How to approach finding life on other worlds will continue to be a challenging issue, but how useful that even as we work out strategies for studying exoplanet atmospheres, we have planets we can actually reach right here in our own Solar System. And if the hunt for life has turned up empty thus far on Mars, we can keep searching there even as we consider the exotic possibility of life in the clouds of Venus. We’ve looked at Venus Life Finder before in these pages. This series of missions is now known as Morning Star, all designed to probe the clouds for signs of a kind of life that would have to endure the most hellish conditions we can imagine. In today’s post, Alex Tolley examines the Morning Star Missions and how they might proceed, depending on the results of that all important first sampling of the atmosphere.
by Alex Tolley
“To boldly seek life, where no terrestrial life has gone before”
The “Morning Star Missions” (formerly Venus Life Finder) group had previously outlined their plans for early life-detecting missions in the possibly habitable, temperate, but highly acidic Venusian clouds, at altitudes of 48-60 km above the searingly hot surface. The first mission, now slated for a 2025 launch, includes an Autofluorescing Nephelometer (AFN) that can detect organic materials, a prerequisite for living organisms.  The instrument emits laser light that causes certain carbon bonds to fluoresce and be detected (in this case, 440 nm is the selected detection wavelength). If no organic material is detected in the cloud droplets, that would eliminate life as we know it. However, there would still be ambiguities regarding whether organic material was detected or not as not all organic matter will fluoresce when stimulated by light. Typically aromatic carbon ring structures fluoresce, whilst linear carbon chains do not. A double-membraned cell wall that could contain a prebiotic metabolic system would probably fail to register. This might well be considered a false negative for what could be a very interesting finding.
It is well known that sulfuric acid (H2SO4) has a deleterious effect on organic matter, and highly concentrated sulfuric acid (CSA) that is expected in the Venusian clouds will rapidly break down organic matter and therefore it would appear that terrestrial life would rapidly succumb to this level of acid condition. [An acid bath is a traditional means by which murderers dispose of the victim’s body.] This would seem to rule out life of a terrestrial nature even in the Venusian clouds.
What about a positive result? Carbon aromatic rings that readily fluoresce may be very common in the clouds as simple carbon molecules are converted as the compounds fall towards the hotter surface. Polyaromatic hydrocarbons [PAH] are common in space and it has been hypothesized that they may be common in Venus’ clouds .
Apart from the simple destruction of living organisms like plants by pouring CSA onto them, prior work  has shown that organic material identified in terrestrial metabolisms is a little more stable than all naturally occurring organic compounds in CSA but far less stable than the space of manufactured organic compounds, as shown in figure 1.
A database of organic compounds and their reactivity to H2SO4 shows compounds with ring structures, especially those with unsaturated carbon-carbon bonds . This implies that any extant organic compounds with these structural features will be more prevalent in the clouds, which includes PAHs. If abiotic aromatic ring carbon compounds are most likely to be resistant to CSA reactions, these abiotic organic molecules may create a false positive result. The search for life must therefore be sure that some biological molecules are resistant to CSA and could theoretically be part of a positive organic molecule detection. Otherwise, this search approach would be futile. That about 10% of the extracted core metabolism compounds are resistant to CSA for greater than 3 years provides support for the possibility that biology may exist in the Venusian clouds.
Which biotic molecules are resistant to CSA and therefore could be present in the clouds? An answer to this issue is provided in a new paper by Seager et al in Proceedings of the National Academy of Sciences  which examines whether any core biological molecules can survive the acid conditions. Information polymers such as DNA and RNA are a central component of terrestrial life. They are composed of nucleic acids of two types: purines (Adenine, Guanine) and pyrimidines (Cytosine, Thymine, Uracil), linked by a sugar (ribose in RNA, and deoxyribose in DNA) and phosphate. As shown in figure 2, the core structures have unsaturated bonds and very limited exposed bonds that could be attacked by CSA. The purpose of the paper was to determine if these nucleic acids are resistant to CSA and therefore possible detectable molecules on Venus.
The researchers performed several tests, including changes in UV spectra, Nuclear Magnetic Resonance (NMR) to detect changes in C-H bonds, NMR to detect the replacement of the hydrogen atoms with deuterium, and NMR to detect the donation of hydrogen ions, H+, to the nucleic acid molecules by the CSA (protonation).
The first series of tests placed these nucleic acids in CSA and tested how the UV spectrum changed over a period of up to 2 weeks in acid concentrations up to 98%. The spectra for the treated nucleic acids were very similar to those in aqueous solutions, indicating that there was no fundamental change in structures or breakdown of the compounds.
The next series of experiments ran NMR tests on the nucleic acids. This detects the state of the carbon atoms and their bonds which are shown by chemical shifts (ppm) in the hydrogens. Once again, the sharp spectral peaks were very similar to the controls, indicating that the structures and identified carbon and nitrogen bonds had not been changed. Figure 4 shows the results.
The last series of experiments used deuterated sulfuric acid [D2SO4] as well as C13 and N15 isotopes in the nucleic acids to determine if any of the bound hydrogen atoms had been replaced, indicating that the structures were capable of having the C-H bonds broken. Again, there was no evidence of bond-breaking and H atoms replacement.
“Taken together the NMR data confirms that the purine ring structure remains intact in 98% w/w D2SO4 in D2O.“
As the nucleic acids were in CSA where H+ ions were abundant, there is the question of whether these ions protonate the compounds. This protonation of the nitrogen and oxygen atoms was detected by NMR. As hydrogen bonds are important in biological functions, most notably the base pairing between purines and pyrimidines in DNA, and pairing of bases in the same RNA strands, protonation would impact these interactions. Figure 5 shows how protonation disrupts this pairing.
