Venus Life Finder Habitability Mission: Motivation, Science Objectives, and Instrumentation

Round 1

Reviewer 1 Report

Review Notes

Venus Life Finder Habitability Mission: Motivation, Science

Objectives, and Instrumentation

Seager et al.

 

 

Overall:

I find the VLF HM mission to be exciting and potentially compelling. Exploring the most likely habitable zone of the Venus atmosphere needs to be done, and the paper describes approaches that appear robust and beg to be executed.

One fundamental point I think this paper and mission need to address is statistical sample. A VLF HM mission negative result will be how certain? This answer appears different for different configurations of the mission, and dependent on the ultimate number and types of instruments selected, but the paper implicitly treats the science question as binary and solvable by the mission on a planetary scale. At least some indication/discussion of the limits is needed.

 

The only reason any of the ratings are not "High" is because additional necessary detail is missing, but can be filled in on revision.

 

Section 1.

 

“Pockets of humidity” – what about the persistence  (duration and extent) of these pockets Does life need to be so robust it can loiter in desiccated form when the pockets dry out? Even a qualitative estimate of the likelihood of these pockets and what kind of global sampling might be required to hit one in a random profile or balloon excursion would go a long way to justifying a mission like VLF HM (and the converse is also true - not having any indication of how rare or common pockets might be limits the appeal of a mission that depends on them, and could imply that *that* is a higher science priority than characterization of any potential pocket once found). A look at references 7-8 does not give a clear impression as to how many measurements of what parts of the Venus atmosphere will give sufficient positive or negative results.

 

The last sentence in Par 2 and first sentence in Par 3 are semi repetitive/awkwardly stated.

 

Section 2.

 

This may be a little picky, but Habitability is a definition in flux, and has meant different things in different papers and contexts. A classic definition is essentially planetary conditions where liquid water is possible, but this paper is going rather deeper than that. What is the exact definition of habitability this paper intends? Sufficient water activity? Quantity? Persistence? These are all implied but not specified. And again, is there a specific quantity of any/all the parameters that would lead to a "Yes/No + confidence level" answer for Habitability? We don't need that exact number, but a statement that that number is sought and an indicator of what the going in assumptions are.

 

2.1.2

Begins with another restatement of concentrated H2SO4, but less certain than in Section “are believed to be”.

 

 

2.2.1

A note or table of possible degeneracies and how they might be resolved would be useful

 

Section 2.2.2. is missing – likely only a formatting error?

 

2.2.3 – degeneracies for organic material vs. inorganic salts are mentioned but left very quantitative.

 

2.3.1 “The Rimmer et al.’s theory” is awkward. Remove the “The” or the apostrophe-s.

 

Section 3

Instrument descriptions are inconsistent. Some mention mass and power, some mention data volume and rate. Either all should mention this in the text, or all should refer to table 3 unless there is special reason the specification is noteworthy.

 

Some mention sample processing. Few mention sample acquisition technique. None mention contamination mitigation (or that it might be important) for the many samples that will be required for a useful total statistical size (or what that statistical number might be). Many of the instruments look like they require a fairly involved inlet, pump, outflow system, but such are not described, and it’s also unclear if those are accounted for in mass and power. DAVINCI has an involved system for preventing clogs and multiple inlets to prevent contamination. This mission intends multiple samples in the very layer and of the very materials that can result in clogs and contamination. A paragraph at minimum that mentions this consideration and at least discusses possible mitigations is warranted.

 

3.1 TOPS is an expendable resource sensor? How much do you need to bring for a robust measurement. How do you avoid contamination place to place? Can it be reset? Same questions for MoOSA. If they are expendable, how many measurements are intended and why? If they are renewable, it should be stated as such.

 

3.2 (Example of instrument parameter point mentioned above) First mention of power & mass requirements is in the Nephelometer. Begins to beg the question of those reqs for TOPS and MoOSA.

 

3.3 “visible at near-infrared wavelength” is a bit of an oxymoron. “Detectable” instead of “visible”.

(instrument parameters) First mentions of data volume in this section.

How does MEMS-A collect its sample? Purely passively?

