tend to absorb most of the UV that hits our atmosphere from the Sun. While the atmosphere of Venus is mostly carbon dioxide, the dark bands absorb even more UV than would be expected.
Some have suggested that there may be a more "boring" explanation for these areas though.
"The particles that make up the dark splotches have been suggested to be ferric chloride, allotropes of sulfur, disulfur dioxide, and so on, but none of these, so far, are able to satisfactorily explain their formation and absorption properties," explained Yeon Joo Lee in an interview with phys.org in 2019.
Yet more intriguing is that the size of the particles is very similar to that of light-absorbing micro-organisms here on Earth. The plot, much like Venus' atmosphere, thickens.
3. "It's [perhaps] life Jim, but not as we know it!"
One thing that appears to be clear is that if life could exist on Venus, it won't be like anything here on Earth. Most life on Earth is water-based to a greater or lesser extent, but this won't be possible on Venus.
That is because, as we previously touched upon, liquid water is unlikely to exist on Venus. While there are lifeforms on Earth that can live in some pretty arid conditions, the paucity of water on Venus would be a stretch too far for any kind of water-based life we know about.
Other commonalities to microbes here on Earth, like cellular structure, might be impossible on Venus. This is because the hydrocarbons that make up components like cell membranes are very unstable in ultra-acidic conditions - like those in the clouds of Venus.
This is obviously a very major problem for any potential Earth-like microbes on Venus.
To solve this very serious problem, it has been proposed that Venusian life may consist of self-replicating molecular components of the aforementioned "red oil". These components could conceivably act in a similar fashion as hydrocarbons here on Earth, offering a way for some type of single-cell organisms to develop.
Of course, this is all purely academic at this point, but future missions to Venus could shed some light on how accurate such theories might just be.
4. Venusian life might have a very strange life cycle
If life does exist it may also have a very odd life cycle compared to life as we know it. According to one research study on just this subject in 2020, microbial life on Venus might have a two-stage life cycle.
The team hypothesized that microbes on Venus may spend the "active" part of their life cycle in the clouds of Venus (as previously mentioned) in droplets to avoid a fatal loss of liquid, whatever that liquid may be. Over time, these droplets would likely grow and begin to sink under the influence of gravity within the cloud column.
As they descend, the temperature gradually increases. the droplets will desiccate, shrink in size, and then be carried back up to the habitable zone of the clouds to start the whole process again.
This theory has not been widely accepted, and most critics point out that the lower haze layers of Venus' atmosphere are more stagnant than the more chaotic upper layers. This would make the mechanism for returning these hypothetical desiccated microbe-laden droplets to higher layers of clouds problematic, to say the least.
However, other studies have theorized that this two-stage life cycle might not actually be necessary anyway. One particular study hypothesized that any lifeforms in the clouds of Venus may have evolved to be "maximally dark". This would mean that they are able to absorb as much solar energy as reasonably practicable, allowing them to be kept aloft in the clouds using thermal uplift currents effectively generated by the microbes themselves.
5. If anything survives on the surface, we may not even recognize it as life
So far, we've only considered lifeforms that could live in the clouds of Venus, but what about the surface? Most theories about life on Venus use Earth-life as a yardstick (and for good reason, as it is the only life we know about), but we may be missing a trick.
After all, if we rule out the possibility altogether, we are less likely to even bother looking.
On Earth, more and more extreme lifeforms are found all the time, and in the most unlikely of places. Some are so well adapted to incredibly hostile environments that their discovery often comes as a huge shock.
Take the microbe Geogemma barossii for example. This hardcore microorganism is able to survive in some of the deepest zinc and copper mines in the world in Canada.
The area they call home is roughly 7,900 feet (2,407m) underground, contaminated with large amounts of heavy metals, and with temperatures in excess of 250 degrees Fahrenheit (121 degrees Celsius).
At these temperatures, most lifeforms would boil and their constituent organic molecules would begin to disintegrate - at least this is what has previously been postulated. Organisms found there are able to not only tolerate these conditions but appear to thrive, "breathing" sulfur compounds and "eating" minerals like pyrite (fools gold).
But they are far from the most extreme forms of life on Earth. Other microorganisms have been found in the depths of the Mariana Trench in the Pacific Ocean. These amazing creatures can tolerate the crushing pressure of 6 miles of water above them, and only really appear to be limited by the increasing temperatures as you approach the deepest parts of the Earth's crust.
In most of these extreme cases, organisms have had to adapt as best they can, and one common theme is their relatively slow metabolism. In some instances, it is so slow that it is almost at a standstill.