Figure 5. The base pairings in aqueous solutions and the impact of H2SO4 protonation that breaks the hydrogen bonds and pairing. Source: Seager et al 2023 
The conclusion is that the purines and pyrimidines of terrestrial information molecules will remain stable in the Venusian clouds in the habitable region. As these molecules will fluoresce, a positive result of organic molecule detection could include these molecules, but follow-on missions would be needed to determine whether these molecules are present.
In summary, these experiments demonstrate that terrestrial information molecules using the core purine and pyrimidine structures are stable in CSA and therefore could potentially be present in the Venusian clouds. Therefore if organic carbon is detected in the first mission, a 2nd mission to characterize the carbon compounds is supported as the presence of organic carbon could include biological molecules.
While the detection of these nucleic acids would be very interesting, it is important to note that to be useful information molecules, they must polymerize in a way that allows their informational function to operate. Otherwise, the nucleic acids are like an alphabet that cannot be composed in text, as the sugar-phosphate links between them would not be stable in CSA. Other molecules would need to be used. Currently, possible linker molecules have not been identified and remain an area of work.
We already know that amino acids are not stable in sulfuric acid, which rules out proteins as the main functional type of molecule of terrestrial life, existing in the Venusian clouds without some mechanism to neutralize the pH.
If amino acids were stable, could the first mission detect them? Amino acids with cyclic rings such as tryptophan fluoresce, albeit with a peak well below the 440 nm detection wavelength of the AFN to be included in the first mission. If subsequently confirmed by other instruments on later missions, it would indicate that the cloud droplet environment is not as unfavorable as assumed. As a side note, the somewhat controversial detection of phosphine suggests the known rapid oxidation by CSA is at least partially avoided, perhaps by either avoiding the cloud droplets or the droplets having a higher pH, or both.
What are the implications for life if nucleic acids are confirmed and in polymer form? The authors offer 3 scenarios:
1. Life may have emerged during the early wet age in Venus’ oceans. As the planet became the hot dry world it is today, that life could have evolved to adapt to the new cloud-borne, temperate, but concentrated sulfuric acid conditions. The DNA/RNA would have had to change links between the nucleic acids to retain their function.
2. During its evolution to the current conditions, life may have evolved the ability to neutralize the acid by excreting ammonia. This would allow it to retain the existing nucleic acid sugar-phosphate links in DNA and RNA, as well as allow proteins to remain stable.
3. Lastly, the abiogenesis of new life in the clouds. Perhaps this is limited to a pre-biotic state with nucleobases only.
In my opinion, scenario 2 seems most likely if there is evidence that terrestrial-analog cellular life exists in the cloud droplets, using polymerized nucleic acids as their information molecules. This is because we know from the evolution of terrestrial life that core metabolism, information storage, and transcription and translation to functional proteins, have remained almost unchanged over billions of years. Extremophiles have been unable to change their core replication and growth biology, despite adapting to their current environments. What they do instead is tinker with the relative production of certain proteins, and evolve new enzymes and pathways to produce new molecules to adapt to the new conditions. Therefore being able to produce ammonia to neutralize the CSA seems a more likely evolutionary path.
If however nucleic acids are found and in a polymerized, functional state, but without accompanying amino acids and functional proteins, is it possible that Venus is in the equivalent condition of the hypothetical pre-biotic RNA World? In this scenario, RNA acts as both the information and functional molecule. We see evidence for its metabolic function as RNA can act as a catalyst and also autocatalyze itself to replicate. On Venus, the RNA analog may be pre-biotic or possibly degenerate, the remaining functional mechanism in a hostile pH environment. Despite this last speculation, it raises the question “How would these ‘nucleic acid bases’ be formed in the clouds?” While these nucleic acids have been shown to have the ability to form from simple molecules like HCN and formamide in aqueous conditions, there is as yet no evidence that they can form in CSA. Unless they can, this would seem to rule out this pre/post-biotic scenario. (See also Bain paper on H2SO4 as a solvent )
In summary, the nucleic acids used in the information molecules DNA and RNA are stable in the acid conditions expected in the Venusian clouds. However, they would not be functional as information molecules unless they can effectively polymerize in a way that allows an analog of the stable form that would allow natural selection to operate. They would need different linker molecules than the sugar-phosphate ones on Earth. Furthermore, protonation of the nitrogens in the nucleic acids would disrupt the hydrogen bonding mechanism for the base pairings. This is another important issue that further constrains the possibility of life on Venus unless it can neutralize the pH of the cloud droplets, with a metabolism that relies on methanogenesis of CO2 like terrestrial archaea, or organic molecules produced in the atmosphere.
If organic molecules are detected in the first 2025 scheduled mission, the stability of nucleic acids in CSA indicates that there is potential for their direct detection in a follow-up mission, holding out the possibility of some sort of life or pre-biotic chemistry on Venus.
Tolley, A (2022) “Venus Life Finder: Scooping Big Science” Centauri-Dreams https://www.centauri-dreams.org/2022/06/03/venus-life-finder-scooping-big-science/
Špaček, J (2021) “Organic Carbon Cycle in the Atmosphere of Venus”, arXiv preprint arXiv:2108.02286.
Bains W, Petkowski JJ, Zhan Z, Seager S. Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid. Life (Basel). 2021 Apr 27;11(5):400. doi: 10.3390/life11050400. PMID: 33925658; PMCID: PMC8145300.
Seager, S et al (2023) “Stability of nucleic acid bases in concentrated sulfuric acid: Implications for the habitability of Venus’ clouds” PNAS 2023 Vol. 120 No. 25 e2220007120 https://doi.org/10.1073/pnas.2220007120
Database of H2SO4 effects on molecules. Reactivity V4.1- release.xlsx Url: https://zenodo.org/record/4467868/files/Reactivity%20V4.1-%20release.xlsx