Biomarkers on the Icy Jovian Moons: Can Europa Also Provide Insights into Life’s Origin?

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This review is dedicated to the discussion of Jovian ice moons, abiogenic evolution and lipid origins of life. Special focus has been made on Europa studies. Beyond the analysis of data procured from the Galileo mission, the identification of chemical compounds, including NH4, is hypothesized to originate from the subsurface ocean of Europa. The manuscript is robustly written and well-organized. However, I recommend major revision since many topics of life origins are important in this context but not mentioned at all. In particular, I advise the authors to enlarge the Introduction section.  Here are my suggestions:

1. Oparin’s lipid-based theory, proposed in the 1920s, suggests life originated from "coacervates"—microscopic, self-assembling spherical aggregates of lipids and proteins. Unfortunately, these studies are not revealed in this manuscript.
2. Moreover, the isotopism and geochemistry of organic compounds in abiogenic context have been largely studied by Galimov and many others, but the authors do not reference to these studies at all.
3. To study the isotopes fractionation of carbon and other elements, beta factor (equilibrium fractionation of isotopes between different chemical species) calculations can be effectively applied. But this technique is never mentioned in this review.
4. Page 6 “we could assume that the evolution of life in the universe can be explained in terms of evolutionary forces that we experience today” – to my opinion, it is a very provocative statement. Michaelian emphasizes thermodynamic dissipation as the driving force and suggest that entropy production might better explain life's origins and evolution, extending beyond Earth-bound Darwinian narratives to universal physical imperatives. Life is a physical phenomenon seeking to dissipate energy, not just a biological one driven solely by reproduction. For this reason, please, revise.
5. I suppose a reference list of 69 items is too small for such a broad topic as abiogenesis and life origins.
6. Finally, I suppose the authors should provide a discussion of beta factor mass-fractionation role in the context of modern studies on isotope fractionation in Jovian ice moons.

Author Response

Replies to Reviewer 1

 

1. Oparin’s lipid-based theory, proposed in the 1920s, suggests life originated from "coacervates"—microscopic, self-assembling spherical aggregates of lipids and proteins. Unfortunately, these studies are not revealed in this manuscript.
   Response: Lines 46-54 - We added text that describes Oparin’s studies.
2. Moreover, the isotopism and geochemistry of organic compounds in abiogenic context have been largely studied by Galimov and many others, but the authors do not reference to these studies at all.
   Response: Lines 55-60 - We added text that references Galimov.
3. To study the isotopes fractionation of carbon and other elements, beta factor (equilibrium fractionation of isotopes between different chemical species) calculations can be effectively applied. But this technique is never mentioned in this review.
   Response: Lines 60-62 - We added text that references beta factor calculations.
4. Page 6 “we could assume that the evolution of life in the universe can be explained in terms of evolutionary forces that we experience today” – to my opinion, it is a very provocative statement. Michaelian emphasizes thermodynamic dissipation as the driving force and suggest that entropy production might better explain life's origins and evolution, extending beyond Earth-bound Darwinian narratives to universal physical imperatives. Life is a physical phenomenon seeking to dissipate energy, not just a biological one driven solely by reproduction. For this reason, please, revise.
   Response: Lines 340-344 - We added text addresses this comment.
5. I suppose a reference list of 69 items is too small for such a broad topic as abiogenesis and life origins.
   Response: We added 11 references to the manuscript.
6. Finally, I suppose the authors should provide a discussion of beta factor mass-fractionation role in the context of modern studies on isotope fractionation in Jovian ice moons. Response: Lines 62-64 - We added text that references this role.

Reviewer 2 Report

Comments and Suggestions for Authors

Submitted review discusses stable isotope excursions (δ34S, δ37Cl) and amphiphile chemistry as potential biosignatures on Europa and other Jovian ocean worlds, framed around JUICE and Europa Clipper instruments. The topic is timely and the mission-instrumentation perspective is valuable. However, several key isotope definitions and equations contain errors, reference quality is uneven with duplicates and unpublished items, and central assumptions about source reservoirs, surface delivery, and measurement precision are not quantified. The Lipid World section is interesting but currently speculative and weakly integrated with the isotope narrative. A strengthened version should correct notation, provide instrument-specific detectability thresholds, and tighten the scope. Recommendation: major revision.