For places like Venus, whether in the clouds or on the surface, Venusian lifeforms' metabolism may appear almost nonexistent, leading to a false negative if we were to ever actively seek life there.
While extreme heat, rather than pressure, is certainly a limiting factor for life on Earth (as far as we know) it might not be a problem for life that evolved on another world. Though this is highly unlikely given our current knowledge of chemistry and biology.
6. The poles might be prime real estate for Venusian life
The high altitude clouds of Venus might not be the only place where life could potentially exist on Venus. While the average temperature of the surface is incredibly hot, relative to Earth, there are some parts that might be cool enough for life to survive.
Back in 2016, some researchers discovered, much to their surprise, that the poles of the planet are a lot cooler than once thought. Very, very cold in fact.
A study released by the European Space Agency (ESA) appears to indicate that some parts of Venus' poles are so cold, that they put out poles to shame. Trawling through data from their now lost Venus Express probe, there are some tantalizing data points that appear to show at least some part of the poles could, perhaps, allow some form of life to exist.
The probe was the first to visit the planet since the 1989 Magellan Mision, and it made many interesting observations about Venus before it ran out of fuel and plummeted to its demise on the planet's surface.
The probe was used to explore the thick atmosphere of Venus to gather information, and it was discovered that the atmosphere appears to be much thinner at the poles than at the rest of the planet. This is in stark contrast to computer models commonly used to model weather patterns, etc, on the planet.
The data the probe collected revealed that the atmosphere above the poles - never studied in situ before - has an average temperature of 114 K (-159 degrees Celsius), i.e. 70 K (203 degrees Celsius) lower than predicted. The atmospheric density is also very low: at 130 and 140 km, the density is 22% and 40% less than predicted, respectively. At 180 km, the atmosphere is half as dense as we thought.
“Concerning uniformity — models are mostly rather smooth while the reality is much more complex and structured,” ESA scientist and lead author Ingo Müller-Wodarg of Imperial College London said in an interview. “We found enormous variability in the atmospheric densities that [are] explained by a combination of local (horizontal) day-night density variations but above all by strong periodicities, atmospheric waves. These are not captured by models.”
The main reason for the models apparently being so far off the mark is that they tend to rely on atmospheric data from more equatorial regions of Venus. Notably, most models are based on the equatorial data collected by the venerable Pioneer Venus mission.
For example, the Japanese Aerospace Exploration Agency has a probe in orbit around Venus called Akatsuki. This probe, unlike Venus Express, is used to mostly study the climate closer to the planet’s equator in the hope of working out what has caused the planet's apparent runaway greenhouse effect.
In the future, other missions, like NASA's planned VERITAS program will map the entire surface of the planet and could tell us more about the geology of the poles.
Müller-Wodarg added that "there may be some relation between the choppy gravity waves (which are a separate phenomenon detected by the much-heralded LIGO study) and geologic activity on the ground near the poles, but it would require further investigation to determine that."
“We can make observations from the ground (and these are continuously being done) but the real motivation would be to launch a new spacecraft to Venus over the coming decade which could explore the polar atmosphere in-situ,” Müller-Wodarg explained.
But, what has this got to do with potential life on the planet? Well, here on Earth, extremophile organisms can survive in some of the most extreme environments you can think of. We've already covered, in part, microbes that can survive intense heat and pressure, but what about the extreme cold (and pressure)?
As it happens there are. One particular example is a bacteria called Planococcus halocryophilus. Found in one the coldest places in the Arctic, this bacteria, as well as 200 other species, all appear to be able to live quite comfortably in conditions of around -13 degrees Fahrenheit (-25 degrees Celsius). These temperatures are similar to those found on Mars but are practically tropical compared to the estimated temperatures at the poles on Venus.
For reference, on Earth at least, the absolute minimum temperature that single-cell life could conceivably survive is about -4 degrees Fahrenheit (-20 degrees Celsius). Below this, single-celled organisms dehydrate, sending them into a vitrified state during which they are unable to reproduce.
So, could some weird cold-loving lifeforms exist in the frigid poles of Venus? For now, we can only really speculate.
And that, Venusian lovers, is your lot for today.
While we can never really be sure if life exists on Venus, it is fun to hypothesize on what, if anything, it would take for native Venusian life to survive there. With the renewed interest in the planet and upcoming missions there, it may only be a matter of time before can finally put to bed the notion that life might exist even there.
If we do find life, however, it would radically change our understanding of what life is and help guide us to find life in other places in the universe. Who knows, life might be far more common than we could even dream of?