Why is MEMS-A superior to XRF? The section does not give basis for comparison argue for MEMS-A’s superiority.

 

3.4

Par 3 has the first mention of the gondola and lacks context at this point. mTLS has data volume and rate but not mass or power mentioned.

 

 

Section 4.

Is the VAIHL study reviewed, or is it an arxiv white paper only? This section does not seem germane to the main objective of this paper, since these instruments are explicitly stated as beyond the scope of the VLF HM. I would much rather see more quantitative analysis of the measurement requirements for the instruments of this mission for the specific science goals in place of this entire section.

 

Section 5.

How much latitudinal travel is expected in a 1-week mission for the fixed altitude balloon? How much variation of the atmosphere is expected over that time? How many different parcels or pockets of atmosphere are expected to be sampled? How long is the duration of the variable altitude balloon mission – also 1 week? The large probe implies 2 vertical profiles are sufficient to get the quantitative robustness needed. This does not appear justified (and if it is, may argue strongly against the need for the more complex balloon missions). 

 

I think this paper should be answering not just the type of measurement that is needed, but how many measurements would be considered a useful minimum cutoff – this speaks directly to the mission concept’s ability to answer the science questions. The implication is 2 latitudinally  separated profiles is a minimum, which one could take to imply a finding “habitable pocket” is a 50/50 proposition all over Venus at all times. If that is an underlying assumption, it should be explicitly stated. If it isn’t then what does 2 (or N) profiles rule in or rule out on a global scale?

Related: how many observations do you need for an acceptable floor for each of the science questions?

For the last 2 paragraphs of questions, I understand that the balloon mission configurations are described in separate papers, but since this paper describes the instruments and justifies the scientific measurements, I think we need some science floor/threshold indication of what is needed for a positive / negative result, and how constraining a negative result would be given the expected sampling.

 

In this section (Table 3), the primary payload appears to be the same for all 3 configurations of the main probe (steady balloon, variable altitude balloon, or descent probe pair). Are there no differences in configuration?

MEMS-G and MoOSA are not present in the main probe. If there are no mini-probes, are they simply not included?

 

Paragraph 2

The call out of Table 3 is incorrect. Table 3 does not refer to the mini-probes, but rater the VHM’s primary instrumentation. Table 3 is not called out where it should be with regard to the primary payloads.

 

In Tables 3 and 4, do the instrument masses include the sample acquisition and preprocessing (piping, vacuums, cleaning, etc?)

Given that TRL 5 is defined by validation in a relevant environment, the TRL asterisk in both tables this application implies pretty strongly that nothing can be considered higher than TRL 4 here, or in reference 33.

Author Response

We provide responses to the Reviewers’ comments on the Aerospace- 1829384 manuscript. Our response is marked in bold font and the reviewers’ comments are in regular font.

Response to the Reviewer 1:

Review Notes

Venus Life Finder Habitability Mission: Motivation, Science

Objectives, and Instrumentation

Seager et al.

Overall:

I find the VLF HM mission to be exciting and potentially compelling. Exploring the most likely habitable zone of the Venus atmosphere needs to be done, and the paper describes approaches that appear robust and beg to be executed.

One fundamental point I think this paper and mission need to address is statistical sample. A VLF HM mission negative result will be how certain? This answer appears different for different configurations of the mission, and dependent on the ultimate number and types of instruments selected, but the paper implicitly treats the science question as binary and solvable by the mission on a planetary scale. At least some indication/discussion of the limits is needed.

The only reason any of the ratings are not "High" is because additional necessary detail is missing, but can be filled in on revision.

Thank you for pointing out these important issues. We have added a few clarifications regarding the detection/non-detection of specific gases in the dedicated table (Table 3). We have also added a new table (Table 2) that discusses scientific hypotheses behind each of the science goals, as well as possible science outcomes, including the potential “non-detection”.

The strategy for this mission concept is to identify the most critical unknown parameters in the clouds and then select the instruments, which already have some level of development, to fulfill the requirements for these measurements. This mission will provide unique data to supplement the outcome of already selected EnVision, DAVINCI and VERITAS, and, as it is mentioned in the text, would outline the need for sample return and exact requirements for such sample. The agglomerate of this information would provide broader picture of Venusian mysteries, but of course, will not answer all of them.