Major comments

1. Several core geochemical statements require correction: sulfur isotope abundances list duplicates 34S, delta notation mixes ‰ and %, and Equations (2)-(3) mis-define Δ33S/Δ36S. Please standardize to VCDT and verify all symbols.
2. The detectability argument needs quantitative treatment. For each relevant instrument (NIM, SUDA, MASPEX) specify achievable isotopic precision, required signal-to-noise, and plausible sampling matrices (sputtered exosphere, plume grains, surface ice), including contamination by radiolysis products.
3. The reference list contains duplicates and several unpublished items. Prioritize peer-reviewed sources and integrate key syntheses on sulfur-isotope biosignatures (Moreras-Marti et al., 2022) plus oceanic crust sulfur-cycling constraints (Ciazela et al., 2017; 2018) to sharpen abiotic baselines.
4. Section 7 introduces the Lipid World and GARD model, but its link to isotopic biosignatures is not explicit. Either integrate via testable, mission-relevant predictions or streamline to maintain a coherent review narrative.

Line-specific minor comments

• L10-14: Specify which biosignature and why it is diagnostic.
• L15-19: Replace “large detectable excursions” with quantitative thresholds and expected precision.
• L23-24: Clarify “primitive chemical phenomena” and provide concrete examples.
• L31-35: Streamline the roadmap sentences; avoid repetition across sections.
• L47-49: Use consistent notation for δ34S; avoid mixing index and parameter.
• L71-73: Isotopic fractionations should be expressed in ‰, not %.
• L78: Sulfur isotope abundances repeat 34S; add 36S and remove duplication.
• L83-88: Equation (1) should reference VCDT; define sa and st in text.
• L94-99: Clarify whether SMOC is appropriate for Europa and its uncertainty.
• L103-105: Rephrase the chondritic Type II sentence; current grammar obscures meaning.
• L112-113: Correct Schidlowski’s field and spelling; avoid inaccurate biographical remarks.
• L115-126: Add a short rationale for selected terrestrial δ34S examples and uncertainties.
• L143-148: Reduce first-person phrasing to a neutral review voice.
• L150-156: The claim that abiotic fractionations are irrelevant for Europa needs evidence.
• L169-171: Equation (2) appears incorrect; Δ33S should depend on δ34S.
• L175-179: Check typographic rendering of δx and Δx terms; several symbols are garbled.
• L181-194: Connect photolysis discussion explicitly to Europa surface irradiation conditions.
• L207-223: Convert Q&A style headings into prose and support with citations.
• L235-241: A concise instrument table would help: resolution, mass range, isotopic capability.
• L265-271: Define the δ34S threshold numerically and justify the chosen cutoff.
• L281-282: Correct “icy shall” to “icy shell”.
• L307-312: The paragraph ends mid-sentence; revise and add a clear conclusion.
• L321-329: Reference [1] is described as intended; provide status or replace.
• L349-357: Figure 1 caption requires explicit assumptions for micelle counts and probabilities.
• L371-376: Replace placeholder “REF Lipid world” with a proper citation.
• L392-414: Split the long technology sentence into shorter units and add key performance metrics.
• L447-452: Replace “///” in Author Contributions with standard CRediT taxonomy.
• L458-459: Abbreviations contain errors; separate BIF from BP and standardize capitalization.
• References: standardize journal formats, and provide DOIs where available

