Venus is considered to be one of the planets most inhospitable to life, because the temperatures on the surface reach a scorching 471° Celsius, well beyond the temperature range that life as we know it can possibly exist. The surface is hot enough to melt lead, and is a scorched and suffocating wasteland where nothing can survive. The atmosphere is thick in carbon dioxide and the clouds blanket the planet in droplets of sulphuric acid, caustic enough to burn holes through human skin.
However, high up in the clouds, the conditions may be just right to offer pockets of habitation for life forms. An international team of researchers have identified a chemical pathway, through which organisms can neutralise the acidic environment in the Venusian atmosphere, and create a self-sustaining environment that is habitable to life.
Co-author of the study, Sara Seager, from MIT says, "No life that we know of could survive in the Venus droplets. But the point is, maybe some life is there, and is modifying its environment so that it is liveable."
The atmosphere of Venus has some puzzling anamolies that are hard to explain. There is a presence of ammonia, which was detected in the 1970s by the Venera 8 and Pioneer probes. There are no known processes that can give rise to the presence of ammonia in the atmosphere. There are also non spherical particles suspended in the atmosphere, that are unlike the droplets of sulphuric acid. There are also small concentrations of oxygen detected. In 2020, researchers detected evidence of phosphine in the upper atmosphere of Venus. The finding was contested and is under review, but phosphine is a gas that is known to be biological in origin, and the phosphorus in the Venusian atmosphere is expected to be oxidised.
Seager says, "The phosphine detection ended up becoming incredibly controversial. But phosphine was like a gateway, and there's been this resurgence in people studying Venus."
The researchers found that ammonia, if present in the atmosphere, can trigger a chemical cascade that neutralises the surrounding sulphuric acid droplets, and explains most of the observed anomalies. The authors suggest a biological origin for the ammonia, as against explanations that propose a non-biological source, such as lightning or volcanic eruptions.
Venus has lingering, unexplained atmospheric anomalies that are incredible. It leaves room for the possibility of life.
Seager explains, "Ammonia shouldn't be on Venus. It has 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. There are very acidic environments on Earth where life does live, but it's nothing like the environment on Venus — unless life is neutralising some of those droplets."
The team notes that there are life forms that manage to survive in acidic environments by producing ammonia on Earth, within human stomachs. On atmosphere, similar reactions can reduce the pH of the clouds from roughly -11 to 0, still resulting in an acidic environment, but one within the tolerable limits of life as we know it.
Life in the Venusian atmosphere could be changing the environment to make it conducive for more life. Co-author of the study, Dr Paul Rimmer identified unexplained chemical signatures from archival data of previous missions to Venus. Many of these anomalies have gone unexplained for decades. In addition to the presence of nonspherical droplets and oxygen, the anomalies also showed a presence of water vapour and sulphur dioxide.
Seager says, "Venus has lingering, unexplained atmospheric anomalies that are incredible. It leaves room for the possibility of life."
One of the possible explanations by Rimmer was mineral dust, swept up from the surface of the planet and into the atmosphere, and interacting with the sulphuric acid droplets, that could produce some of the observed anomalies, but not all. However, the physical requirements for such a scenario is unfeasible, with a massive amount of dust required to be lofted into the atmosphere to induce the observed anomalies.
The presence of ammonia can explain most of the observed anomalies on Venus.
Rimmer explains, "The hypothesis requires either large amounts of water-rich volcanism or transport of a lot of dust rich in hydroxide salts. So far, I have been unable to identify a plausible mineralogy for this mechanism."
The researchers then investigated the possibility of a biological origin of ammonia, and if such an origin could also explain the other observed anomalies in the atmosphere of Venus. The researchers found that if life was producing ammonia in the most efficient way possible, then the associated chemical reactions would yield oxygen.
The ammonia, once introduced to the atmosphere could dissolve within the droplets of sulphuric acid and neutralising them. The introduction of ammonia would also transform the round, spherical droplets into a nonspherical droplets of salty slurry. The resulting reaction would trigger any surrounding sulphuric acid droplets to dissolve as well. The presence of ammonia can explain most of the observed anomalies on Venus.
Co-author of the study, Dr William Bains says, "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 neutralise some of the droplets, making them potentially more habitable. There are many other challenges for life to overcome if it is to live in the clouds of Venus. There is almost no water there for a start, and all life that we know of needs water. But if life is there, then neutralising the acid will make the clouds just a bit more habitable than we thought."
The researchers also showed that other proposed sources for ammonia, including volcanism, lightning strikes and even meteorite impacts cannot produce the amount of ammonia required to explain the observations.
Rimmer explains, "This hypothesis predicts that the tentative detection of oxygen and ammonia in Venus's clouds by probes will be confirmed by future missions, and that both life and ammonium sulphite and sulphate are present in the largest droplets in the lower part of the cloud. There are also several remaining mysteries: if life is there, how does it propagate in an environment as dry as the clouds of Venus? If it is making water when neutralising the droplets, what happens to that water? If life is not in the clouds of Venus, what alternative abiotic chemistry is taking place to explain this depletion of sulphur dioxide and water? Future lab experiments and missions will be able to test these predictions and may shed light on these outstanding mysteries."
The team hopes to gather more data through the privately funded Venus Life Finder missions, a series of probes planned to investigate the permanent cloud deck in the Venusian atmosphere for signs of life.
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