Review Reports - An Experimental Approach to Inform Venus Astrobiology Mission Design and Science Objectives

Round 1

Reviewer 1 Report

In this well-written and succinct Perspective, the authors articulate the need for a robust experimental paradigm to complement and supplement (putative) future missions to Venus and other astrobiological targets in the Solar system. The manuscript is interesting, quite comprehensive, and well-suited for the journal's Special Issue. However, there are a number of moderate edits that the authors are encouraged to undertake, which will help improve the manuscript further; they are adumbrated below.

1. In the Abstract and the paper, the authors make a persuasive case for lab experiments to complement missions, but I think some further finessing and exposition - in the Abstract and/or Conclusion - will help strengthen their case as described below.
One can envision two broad approaches, to wit, "top-down" and "bottom-up". On the one hand, lab experiments can help us guide what, where, and how to search for in situ biosignatures. On the other hand, data from missions that probe (potentially) habitable environments can sharpen our focus and understanding of what types of assays and experiments are needed. Thus, these two strands are deeply interlinked and have a synergistic relationship with one another.

2. With regards to line 59 (pg. 2), not just the biosphere but also the origin of life can be conceived of as a planetary process, as summarized in Smith & Morowitz (2016) and Sasselov et al. (2020).
https://www.cambridge.org/de/academic/subjects/physics/biological-physics-and-soft-matter-physics/origin-and-nature-life-earth-emergence-fourth-geosphere?format=HB&isbn=9781107121881
https://www.science.org/doi/10.1126/sciadv.aax3419

3. Line 77 that alludes to "universal" biochemistry should be contrasted against the other school of thought, which allows for a diverse spectrum of biochemistries. I believe this contradistinction is important in the Introduction, because it sets the stage for life on Venus and other "exotic" worlds in the Solar system covered later in the manuscript (Sections to 4). Moreover, if a wide array of biochemistries are feasible, this may translate to higher odds of life in the Solar system and our Galaxy. Some references that warrant citation in this context are Bains (2004), Chapters 6 & 7 of Schulze-Makuch & Irwin (2018), and Chapters 1 & 5 of Lingam & Loeb (2021).
https://www.liebertpub.com/doi/10.1089/153110704323175124
https://link.springer.com/book/10.1007/978-3-319-97658-7
https://www.hup.harvard.edu/catalog.php?isbn=9780674987579

4. Somewhere in the manuscript (e.g., lines 120-122) the authors should consider articulating the need for a strong and sustained program of Solar system exploration by robotic probes, perhaps at a level higher than today. This addition is, however, purely optional.

5. I concur with lines 128-131. In fact, a null result can actually be useful in placing some constraints on the probability of abiogenesis, as shown in Balbi & Grimaldi (2020) using a Bayesian framework.
https://www.pnas.org/doi/abs/10.1073/pnas.2007560117

6. With regards to line 146 (Venus Life Finder), other precursor astrobiology missions have been suggested in the literature such as Hein et al. (2020); such publications should be acknowledged for the sake of completeness.
https://iopscience.iop.org/article/10.3847/2041-8213/abc347/meta

7. Although the odds may be very slim, the notion that microbes from Earth were transported to Venus (via rocks) in the past, and took root there (and adapted) cannot be discounted altogether; it can be mentioned in passing in a footnote near lines 215-219 or 271-274. Simulations and theory on the dynamical aspects of meteorite transfer to/from Venus can be found in Cabot & Laughlin (2020) and Chapter 10 of Lingam & Loeb (2021).
https://iopscience.iop.org/article/10.3847/PSJ/abbc18

8. In Table 1, are the Venusian clouds confirmed to have 15% (by weight?) water? This fraction seems high given the anticipated very low water activity.

9. In the paper (especially vis-a-vis Section 3 and Table 2), there is an emphasis on organic chemistry. While the reasons are apparent to many astrobiologists, this journal is not dedicated to that field - therefore, I would suggest briefly commenting on why silicon-based chemistries would be disfavored in the Venusian atmosphere.

10. In connection with lines 231-233, I would imagine that the spate of experiments on the so-called "cyanosulfidic metabolism" could be performed in Venusian conditions.

11. Ref. [52] does not appear to be published as of yet; thus, the statement(s) in lines 311-314 should be slightly reworded accordingly.

-- Manasvi Lingam

Author Response

We provide responses to the Reviewers’ comments on the Aerospace-1914358 manuscript. Our responses are marked in bold font and the reviewers’ comments are in regular font.

Reviewer 1:

We appreciate the reviewer's positive assessment and helpful comments.