We have also provided data and a short discussion on the potential sample collection of liquid (as a part of the new Section 4 where we discuss different architectures of the Habitability Mission). While this is out of the scope of this paper, the collection of a few milliliters of liquid stored in separate vials would provide access to billions of droplets for dedicated measurements. Further definition of statistical significance for the measurements would be defined during the mission adaptation phase and would be limited due to engineering challenges, not the scientific objectives.

 Section 1.

 “Pockets of humidity” – what about the persistence  (duration and extent) of these pockets Does life need to be so robust it can loiter in desiccated form when the pockets dry out? Even a qualitative estimate of the likelihood of these pockets and what kind of global sampling might be required to hit one in a random profile or balloon excursion would go a long way to justifying a mission like VLF HM (and the converse is also true - not having any indication of how rare or common pockets might be limits the appeal of a mission that depends on them, and could imply that *that* is a higher science priority than characterization of any potential pocket once found). A look at references 7-8 does not give a clear impression as to how many measurements of what parts of the Venus atmosphere will give sufficient positive or negative results.

We have clarified in the text that life can survive complete desiccation for prolonged periods (thousands of years on Earth), and that the challenge of low water activity affects the ability of the cell to divide, i.e. the cell division in desiccated state is not possible for life as we know it.

The measurements for water that gave the unusually high values were obtained by both Venera and Pioneer Venus probes. The so called “contact methods” of water detection (CM methods) give H2O abundances in a range of thousands of ppm. For example, Vega estimates 1000 ppm H2O at 50-60 km, decreasing to 150 ppm at 25-30 km. Venera 14 humidity sensor gave the value of 2000 ppm at 46-50 km while Venera 14 GC gave 700 ppm at 49-58 km. Pioneer Venus Gas Chromatograph (PVGC) also suggested high H2O water mixing ratios <600 ppm at 51.6 km, ~5000 ppm at 41.7 km and 1350 ppm at 21.6 km. Venera 12 GC on the other hand does not agree with such a high H2O abundances and provides upper limits of 200 ppm below 42 km.
This suggest that such localized anomalously high water abundances could be relatively common as both the US Pioneer as well as some Venera probes did detect them, although ascribing probability to such observations is difficult.
The measurements are highly variable and are often inconsistent with each other and the spectroscopic methods. “Contact methods” (CM) give generally much higher H2O abundances than other in situ methods, including spectroscopy.

We have a brief discussion of the Venera, VeGa and Pioneer water vapor measurements in Section 2.1.1. We did not expand on it further in the text of the paper as the detailed discussion of such anomalous and unexplained observations is the topic of the separate paper that is currently in review in the Special Issue devoted to Venus (Venus Special Collection 2) in the Astrobiology journal. We do however provide a citation to that paper in the text instead of the previous refs. [7] and [8].
We have also included, in the same section 2.1.1., a sentence that comments on the probability of the existence of pockets:

“If correct, they could suggest that such localized anomalously high water abundances are relatively common, as both the Pioneer Venus as well as Venera probes did detect them, although ascribing probability to such observations is difficult.”

Regarding “Even a qualitative estimate of the likelihood of these pockets and what kind of global sampling might be required to hit one in a random profile or balloon excursion would go a long way to justifying a mission like VLF HM” this is beyond the scope of our paper because there is not enough existing information available to calculate the likelihood, but we will take this seriously for future work. However, the fact that both Venera and Pioneer detected such pockets of high humidity makes it fairly likely the pockets, if they actually exist and were not caused by experimental errors, would be encountered again.

Followed by a concluding sentence:

“Ultimately such discrepancies can only be resolved by new in situ measurements of water abundance at multiple locations in the atmosphere.”

Cited paper:
Petkowski, J.J.; Seager, S.; Grinspoon, D.H.; Bains, W.; Ranjan, S.; Rimmer, P.B.; Buchanan, W.P.; Agrawal, A.; Mogul, R.; Carr, C.E. Venus’ Atmosphere Anomalies as Motivation for Astrobiology Missions. Astrobiology 2022, in review.

