Our Next Door Neighbours
On the Detection of Phosphine and the Possibility of Life on Venus
The surprising detection of signs of life high in the atmosphere of Venus reignited interest in Earth’s hellish sister. On 14 September, a team of researchers led by Jane Greaves announced that they had detected phosphine, a gas flagged by astrobiologists as a potential marker for life, 55 km above the surface of Venus.
The Ingredients for Life
Before we even consider the existence of life, we need to determine if it is even possible for it to even survive on Venus. In general, for planets to be a potential host for life as we know it, it must have 3 vital ingredients.
One of the guiding policies in the search for alien life is “follow the water”. Why? Well, water does it all. Not only does it support cell structure, but it also acts as a universal solvent. This allows it to transport a variety of important chemicals in cells and remove waste. Water’s many unique qualities make it vital for life as we know it.
A certain 6 elements make up all of life on earth: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (CHNOPS). These elements exist in great abundance in the universe and account for 97% of our bodies. Especially important is carbon, the building block of life as we know it. Carbon bonds easily with a variety of atoms, allowing it to form tons of different compounds. In fact, carbon forms more compounds than all the other elements put together.
All life uses energy to drive chemical reactions and fuel processes essential for their survival. This includes the break-up, transport and assembling of molecules. On Earth, all energy comes directly or indirectly from the sun. Plants use energy from the sun for photosynthesis, converting water and carbon dioxide to glucose to use for energy, and releasing oxygen as a byproduct.
Venus, the Goddess of… Love?
Being outside the habitable zone of our sun, the Earth-sized planet is often described as hellish, and with good reason. However, research suggests that Venus was once home to an ocean of liquid water on its surface, and was very similar to Earth for 2 billion years of its history. How could these 2 planets go on such different paths? The answer lies in Venus’s atmosphere.
Its toxic atmosphere now consists mostly of carbon dioxide, with clouds of mostly sulfuric acid, trapping heat and causing a runaway greenhouse effect. This would have evaporated the water, leaving only a very tiny amount of water vapour in the atmosphere. Now, its surface temperature reaches 462°C (863 °F), hot enough to melt lead. Its atmospheric pressure approaches a crushing 90 times that of Earth. All this makes it impossible for liquid water to exist on its surface. Thus, astronomers have long looked away from Venus as a possible host for life and turned their heads to Mars.
Armed with the JCMT and ALMA radio telescopes, the team detected the absorption at a specific wavelength which can only be explained by the presence of phosphine. The data suggests a phosphine concentration of 20 parts per billion in the atmosphere of Venus, 1,000 to a million times larger than found in Earth’s atmosphere.
Phosphine is a toxic, flammable gas which is only produced by industrial means or through the anaerobic (oxygen-free) decay of phosphorus-containing organic material on earth. It is commonly associated with rotting microbial activity, feces, and yes… farts.
It is a relatively simple molecule, consisting of 1 phosphorus atom and 3 hydrogen atoms, however, it takes a lot of energy to make. Phosphorus would much more readily bind with oxygen.
The atmospheres of the gas giants Jupiter and Saturn contain phosphine because of their hot interior, which is energetic enough to produce the molecule, and because their atmospheres consist mostly of hydrogen and helium. Compared to the interiors of Jupiter and Saturn, the Venusian surface is relatively cold, and with the amount of oxygen-containing molecules (CO2) in its atmosphere, the detection of that amount of phosphine was shocking and suggests that the gas is being produced on the planet itself.
Scientists have looked into potential reactions or processes in which phosphine can be formed in the atmosphere of Venus, including volcanic activity, lightning, gas reactions, photochemistry and many others. Guess what they said.
“None of these potential phosphine production pathways are sufficient to explain the presence of ppb phosphine levels on Venus” — Bains et al.
Yes, NONE of these methods produces enough phosphine to explain the amount found in the Venusian atmosphere. In other words, we have no idea where the phosphine is coming from.
Assuming all the observations and calculations are correct, there are currently only 2 possibilities: either there is a chemical or geological process producing the phosphine that we simply don’t know about, or there is alien life in the atmosphere of Venus.
Life in the Clouds
Although astronomers had recently focused on Mars and moons in the outer solar system, Venus has not entirely been neglected in the search for life. The idea of life in the clouds of Venus has floated around (pun intended) for quite some time. In 1967, astrobiologist Harold Morowitz and famous astronomer Carl Sagan speculated that there may be life in the clouds of Venus.
There is in fact a layer of atmosphere, 50–60 km above the scorching Venusian surface, where the temperature and pressure are similar to that of Earth. There is chemistry. There is energy. There is water (albeit water vapour). One may even call this environment comfortable for life, if not for the sulfuric acid clouds. Nevertheless, there are organisms which are able to not only survive, but thrive in such acidic environments.
Microbes are known to be able to float and survive in Earth’s atmosphere, and though it is a difficult place to survive, it is not impossible. With the abundance of water, sunlight, and carbon dioxide — the prerequisites for photosynthesis– in the clouds of Venus, there is definitely a possibility of life within this cloud layer.
Before getting too far ahead of ourselves, we need to first confirm that there actually is phosphine in the clouds of Venus. On Earth, astronomers are now following up on the detection to see if they can replicate the results observed by Greaves and her team, and to use different parts of the spectrum to see if they can get other absorption lines associated with phosphine.
Away from Earth, 3 different missions are scheduled to fly near Venus in the next few months. However, these missions were not designed to look for phosphine or life on Venus; one is on their way to Mercury, while the other 2 are heading for the Sun. It is unclear whether the instruments have the right sensitivity to detect phosphine in the atmosphere of Venus.
More promising are the future missions that can be designated to search for phosphine and life on Venus. India’s ISRO already has a Venus orbiter planned to launch in 2025, with the possibility of adjusting its instruments to try to detect phosphine in Venus. The US and Europe are also considering missions to Venus to confirm the presence of phosphine and learn more about the planet.
Now let's say there is phosphine in the Venusian atmosphere. How would we know if there is life? Well, the only definitive answer would be a sample return, gathering potentially organic material in Venus’s habitable cloud layer, and bringing it back to Earth. However, that is extremely far in the future, so sit tight!
The potential of life on Venus raises many intriguing questions. How did life get there? Is this life relatively new, emerging within the past few thousand years? Or is it ancient life, evolved from the once possibly habitable Venus from billions of years ago? If so, is life on Earth somehow related to Venusian life? Or, is it a completely different form of life, unknown to humans? Is there more life in our solar system? Our galaxy?
Overwhelmed? Me too.
Nevertheless, the existence of life on Venus would be monumental, marking the first form of alien life ever discovered. It will demonstrate the ability for life to adapt and evolve, and how life can exist where no one expects it to be. It could teach us about ourselves, our planet, and how life came to be on Earth. Or, it could not. If there is life literally right next to us, who knows how much life there is in our solar system, our galaxy, or our universe. All this is still purely speculative; before we learn more about what lies within those 10 kilometres of the Venusian atmosphere, we can only guess.
At the end of the day, there are 3 scenarios:
“Worst” case scenario? There is no phosphine. Life goes on.
“Meh” scenario? We find a new way to make phosphine, and learn more about chemistry on Venus. Life goes on.
“Best” case scenario? We discover life on Venus. We study it.
A great episode of StarTalk, answering many questions relating to life on Venus.
The original paper announcing the detection of phosphine on Venus: https://www.nature.com/articles/s41550-020-1174-4
An overview of what life could look like on Venus: https://www.liebertpub.com/doi/10.1089/ast.2020.2244