In the Abstract and the paper, the authors make a persuasive case for lab experiments to complement missions, but I think some further finessing and exposition - in the Abstract and/or Conclusion - will help strengthen their case as described below.
One can envision two broad approaches, to wit, "top-down" and "bottom-up". On the one hand, lab experiments can help us guide what, where, and how to search for in situ biosignatures. On the other hand, data from missions that probe (potentially) habitable environments can sharpen our focus and understanding of what types of assays and experiments are needed. Thus, these two strands are deeply interlinked and have a synergistic relationship with one another.

We agree. We had previously only alluded to this point but have now reiterated it in the conclusion as suggested. "Such experiments will inform mission design directly, and successful missions will in turn iteratively refine future experiments."

With regards to line 59 (pg. 2), not just the biosphere but also the origin of life can be conceived of as a planetary process, as summarized in Smith & Morowitz (2016) and Sasselov et al. (2020).
https://www.cambridge.org/de/academic/subjects/physics/biological-physics-and-soft-matter-physics/origin-and-nature-life-earth-emergence-fourth-geosphere?format=HB&isbn=9781107121881
https://www.science.org/doi/10.1126/sciadv.aax3419

Indeed. We have appended these references and agree that they make a subtle and interesting additional point.

Line 77 that alludes to "universal" biochemistry should be contrasted against the other school of thought, which allows for a diverse spectrum of biochemistries. I believe this contradistinction is important in the Introduction, because it sets the stage for life on Venus and other "exotic" worlds in the Solar system covered later in the manuscript (Sections to 4). Moreover, if a wide array of biochemistries are feasible, this may translate to higher odds of life in the Solar system and our Galaxy. Some references that warrant citation in this context are Bains (2004), Chapters 6 & 7 of Schulze-Makuch & Irwin (2018), and Chapters 1 & 5 of Lingam & Loeb (2021).
https://www.liebertpub.com/doi/10.1089/153110704323175124
https://link.springer.com/book/10.1007/978-3-319-97658-7
https://www.hup.harvard.edu/catalog.php?isbn=9780674987579

This is an important but divisive issue that we intentionally avoided because it may perhaps be distracting. There is wide disagreement on this topic in the field: our focus on experimental approaches emphasizes leveraging life as we know it (though we allow that to encompass a very broad interpretation of "life") to design experiments, and that sets some limits on what is reasonable to explore in the laboratory, exotic biochemistry state space being effectively infinite. To address the reviewer's concern directly, however, we note (1) that the line in question does not adhere to one school or there other, it merely notes that, "we cannot rule out that some or even all of Earth-life's fundamental features are in fact universal." And (2) we have appended the indicated references so that the curious reader can explore these topics.

Somewhere in the manuscript (e.g., lines 120-122) the authors should consider articulating the need for a strong and sustained program of Solar system exploration by robotic probes, perhaps at a level higher than today. This addition is, however, purely optional.

We wholeheartedly agree and have added a sentence after the suggested lines. "We advocate for a strong, sustained, and international program of robotic space probes to systematically explore the solar system and provide the observations needed to push astrobiology into a new experimental era."

I concur with lines 128-131. In fact, a null result can actually be useful in placing some constraints on the probability of abiogenesis, as shown in Balbi & Grimaldi (2020) using a Bayesian framework.
https://www.pnas.org/doi/abs/10.1073/pnas.2007560117

We have appended the indicated reference and thank the reviewer for bringing it to our attention.

With regards to line 146 (Venus Life Finder), other precursor astrobiology missions have been suggested in the literature such as Hein et al. (2020); such publications should be acknowledged for the sake of completeness.
https://iopscience.iop.org/article/10.3847/2041-8213/abc347/meta

We thank the reviewer for bringing this reference to our attention, and have now included it as well as two other papers that postulate astrobiological missions to Venus.

Although the odds may be very slim, the notion that microbes from Earth were transported to Venus (via rocks) in the past, and took root there (and adapted) cannot be discounted altogether; it can be mentioned in passing in a footnote near lines 215-219 or 271-274. Simulations and theory on the dynamical aspects of meteorite transfer to/from Venus can be found in Cabot & Laughlin (2020) and Chapter 10 of Lingam & Loeb (2021).
https://iopscience.iop.org/article/10.3847/PSJ/abbc18

Lines 215-216 now include "such as Earth microbes from the spacecraft or historical meteoritic transfer" and the indicated reference.

In Table 1, are the Venusian clouds confirmed to have 15% (by weight?) water? This fraction seems high given the anticipated very low water activity.