The last sentence in Par 2 and first sentence in Par 3 are semi repetitive/awkwardly stated.

We have rearranged the sentences in Par 2 and Par 3 of the Introduction to avoid repetition.

Section 2.

 This may be a little picky, but Habitability is a definition in flux, and has meant different things in different papers and contexts. A classic definition is essentially planetary conditions where liquid water is possible, but this paper is going rather deeper than that. What is the exact definition of habitability this paper intends? Sufficient water activity? Quantity? Persistence? These are all implied but not specified. And again, is there a specific quantity of any/all the parameters that would lead to a "Yes/No + confidence level" answer for Habitability? We don't need that exact number, but a statement that that number is sought and an indicator of what the going in assumptions are.

We have added a paragraph at the end of Section 1, Introduction on the definition of “habitability” in the context of the VLF Habitability mission science objectives.

We have also added a new Table 2 where we discuss the possible science outcomes for each of the science goals of the Habitability mission, how each science outcome, including a negative ones, like non-detections, would impact the habitability of the clouds.

2.1.2

Begins with another restatement of concentrated H2SO4, but less certain than in Section “are believed to be”.

We have deleted the “are believed to be” statement and instead emphasized that the particles are predominantly liquid concentrated sulfuric acid droplets.

2.2.1

A note or table of possible degeneracies and how they might be resolved would be useful

We have updated Table 3 with a separate column summarizing scientific outcomes of possible detection or non-detection of disequilibrium gases. We have also added a dedicated Table 2 that connects the Science Objectives of the VLF Habitability Mission, testable Hypotheses and Mission Outcomes. 

Section 2.2.2. is missing – likely only a formatting error?

 Corrected.

2.2.3 – degeneracies for organic material vs. inorganic salts are mentioned but left very quantitative.

 The laboratory tests on Autofluorescence Nephelometer (AFN) detection of fluorescence of organic compounds, including potential contaminants and mineral “false positives”, are currently underway and are aimed to guide the data analysis and interpretation.
We have added citations throughout the paper to the new article that describes the AFN instrument development in detail.
Citation added:
Baumgardner, Darrel, et al. "Deducing the Composition of Venus Cloud Particles with the Autofluorescence Nephelometer (AFN)." Aerospace 9.9 (2022): 492.

2.3.1 “The Rimmer et al.’s theory” is awkward. Remove the “The” or the apostrophe-s.

 Corrected.

Section 3

Instrument descriptions are inconsistent. Some mention mass and power, some mention data volume and rate. Either all should mention this in the text, or all should refer to table 3 unless there is special reason the specification is noteworthy.

 We have made the description of each individual instrument more consistent with each other. Each instrument described in Section 3 has now mass and power mentioned directly in the text, while other characteristics like data volume are mentioned in Table 4. Table 4 is now referenced in each instrument section to make the data more easily accessible.
We have also updated citations, where possible, to dedicated papers that describe each instrument development.

Some mention sample processing. Few mention sample acquisition technique. None mention contamination mitigation (or that it might be important) for the many samples that will be required for a useful total statistical size (or what that statistical number might be). Many of the instruments look like they require a fairly involved inlet, pump, outflow system, but such are not described, and it’s also unclear if those are accounted for in mass and power. DAVINCI has an involved system for preventing clogs and multiple inlets to prevent contamination. This mission intends multiple samples in the very layer and of the very materials that can result in clogs and contamination. A paragraph at minimum that mentions this consideration and at least discusses possible mitigations is warranted.

We have included a paragraph at the end of Section 3 that discusses the clogging problem and resolution by the former Soviet Union and the NASA DAVINCI teams.

We have also clarified, in the instrument section, the information on inlets, pumps etc. where appropriate.

3.1 TOPS is an expendable resource sensor? How much do you need to bring for a robust measurement. How do you avoid contamination place to place? Can it be reset? Same questions for MoOSA. If they are expendable, how many measurements are intended and why? If they are renewable, it should be stated as such.

