Sulfur-Metabolizing Aerial Life Cannot Explain Composition of Venus’ Atmosphere, Scientists Say

In new research, a team of scientists at the University of Cambridge has tested the ‘life in the Venusian clouds’ hypothesis.

Jordan et al. demonstrate that the three proposed sulfur-based metabolic pathways for Venusian aerial life are capable of reproducing the observed sulfur dioxide-depletion in the cloud layer of Venus, but in each case they require a source of chemical reducing power roughly equal in abundance to below- cloud sulfur dioxide: the atmosphere otherwise does not possess enough reducing power for life to exploit to generate the observed sulfur dioxide-depletion. Image credit: Mattias Malmer/NASA.

Life in the clouds of Venus, if present in sufficiently high abundance, must be affecting the atmospheric chemistry.

It has been proposed that abundant Venusian life could obtain energy from its environment using three possible sulfur energy-metabolisms.

These metabolisms raise the possibility of Venus’ enigmatic cloud-layer depletion of sulfur dioxide (SO2) being caused by life.

“We’ve spent the past two years trying to explain the weird sulfur chemistry we see in the clouds of Venus,” said Dr. Paul Rimmer, a researcher in the Department of Earth Sciences and Cavendish Laboratory at the University of Cambridge and the MRC Laboratory of Molecular Biology.

“Life is pretty good at weird chemistry, so we’ve been studying whether there’s a way to make life a potential explanation for what we see.”

In the study, Dr. Rimmer and his colleagues used a combination of atmospheric and biochemical models to study the chemical reactions that are expected to occur, given the known sources of chemical energy in Venus’ atmosphere.

“We looked at the sulfur-based ‘food’ available in the Venusian atmosphere — it’s not anything you or I would want to eat, but it is the main available energy source,” explained Dr. Sean Jordan, a researcher at the Institute of Astronomy at the University of Cambridge.

“If that food is being consumed by life, we should see evidence of that through specific chemicals being lost and gained in the atmosphere.”

The team’s models looked at the abundance of sulfur dioxide in the Venusian atmosphere.

On Earth, most sulfur dioxide in the atmosphere comes from volcanic emissions. On Venus, there are high levels of sulfur dioxide lower in the clouds, but it somehow gets ‘sucked out’ of the atmosphere at higher altitudes.

“If life is present, it must be affecting the atmospheric chemistry. Could life be the reason that sulfur dioxide levels on Venus get reduced so much?” said Dr. Oliver Shorttle, a researcher in the Department of Earth Sciences and the Institute of Astronomy at the University of Cambridge.

The models include a list of metabolic reactions that the life forms would carry out in order to get their ‘food,’ and the waste by-products.

The researchers ran the model to see if the reduction in sulfur dioxide levels could be explained by these metabolic reactions.

They found that the metabolic reactions can result in a drop in sulfur dioxide levels, but only by producing other molecules in very large amounts that aren’t seen.

The results set a hard limit on how much life could exist on Venus without blowing apart our understanding of how reactions work in planetary atmospheres.

“If life was responsible for the sulfur dioxide levels we see on Venus, it would also break everything we know about Venus’ atmospheric chemistry,” Dr. Jordan said.

“We wanted life to be a potential explanation, but when we ran the models, it isn’t a viable solution.”

“But if life isn’t responsible for what we see on Venus, it’s still a problem to be solved — there’s lots of strange chemistry to follow up on.”

A paper on the findings was published in the journal Nature Communications.


S. Jordan et al. 2022. Proposed energy-metabolisms cannot explain the atmospheric chemistry of Venus. Nat Communion 13, 3274; doi: 10.1038/s41467-022-30804-8

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