We understand this value to be putative, as indicated in the table. An important goal of upcoming missions will be to pin this down. Nonetheless, we have appended a reference that asserts this value:

https://www.researchgate.net/publication/329220197_Clouds_and_Hazes_of_Venus

In the paper (especially vis-a-vis Section 3 and Table 2), there is an emphasis on organic chemistry. While the reasons are apparent to many astrobiologists, this journal is not dedicated to that field - therefore, I would suggest briefly commenting on why silicon-based chemistries would be disfavored in the Venusian atmosphere.

From the astrobiological perspective, silicon as a sole building-block of complex organic chemistry or biochemistry (where it substitutes carbon as a main scaffolding element of organic molecules) is most certainly impossible, especially in water where most organosilicons would eventually react away to form an insoluble and highly unreactive silica. Silicon could be utilized in biochemistry as a rare and specialized heteroatom, but not as a main building-block of organic chemistry. In concentrated sulfuric acid, rather unexpectedly, more silicon-containing molecules could be stable, as previously discussed by Petkowski et al. 2020, however we believe that in the context of our perspective paper going into details on organo-silicon chemistry is not required and might distract the reader from the main point of the article.

Reference already cited in the paper: Bains, W.; Petkowski, J.J.; Zhan, Z.; Seager, S. Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid. Life 2021, 11, 400, doi:10.3390/life11050400. follows up on the previous work of Petkowski et al. 2020 and discusses in detail the stability of the silicon organo-chemistry in concentrated sulfuric acid.

In connection with lines 231-233, I would imagine that the spate of experiments on the so-called "cyanosulfidic metabolism" could be performed in Venusian conditions.

The Sutherland cyanosulfidic model (if that is what the reviewer is referring to) does rather specifically rely on water solvent, is highly dependent on solvated electrons and specific pH values (because of pKa - dependencies), and suggests subaerial UV-light exposed pools that can be dehydrated. It may be worth reflecting on how the system could in principle be modified to accommodate Venusian conditions, but on the face of it, cyanosulfidic systems chemistry is incompatible with current Venusian conditions.

Ref. [52] does not appear to be published as of yet; thus, the statement(s) in lines 311-314 should be slightly reworded accordingly.

We have adjusted the wording from "if there is organic carbon in the Venus atmosphere, it is expected to react" to "if there is organic carbon in the Venus atmosphere, it may react."

Reviewer 2 Report

See attached review comments.

Comments for author File: Comments.pdf

Author Response

We provide responses to the Reviewers’ comments on the Aerospace-1914358 manuscript. Our responses are marked in bold font and the reviewers’ comments are in regular font.

Reviewer 2:

Comments on “An Experimental Approach to Inform Venus Astrobiology Mission Design and

Science Objectives” by Duzdevich et al.

Duzdevich et al. present a rationale for experiments that can be used to learn about the astrobiology aspects of the Venus atmosphere/cloud layer. Previously a case for astrobiology investigation has been presented by (Limaye et al. 2021). Interestingly Duzdevich et al. do not discuss a sample return possibility. The paper is well written and should be published with minor revisions addressing comments below.

We appreciate the reviewer's positive response. Sample return will be discussed in another paper of the special issue by some of the authors, and the suggested reference has been added.

Line # Comment 38-42 The authors may not be aware of “Venus: an astrobiology target”

https://doi.org/10.1089/ast.2020.2268 which also provides a rationale for investigation

We chose to avoid references in the abstract, but have added this excellent reference at the first mention of Venus as an astrobiological target.

109-110 A planet-wide signal that can be monitored remotely is through the contrasts in the cloud cover of Venus which was in a paper already cited (#13, Limaye et al. (2018).

We have now included this citation here as well.

138-139 The environmental values for the cloud layer are not quite correct. The clouds are believed to extend up to ~ 70 km above the mean surface in low latitudes and to ~ 67 km in polar regions (Ignatiev et al. 2009). Please also see and the cloud structure and thermal structure review chapters in “”Venus III: The View After Venus Express” by Bezard et al. (Springer) book. The currently cited paper gives only partial information and not very representative of the global structure and should be replaced.

We have added “e.g. accordingly to [11] (see also [12] (…)” and retained ref. to Paetzold et al. 2007, after which we have provided a reference to Venus III collection paper by Limaye et al. 2018 on the Venus thermal structure to acknowledge the reviewer’s point that there is a variability in the global characteristics of Venus’s clouds. The sentence now reads:
“However, ~50 km above the surface, in the clouds, the temperatures and pressures are much lower: ~60 °C and ~1 bar, according to [11] (see also [12] for a discussion of the thermal structure of the Venusian atmosphere and its variability).”

