Ammonia may be the Key to Life in the Venusian Clouds
One challenge for life resolved (perhaps), but others remain.
In a new paper published in the Proceedings of the National Academy of Sciences (USA), William Bains from MIT and colleagues suggested that ammonia could be used by microbial life to neutralize the acidity in the Venusian clouds. The presence of ammonia would also explain some of the chemical anomalies observed in the Venusian atmosphere.
The possibility of life in the Venusian clouds is highly speculative but has recently come into the focus of scientific discussions, especially after the claimed detection of the biomarker phosphine in the lower Venusian atmosphere. I summarized the pro and cons for Venusian life in a recent commentary article in the journal Life. The main arguments against Venusian life are the extreme acidity in the clouds, and the very low availability of water and hydrogen. For example, based on the modeled sulfuric acid concentrations at Venus, the acidity in the Venusian clouds is thought to be so low that no organism on Earth could thrive there.
However, Bains found a solution to the problem in the form of ammonia, which would raise the pH-value in the cloud droplets to a value of about 1. Environmental conditions would still be very acidic, but they would be in the range that acidophilic organisms on Earth use as a habitat. Bains and his team went one step further and suggested that the ammonia could be biologically produced to make the cloud droplets a favorable habitat. They provide various examples in which microorganisms on Earth secrete ammonia to neutralize their immediate environment if they live in small droplets. I find this a fascinating suggestion, and it would be in line with how life on Earth transforms an otherwise hostile environment into a suitable habitat. The authors also pointed out that the Venera 8 and Pioneer 13 missions tentatively reported a significant amount of ammonia in the lower Venusian atmosphere. If the detections can be confirmed, it would also resolve chemical anomalies which are difficult to explain otherwise, such as the presence of molecular oxygen in the clouds and the vertical sulfur dioxide profile in the Venusian atmosphere.
If Bains is correct, then one large hurdle for microbial life at Venus is resolved. However, other challenges remain. Foremost, the expected low water and hydrogen availability in the Venusian clouds. The authors addressed these issues by stating that in-cloud water vapor abundances range widely (between 5 ppm and 0.2%), suggesting that there may be much more benign local conditions within otherwise extreme dryness. Perhaps, islands of habitability exist somewhat analogous to microbial hotspots that are present within the hyperarid Atacama desert on Earth? The scarcity of hydrogen may even be more difficult to deal with, because biochemical compounds on Earth incorporate lots of hydrogen atoms. We might even have to consider the idea that Venusian microbes would need to have a substantially different biochemistry than life on Earth to overcome that challenge.
However, I have a difficult time envisioning that life on Earth and putative life at Venus would be very different. They might actually be related, but at the minimum started out similarly in a comparable origin-of-life environment. Perhaps the ancestors of current Venusian cloud life (if it exists) have been transported from Earth to Venus at a time when oceans on both Earth and Venus existed? Given the drastic environmental changes in the Venusian environment, former ocean life on Venus may have chosen very different adaptation strategies than their “distant cousins” on Earth and evolved a few new biochemical tools we don´t see on Earth. I consider this possibility more likely and also the presence of early oceans on Venus, but even the existence of the oceans is controversial. The newly planned Venus missions in the late 2020s will provide at least some of the answers, particularly regarding the chemical environment of the Venusian atmosphere and the natural history of Venus.