Author Response

Replies to Reviewer 2

Major comments

1. Several core geochemical statements require correction: sulfur isotope abundances list duplicates ³⁴S, delta notation mixes ‰ and %, and Equations (2)-(3) mis-define Δ³³S/Δ³⁶S. Please standardize to VCDT and verify all symbols.
   Response: On line 102 we corrected the duplication of ³⁴S isotope abundance as well as elsewhere; the mixes of ‰ and % were amended; and in lines 199-202 equations (2)-(3) were amended.
2. The detectability argument needs quantitative treatment. For each relevant instrument (NIM, SUDA, MASPEX) specify achievable isotopic precision, required signal-to-noise, and plausible sampling matrices (sputtered exosphere, plume grains, surface ice), including contamination by radiolysis products.
   Response: Lines 289-334 - We added text and a table addressing this point.
3. The reference list contains duplicates and several unpublished items. Prioritize peer-reviewed sources and integrate key syntheses on sulfur-isotope biosignatures (Moreras-Marti et al., 2022) plus oceanic crust sulfur-cycling constraints (Ciazela et al., 2017; 2018) to sharpen abiotic baselines.
   Response: We amended the references and the reference list and added the ones mentioned above, Moreras-Marti et al., 2022 Ref 32 (lines 260-269), Ciazela et al., 2017; 2018 Ref 37 (lines 233-246).
   
   
4. Section 7 introduces the Lipid World and GARD model, but its link to isotopic biosignatures is not explicit. Either integrate via testable, mission-relevant predictions or streamline to  maintain a coherent review narrative.
   Response: Lines 464-468 - We added a section that integrates the above.
   
   

Line-specific minor comments

L12-16: Specify which biosignature and why it is diagnostic.

Done. We added abstract style short text for that.

1. L16-19: Replace “large detectable excursions” with quantitative thresholds and expected precision.
   We added text that refers to newly added text in lines 377-380 addressing this point.
2. L25-28: Clarify “primitive chemical phenomena” and provide concrete examples.
   We added in the text that clarifies that the requested details required here are in section 7.
3. L38-45: Streamline the roadmap sentences; avoid repetition across sections.
   Done.
4. L71-73: Use consistent notation for δ34S; avoid mixing index and parameter.
   Done.
5. L95-97: Isotopic fractionations should be expressed in ‰, not %.
   Done.
6. L102: Sulfur isotope abundances repeat 34S; add 36S and remove duplication.
   Done.
7. L107-117: Equation (1) should reference VCDT; define sa and st in text.
   Done.
8. L123-125: Clarify whether SMOC is appropriate for Europa and its uncertainty.
   Done.
9. L129-131: Rephrase the chondritic Type II sentence; current grammar obscures meaning.
   Done.
10. L137-142: Correct Schidlowski’s field and spelling; avoid inaccurate biographical remarks.
    Done.
11. L143-155: Add a short rationale for selected terrestrial δ34S examples and uncertainties.
    This was done in lines 249-263.
12. L171-177: Reduce first-person phrasing to a neutral review voice.
    Done.
13. L178-185: The claim that abiotic fractionations are irrelevant for Europa needs evidence.
    Done.
14. L199-202: Equation (2) appears incorrect; Δ33S should depend on δ34S.
    Amended.
15. L207-211: Check typographic rendering of δx and Δx terms; several symbols are garbled. Done.
16. L213-218: Connect photolysis discussion explicitly to Europa surface irradiation conditions. Done.
17. L252-259: Convert Q&A style headings into prose and support with citations.
    Done.
18. L289-334: A concise instrument table would help: resolution, mass range, isotopic capability. We added an instrumentation table and relevant text.
19. L369-372: Define the δ34S threshold numerically and justify the chosen cutoff.
    Done.
20. L382: Correct “icy shall” to “icy shell”.
    Done.
21. L413-415: The paragraph ends mid-sentence; revise and add a clear conclusion.
    Done.
22. L424-429: Reference [1] is described as intended; provide status or replace.
    Replaced with ref 58.
23. L453-454: Figure 1 caption requires explicit assumptions for micelle counts and probabilities. 
    We added text with explicit assumptions.
24. L481-485: Replace placeholder “REF Lipid world” with a proper citation.
    Amended.
25. L506-524: Split the long technology sentence into shorter units and add key performance metrics.
    We made the sentence more readable by cutting the paragraph into separate sentences. The exact capacities of these technologies are described in detail in the appearing references.
26. L558-562: Replace “///” in Author Contributions with standard CRediT taxonomy. Amended.
27. L573-574: Abbreviations contain errors; separate BIF from BP and standardize capitalization.
    Amended.
28. References: standardize journal formats, and provide DOIs where available
    Amended

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have carefully answered to my critics. Many important citations have been added and discussed. Now the manuscript can be published as is.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has improved and I accept its publication