We have also cited Table 1. The purpose of Table 1 is to provide a very general summary, but we agree with the reviewer that the reader should have access to references that discuss variability in atmospheric characteristics of the planet. We have provided a short footnote beneath Table 1 in relation to the clouds’ altitude range, thermal structure etc.. We have added "Clouds – avg. altitude range", to emphasize that the altitude of the cloud decks does vary, and we have added the text in the Table 1 footnote.
We have also cited the suggested literature.

155-157 There are suggestions presented by Rimmer et al. (2021) and Mogul et al. (2021) that suggest that the cloud droplet pH may be different due to presence of some contaminants.

This is an important point, and we have appended the indicated references.

163-164 Table 1 lists the environmental values for clouds (altitude, pressure, temperature) which are not what is found in most relevant papers. Please refer to the appropriate chapters (Titov et al. 2018)in Venus III book. Another minor point is that the bulk composition of Venus atmosphere has been discovered to be altitude dependent such that there is a greater amount of nitrogen at higher altitudes and lower at lower altitudes from measurements between 60 km and 22 km (Peplowski et al. 2020).

As it is in the case of the comment above, Table 1 is a summary and not a detailed overview. We do however agree with the reviewer and have directed the reader (in the Table 1 footnote) to literature detailing the discussion on the characteristics of Venus’ clouds and the atmosphere and its variability.

168-170 Adaptations of any life if it originated on ancient Venus has been discussed briefly by (Limaye

et al. 2021)

We have noted this reference here too. We are pleased to find that others are thinking along similar lines.

365-367 The citation for Griffin et al. paper (#2) is incomplete. A web search did not yield any successful results for the full text.

Thank you for pointing this out. This is a Lovelock and Giffin paper, not Griffin, submitted to the American Astronautical Society and published in Advances in the Astronautical Sciences, 25, pp.179-193, 1969.
The full text can be found here: http://www.jameslovelock.org/planetary-atmospheres-compositional-and-other-changes-associated-with-the-presence-of-life/

We have updated the citation in the reference list.

381-382 There are better works to cite, e.g. the VIRA papers describing thermal structure and cloud models.

We have cited VIRA papers as References to Table 1.

412-415 Duplicate citation for the Venus Fact sheet

Those references are not duplicated. One specifically refers to the Earth Fact Sheet, and the other to Venus Fact Sheet.

Publications cited:

Ignatiev N. I., Titov D. V., Piccioni G., Drossart P., Markiewicz W. J., Cottini V., Roatsch T., Almeida M., and Manoel N. (2009) Altimetry of the Venus cloud tops from the Venus Express observations. Journal of Geophysical Research: Planets, 114.10.1029/2008je003320

Limaye S. S., Mogul R., Smith D. J., Ansari A. H., Słowik G., and Vaishampayan P. (2018) Venus' Spectral Signatures and the Potential for Life in the Clouds. Astrobiology, 18: 1181- 1198.10.1089/ast.2017.1783

Limaye S. S., Mogul R., Baines K. H., Bullock M. A., Cockell C., Cutts J. A., Gentry D. M., Grinspoon D. H., Head J. W., Jessup K.-L., Kompanichenko V., Lee Y. J., Mathies R., Milojevic T., Pertzborn R. A., Rothschild L., Sasaki S., Schulze-Makuch D., Smith D. J., and Way M. J. (2021) Venus, an Astrobiology Target. Astrobiology, 21: 1163.10.1089/ast.2020.2268

Mogul R., Limaye S. S., Way M. J., and Cordova J. A. (2021) Venus' Mass Spectra Show Signs of Disequilibria in the Middle Clouds. Geophysical Research Letters, 48: e2020GL091327.https://doi.org/10.1029/2020GL091327

Peplowski P. N., Lawrence D. J., and Wilson J. T. (2020) Chemically distinct regions of Venus's atmosphere revealed by measured N<SUB>2</SUB> concentrations. Nature Astronomy, 4: 947.10.1038/s41550-020-1079-2

Rimmer P. B., Jordan S., Constantinou T., Woitke P., Shorttle O., Hobbs R., and Paschodimas A. (2021) Three Different Ways to Explain the Sulfur Depletion in the Clouds of Venus. pp arXiv:2101.08582, https://ui.adsabs.harvard.edu/abs/2021arXiv210108582R

Titov D. V., Ignatiev N. I., McGouldrick K., Wilquet V., and Wilson C. F. (2018) Clouds and Hazes of Venus.

Space Science Reviews, 214: 126.10.1007/s11214-018-0552-z