 The TOPS sensor is a one-use instrument and it cannot be reused. The sensor film has to be protected during the voyage to Venus and is be exposed to the atmosphere only during the measurement of the acidity of the single droplets hitting the sensor plate. We have added clarification, that multiple uses require an array of sensor plates for samples in various atmospheric locations at the cost of increased power and mass.

The MoOSA sensor can be refreshed by applying a small voltage to the sensitive film. We have also added more information on the MoOSA sensor.

We have added a paragraph in the TOPS and MoOSA instrument section that clarifies that.

We have also added a citation to the dedicated paper on the TOPS sensor development:
Kaasik, L.; Rahu, I.; Roper, E.M.; Seeba, R.; Rohtsalu, A.; Pajusalu, M. Sensor for determining single droplet acidities in the Venusian atmosphere. Aerospace 2022, 9(10), 560 .

3.2 (Example of instrument parameter point mentioned above) First mention of power & mass requirements is in the Nephelometer. Begins to beg the question of those reqs for TOPS and MoOSA.

We have included the peak power and mass values for both TOPS and MoOSA in Section 3.1. As mentioned above, we have made the description of each individual instruments more consistent with each other.

3.3 “visible at near-infrared wavelength” is a bit of an oxymoron. “Detectable” instead of “visible”.

Corrected.

(instrument parameters) First mentions of data volume in this section.

To standardize the description of the instruments we have removed the data volume mentions from the main text and included data volume values in Table 4 instead.

How does MEMS-A collect its sample? Purely passively?

The sample inlet system consists of a capillary channel with a widened inlet. Its microfluidic channel and a mechanical MEMS micropump allow for the flow of the sample to the sample ionizer.
We have clarified that in the text.

Why is MEMS-A superior to XRF? The section does not give basis for comparison argue for MEMS-A’s superiority.

MEMS-A is not superior to XRF. XRF can be considered a possible alternative to MEMS-A, as both instruments could answer similar science questions, e.g. about the elemental composition of the could particles.

3.4

Par 3 has the first mention of the gondola and lacks context at this point. mTLS has data volume and rate but not mass or power mentioned.

We have changed the word “gondola” to “mission”, as in this context, the word “mission” is more appropriate.

To standardize the description of the instruments we have removed the TLS data volume mentions from the main text and included data volume values in Table 4 instead. We added mass and peak power requirements in the text.

Section 4.

Is the VAIHL study reviewed, or is it an arxiv white paper only? This section does not seem germane to the main objective of this paper, since these instruments are explicitly stated as beyond the scope of the VLF HM. I would much rather see more quantitative analysis of the measurement requirements for the instruments of this mission for the specific science goals in place of this entire section.

We agree with the reviewer and we have removed this section from the paper. Instead we have included a new Section 4 that expands on the alternative architectures of the VLF Habitability mission, including alternative science instrumentation.

Section 5.

How much latitudinal travel is expected in a 1-week mission for the fixed altitude balloon? How much variation of the atmosphere is expected over that time? How many different parcels or pockets of atmosphere are expected to be sampled? How long is the duration of the variable altitude balloon mission – also 1 week? The large probe implies 2 vertical profiles are sufficient to get the quantitative robustness needed. This does not appear justified (and if it is, may argue strongly against the need for the more complex balloon missions). 

We have included more detailed discussion of mission implementation concepts in the new Section 4. This includes more details on the fixed altitude balloon variant (Section 4.1) and the Parachute probe (Section 4.3). The fixed and variable altitude balloon mission implementation concepts have also their dedicated companion papers in the same special issue that we have cited accordingly.

I think this paper should be answering not just the type of measurement that is needed, but how many measurements would be considered a useful minimum cutoff – this speaks directly to the mission concept’s ability to answer the science questions. The implication is 2 latitudinally  separated profiles is a minimum, which one could take to imply a finding “habitable pocket” is a 50/50 proposition all over Venus at all times. If that is an underlying assumption, it should be explicitly stated. If it isn’t then what does 2 (or N) profiles rule in or rule out on a global scale?

Related: how many observations do you need for an acceptable floor for each of the science questions?

