Venus is often described as a hellscape. The surface temperature exceeds the melting point of lead, and although its atmosphere is dominated by carbon dioxide, it contains enough sulfuric acid to meet the comparison with Hades.
But conditions in Venus' vast atmosphere are not uniform. There are places where some of life's building blocks could withstand the planet's inhospitable nature.
Among the rocky planets, Venus has by far the largest atmosphere in terms of volume. While its surface is inhospitable, its atmosphere features regions that are most Earth-like compared to any other part of the solar system. Scientists have wondered whether life could survive in parts of the planet's upper atmosphere, and the discovery of the potential biomarker phosphine (although this was later disproved) sparked further interest.
Some research suggests that life may exist in Venus' voluminous clouds. Photo credit: Abreu et al. 2024.
One reason Venus keeps coming up in discussions about habitability is that it is accessible, whereas exoplanets are not. Venus is easy to reach and we currently have an orbiter in place, the Japanese Akatsuki spacecraft. Three more missions to Venus are planned for the mid-2030s: NASA's Veritas and DAVINCI and ESA's EnVision.
Nobody is convinced that we will find life on Venus. But the planet can teach us a lot about chemistry and biology and their limitations.
In new research, a team of scientists tested various building blocks in Venus-like conditions to see if they could withstand the planet's dangerous nature. The research is: “Simple lipids form stable higher-order structures in concentrated sulfuric acid.” The lead author is Daniel Duzdevich from the Department of Chemistry at the University of Chicago. The paper is currently in pre-print and has been submitted to the journal Astrobiology.
The surface of Venus is not a candidate for habitability. But there may be regions in its atmosphere. The problem is that much of Venus's sulfuric acid is concentrated in individual clouds rather than distributed throughout the atmosphere.
“The surface of Venus is sterilizing, but the cloud cover includes regions with temperatures and pressures conventionally considered to be compatible with life. However, it is believed that Venus clouds consist of concentrated sulfuric acid,” the authors explain.
Cloud structure in Venus's atmosphere in 2016, revealed by observations in Akatsuki's two ultraviolet bands. Photo credit: Kevin M. Gill
They wanted to test whether some of the “fundamental features” of life could withstand the challenging environment of Venus. Can the chemistry of life resist sulfuric acid?
“Organic chemistry in concentrated sulfuric acid is rarely studied, but is surprisingly extensive. “Recent work supports the idea that complex organic molecules, including amino acids and nucleobases, can be stable in this unusual solvent,” the authors write.
If simple organic molecules can remain stable in sulfuric acid, it is an interesting observation in favor of life. But more complexity is needed, and that is exactly what this research focuses on.
“A fundamental feature of life is cellularity: the distinction between “inside” (the contents of a cell, including information, molecules, and all their interactions) and “outside” (the environment), as well as a mechanism for communication and exchange between the two.” , write Duzdevich and his co-researchers.
The researchers focused on lipids, the membranes that define cells. Lipids are the basis of cell structure, not only as membranes between cells, but also as membranes that form certain parts of the cell's interior. “The cell membrane is particularly important in extreme environments because it must help maintain the homeostasis of the intracellular environment against otherwise harsh external conditions,” the authors write.
The researchers conducted laboratory experiments to determine whether lipids could withstand the harsh environment of Venus. They asked two questions: Can simple lipids resist degradation by sulfuric acid and can the lipids form stable higher-order structures, as they do in cells?
The researchers put large amounts of lipids into vials, exposed them to different concentrations of sulfuric acid, and measured each vial at specific time intervals. Their results show that some lipids can survive exposure to acid and even form structures.
This research illustration shows the vesicle-like structures that formed after concentrated sulfuric acid was added to solid lipids. Each panel is a different area of the same sample taken on the same day. Subsequent images showed that the structures were still intact after seven days. Image source: Duzdevich et al. 2024.
Interested readers can explore the detailed chemistry themselves.
In summary, the results indicate that stable membranes can form and persist in the presence of sulfuric acid. Life uses water as a solvent because it is a polar molecule, can form networks of hydrogen bonds, has a high heat capacity, and is naturally abundant on Earth. But it is not abundant everywhere.
Crucially, this study shows that some aspects of the chemistry of life do not require water as a solvent. Instead, they can tolerate and use sulfuric acid as a solvent. “Here we demonstrate the unexpected stability of complex membrane structures in another polar solvent: concentrated sulfuric acid,” the authors write.
What does this mean for exoplanet habitability and astrobiology?
“Concentrated sulfuric acid as a planetary solvent could be widespread on exoplanets, either on exo-Venuses or on other rocky planets that have dried out due to the stellar activity of their parent star,” the researchers explain.
And of course sulfuric acid is present in large quantities on Venus.
“Concentrated sulfuric acid is also present in our immediate planetary environment as the dominant liquid in the clouds of Venus, further underscoring its importance to planetary science, planetary habitability, and astrobiology,” the authors write.
The question of whether life could somehow survive in the clouds of Venus will not go away. We're new to astrobiology and just can't rule everything out. It may seem far-fetched, but science is a game of evidence, and evidence can be surprising.
This study doesn't provide evidence that can answer the question – big questions like this are answered step by step – but it does provide an interesting result.
“By demonstrating the stability of lipid membranes in this aggressive solvent, we have taken a significant step forward in exploring the potential habitability of the concentrated sulfuric acid cloud environment on Venus,” the authors conclude.
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