Venus is our closest neighbor (or it comes closest to us in its orbit link). We discovered the Venusian atmosphere in 1761. Early knowledge of an atmosphere, combined persistent cloud cover of the planet lending it a mysterious air, made it a favorite of science fiction authors. Up until the Mariner probe proved the surface of Venus inhospitable to life, stories about life on Venus were almost as popular as those on Mars.
This popularity, of course, changed after our early probes. Venus is a hell planet. Surface temperatures are hot enough to melt lead. Even the clouds that made the planet so appealing to write about are composed of sulfuric acid. While liquid water can exist at high altitudes, even the wettest areas are more arid than the driest deserts on Earth. Yet theories of life persist.
Possible Signs of Life Found in Venusian Atmosphere
On September 14, 2020, Nature Astronomy published an article reporting the discovery of possible signs of life on Venus. Specifically, they found phosphine at a concentration of about 20 ppb in the atmosphere. On Earth, phosphine is made almost solely by life. Anaerobic bacteria, bacteria that don’t require oxygen, such as those that live in swamps or the guts of animals, naturally produce the chemical. Some forms of modern industry also produce the chemical. Barring the existence of meth labs on Venus, anaerobic life remains the most likely known cause of phosphine.
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Of course, this does not mean that there is life on Venus. Astronomers will need to confirm the existence of phosphine, and it’s quantity. We may find the original study to be in error, and no phosphine exists at all. We may discover that, while the chemical does exist, it is in a low enough concentration that abiotic chemical processes can more readily explain it’s existence.
Even if astronomers confirm the existence of phosphine, it does not guarantee the presence of life. As much as it pains me personally to admit this, there are many things we just don’t know. One of those happens to be the exact chemical processes that occur on a different planet. Scientists are already working on other explanations. For instance, there are known chemical processes in gas giants that produce phosphine without requiring life. We just don’t know of any processes that can produce phosphine in these quantities on a rocky planet such as Venus.
Given the hellscape on the planet Venus, life remains an implausibility. But it’s not, as we once thought, impossible. As our good Sherlock Holmes once said, “When you eliminate the impossible, whatever remains, however implausible, must be the truth.” We must now eliminate the impossible.
What Would Life on Venus Look Like?
Finding signs of life on Venus begs the question, just what would life on Venus look like? Is it something we would even recognize as life? Luckily for us, scientists have considered this for many years. Even after we learned the environment on Venus, some scientists never gave up on the possibility of life on our nearest neighbor. Just a month before the discovery of phosphine, another article was published theorizing a hypothetical life cycle in the Venusian clouds.
This life would begin as a desiccated spore. The planet Venus is incredibly dry. It’s so dry that any lifeforms would have the moisture sucked out of their cells outside of the cloud layer. It’s theorized that life would survive dropping below the cloud layer by becoming dried out spores. This sporation is similar to how some life forms on Earth survive extreme environments. We already know of spores that can be dormant for years before coming back to life when introduced to the right conditions. It is inside the realm of possibility that spores on Venus could survive as spores for decades. To be considered life they would need to reproduce. Reproduction wouldn’t only serve the crucial function of replacing spores that died, but reproduction of some form is also necessary in most definitions of life.
Water exists in the cloud layer. According to this theory, spores would be lifted from below into the cloud layer where they can seed clouds. Similar to known cloud seeding on Earth, these spores would have an outer layer that attracts water. As a water droplet forms around the spore, it would come to life. Surviving in the nutrient-poor and highly acidic environment of the Venusian cloud layer would be extremely energy-intensive. This energy is theorized to be derived from the sun. In short, this life would perform photosynthesis similar to plants on Earth. Inside the droplet, life could exist and reproduce.
Finally, the water droplet would become too heavy to remain in the cloud layer. As the life-giving water falls out of the cloud, the life it contains would dry out and become a spore. The heat and pressure would quickly cause the water to dissipate, leaving the spores. The spores would inevitably separate and, hopefully, be lifted back into the cloud layer.
How Much Life Would Be Required to Make This Biosignature?
Finding a biosignature is an important first step in determining the plausibility of life on an alien world. The second step is to determine the likelihood that the biosignature is really created by life. I previously pointed out that we simply don’t know enough to determine if the phosphine discovered on Venus can be formed abiotically. However, a new study from Mansavi Lingam and Abraham Loeb examined how much life would be needed to create this biosignature on Venus.
Their study found that it wouldn’t require that much life in order to create the amounts of phosphine found in the Venusian atmosphere. While the math provides only a rough estimate, Lingam and Loeb show that the required life is orders of magnitude less than what exists in the atmosphere on Earth. This is important as the conditions on Venus, even in the relatively hospitable clouds, is still vastly harsher than the conditions in the most unlivable places on Earth. If much more life were required to form this biosignature, that would make life on Venus much more implausible.
As it stands, life on Venus is still a plausible explanation for the presence of phosphine on Venus.