This is an excellent comment and something that needs to be completed. We have decided to leave this for future work for a mission proposal when we are ready to formulate science requirements.

For the last 2 paragraphs of questions, I understand that the balloon mission configurations are described in separate papers, but since this paper describes the instruments and justifies the scientific measurements, I think we need some science floor/threshold indication of what is needed for a positive / negative result, and how constraining a negative result would be given the expected sampling.

We have provided those for each science objective/goal in Table 2 and Table 3. This includes science outcomes that are negative.

In this section (Table 3), the primary payload appears to be the same for all 3 configurations of the main probe (steady balloon, variable altitude balloon, or descent probe pair). Are there no differences in configuration?

The primary payload is basically the point—and different mission architectures can support the same science objectives. While our most preferred mission is the balloon option because of the time it can spend in the Venus atmosphere, we consider the probe with parachute because it would be cheaper and less complex than a balloon.

MEMS-G and MoOSA are not present in the main probe. If there are no mini-probes, are they simply not included?

The advantage of MEMS-G and MoOSA in the context of the mini probes is the small size of these instruments that makes them particularly suitable for the mini probe payload. The alternative instruments that could address the same science objectives are TLS and TOPS. TLS and TOPS take more mass and volume hence are more suitable for the gondola instrument suite. The TOPS instrument is an exception, as even if it is larger than its scientific counterpart – MoOSA – it could be in principle implemented on both the gondola and the mini probes.

We have added a sentence to that effect in the footnote of Table 5:
“Due to their relatively small mass and size MEMS-G and MoOSA instruments are particularly suited as payload for the mini probes.”

Paragraph 2

The call out of Table 3 is incorrect. Table 3 does not refer to the mini-probes, but rater the VHM’s primary instrumentation. Table 3 is not called out where it should be with regard to the primary payloads.

 Corrected. We have also cited Table 4 throughout the paper where needed.

In Tables 3 and 4, do the instrument masses include the sample acquisition and preprocessing (piping, vacuums, cleaning, etc?)

Yes, they do. We have made that point explicitly.

Given that TRL 5 is defined by validation in a relevant environment, the TRL asterisk in both tables this application implies pretty strongly that nothing can be considered higher than TRL 4 here, or in reference 33.

Agreed. We have changed the TRL levels to 4.

Reviewer 2 Report

General comments

* The main concern I have regarding this mission concept is methodological: the chemistry in the cloud region is very poorly constrained. No single chemical model is able to reproduce both upper and lower atmospheric profiles involved in sulfur or water chemistry without ad-hoc assumptions or parameterizations. In such a case, it would be very hard, almost impossible, to interpret any discoveries regarding gaseous or cloud droplet chemistry from an astrobiological perspective, since the null hypothesis (abiotic scenario) is not known well enough. 

One should therefore assume that substantial progress should be achieved in this domain prior to the mission launch, which brings a major scientific risk. Or reformulate the mission goals towards aiming to better understand homogeneous and heterogeneous chemistry in the clouds of Venus, including (but not limited to) habitability or extant biological reactions. 

This important point should be discussed extensively in a revised version, with possible mitigations stated explicitly. If not possible, then the mission scientific case is ill-defined, and I would not support publication.

* Expected data rates are indicated for the various instruments, but the data link with Earth is not discussed at all. I suppose that, for cost reasons, direct datalink with Earth is considered. Are the indicated data volumes compatible with direct data link with Earth? 

Specific comments

* "New work by [7] ... (see also [8,10])": it should however be stated here or in Sec. 2.1.2 that this raise in pH comes at a high price for water activity, due to the high sulfate salt concentration required.

* "global average of 30 ppmv [12]" this value is representative of the lower atmosphere below the clouds. Above the clouds, the average value is 10 times lower than that [Encrenaz et al. 2016; Cottini et al., 2012; Fedorova et al., 2015; Marcq et al. 2018].

* "If water vapor can be as high as some measurements suggest". Please state the enhancement factor relative to the average background level, based on the discussion in next paragraph. One could also quote the tentative 200 ppmv value measured by Bell et al. (1991) paper near 2.4 µm. 

* NOx should be added to Table 2. If it were produced (by e.g. electrical discharges), then it would be highly reactive and interact with other gases and cloud particles.

* "one-in-a-million particles". It would be interesting to make this statement more quantitative and compare with microbes founds in Earth's clouds here. What is the probability distribution for an Earth droplet to host microbes with respect to e.g. altitude, droplet size, etc.? It would most likely set an upper limit for the Venusian case.

* "on the order of 6 wavenumbers". I don't understand. 6 cm-1? ratio between largest/smallest wavenumber?

* "ESA/Roscosmos" it will probably be launched without any Roscosmos involvement...

Author Response

We provide responses to the Reviewers’ comments on the Aerospace- 1829384 manuscript. Our response is marked in bold font and the reviewers’ comments are in regular font.

Response to the Reviewer 2:

General comments

* The main concern I have regarding this mission concept is methodological: the chemistry in the cloud region is very poorly constrained. No single chemical model is able to reproduce both upper and lower atmospheric profiles involved in sulfur or water chemistry without ad-hoc assumptions or parameterizations. In such a case, it would be very hard, almost impossible, to interpret any discoveries regarding gaseous or cloud droplet chemistry from an astrobiological perspective, since the null hypothesis (abiotic scenario) is not known well enough. 

We have clarified what various scientific outcomes of the mission would mean from the point of view of habitability of the clouds and broader astrobiology. We have included a dedicated table (Table 2 that shows possible scientific outcomes, including the negative results. Similar assessment of scientific outcomes is added to Table 3 which specifically addresses the detection or non-detection of atmospheric gases and how such results could be interpreted.

The reviewer is correct that the chemistry in the cloud region is very poorly constrained. In fact, it is for this reason why the VLF Habitability mission is needed. The VLF Habitability mission is an exploratory/discovery mission at its core. No other planned mission, with the exception of the Rocket Lab mission to Venus, aims to study cloud particles and their chemical composition. The main objectives of the VLF Habitability mission is to confirm or refute decades-long anomalies that could be tied to the habitability of the clouds, finding out which of those anomalous measurements are correct and which are erroneous contributes to constraining the chemistry of the clouds, and also informs any chemical mechanisms that are responsible for their existence, including constraining any abiotic scenarios. 

One should therefore assume that substantial progress should be achieved in this domain prior to the mission launch, which brings a major scientific risk. Or reformulate the mission goals towards aiming to better understand homogeneous and heterogeneous chemistry in the clouds of Venus, including (but not limited to) habitability or extant biological reactions. 

This important point should be discussed extensively in a revised version, with possible mitigations stated explicitly. If not possible, then the mission scientific case is ill-defined, and I would not support publication.

Prior to the VLF Habitability mission launch, another mission is scheduled for launch, the Rocket Lab mission to Venus. A series of laboratory experiments are also underway (that are beyond the scope of this paper) that are aimed at better understanding the chemical environment of the clouds. Those include laboratory experiments on the basic chemistry of organic molecules in concentrated sulfuric acid, laboratory testing of the mission instrumentation, and other experiments.

All of those tests, including the prior mission to Venus developed by the Rocket Lab company, serve as an information-gathering endeavor for the VLF Habitability mission, minimizing its scientific risk and guiding necessary mission modifications, if needed.

We have added citations to individual papers describing the Rocket Lab mission. In the instrument section we have also added citations to the dedicated papers describing the scientific instruments that are considered for the mission, including their laboratory testing (e.g. the TOPS acidity sensor). We have also added citations to preliminary papers describing experiments on the reactivity of organic chemicals in concentrated sulfuric acid. All of those endeavors are aimed at scientific risk mitigation and gaining crucial information about the cloud environment ahead of the mission launch.

To address the possible scientific risk of the mission we have also discussed the possible range of scientific outcomes (included in Table 2 and Table 3), including the possible scientific value of non-detections.

* Expected data rates are indicated for the various instruments, but the data link with Earth is not discussed at all. I suppose that, for cost reasons, direct datalink with Earth is considered. Are the indicated data volumes compatible with direct data link with Earth? 

We have added the short discussion of the data volume limitations in the legend of Table 4.

Specific comments

* "New work by [7] ... (see also [8,10])": it should however be stated here or in Sec. 2.1.2 that this raise in pH comes at a high price for water activity, due to the high sulfate salt concentration required.

That is correct. We have added an additional sentence that aims to clarify that:

“However, while the hypothesized neutralization of at least a fraction of the cloud particles makes them more habitable from the acidity point of view it does not mitigate the extremely low water activity of the cloud habitat.” 

* "global average of 30 ppmv [12]" this value is representative of the lower atmosphere below the clouds. Above the clouds, the average value is 10 times lower than that [Encrenaz et al. 2016; Cottini et al., 2012; Fedorova et al., 2015; Marcq et al. 2018].

That is correct. We have made that distinction explicitly in this paragraph and cited the papers above.

* "If water vapor can be as high as some measurements suggest". Please state the enhancement factor relative to the average background level, based on the discussion in next paragraph. One could also quote the tentative 200 ppmv value measured by Bell et al. (1991) paper near 2.4 µm. 

We have cited: [7] Bell III, J.F.; Crisp, D.; Lucey, P.G.; Ozoroski, T.A.; Sinton, W.M.; Willis, S.C.; Campbell, B.A. Spectroscopic observations of bright and dark emission features on the night side of Venus. Science (80-. ). 1991, 252, 1293–1296., as requested.
The sentence now reads: “However, measurements of atmospheric water vapor indicate significant variations in vertical H₂O abundance (e.g. [7]; reviewed in [8]). If water vapor can be as high as some measurements suggest, i.e. hundreds to even few thousands of ppm, Venus may have pockets of high-enough humidity for life to thrive (reviewed in [8])”.

* NOx should be added to Table 2. If it were produced (by e.g. electrical discharges), then it would be highly reactive and interact with other gases and cloud particles.

Agreed. We have added NOx to Table 3 (which used to be Table 2).

* "one-in-a-million particles". It would be interesting to make this statement more quantitative and compare with microbes founds in Earth's clouds here. What is the probability distribution for an Earth droplet to host microbes with respect to e.g. altitude, droplet size, etc.? It would most likely set an upper limit for the Venusian case.

We have added a brief statement about liquid sample collection that would provide billions to trillions of droplets per one day of sampling in the new Section 4.4 where we discuss alternative instrument variants of the Habitability mission, while emphasizing numbers on Venus are speculative.

We reference our past paper with details on Earth’s aerial biosphere in this context (Seager, S.; Petkowski, J.J.; Gao, P.; Bains, W.; Bryan, N.C.; Ranjan, S.; Greaves, J. The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: a Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology 2021, 21, 1206–1223).

For specifics, Earth's water clouds have 30,000 cells per cubic meter of cloud. In terms of actual liquid water collected from the clouds, people find 100,000 cells per ml of cloud water  [Amato P et al 2005].

Citations:

Amato, Pierre, et al. "Microbial population in cloud water at the Puy de Dôme: implications for the chemistry of clouds." Atmospheric Environment 39.22 (2005): 4143-4153.

* "on the order of 6 wavenumbers". I don't understand. 6 cm-1? ratio between largest/smallest wavenumber?

The spectral width of the instrument is 6 wavenumbers. For example, from 3050 to 3056 cm^-1. We changed text to, “a spectral width of about 6 cm^-1”

* "ESA/Roscosmos" it will probably be launched without any Roscosmos involvement...

We have deleted “ESA/Roscosmos”.

Round 2

Reviewer 1 Report

Thanks to the authors. The revisions address all of my concerns and questions.

My only remaining comment is mostly stylistic - nearly all of the bold text should be eliminated, save for 2 or at most 3 critical points that you'd like to highlight above everything else presented in the paper. Anything more reduces the efficacy of the emphasis.

Reviewer 2 Report

The authors have answers most of my previous concerns in their updated manuscript version -- I am not always convinced by the provided answers, but their arguments are now substantiated enough to be debated in the broader scientific community, and therefore should be published.