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Molecules to Microbes
Open AccessArticlePost Publication Peer ReviewVersion 3, Revised

Organic Molecules: Is It Possible to Distinguish Aromatics from Aliphatics Collected by Space Missions in High-Speed Impacts? (Version 3, Revised)

Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Kent, Canterbury CT2 7NH, UK
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Author to whom correspondence should be addressed.
Received: 22 August 2019 / Accepted: 29 August 2019 / Published: 8 June 2020
(This article belongs to the Special Issue Molecules to Microbes)
Peer review status: 3rd round review Read review reports

Reviewer 1 Vladimir Airapetian NASA GSFC and American University Reviewer 2 Marta Filipa Simões State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau. Reviewer 3 Frédéric Foucher CNRS, Centre de Biophysique Moléculaire
Version 1
Original
Not approved
Authors' response
Not approved
Authors' response
Not approved
Authors' response
Version 2
Revised
Approved with revisions
Authors' response
Approved with revisions
Authors' response
Version 3
Revised
Reviewer invited Approved
Version 3, Revised
Published: 8 June 2020
DOI: 10.3390/sci2020041
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Version 2, Revised
Published: 13 March 2020
DOI: 10.3390/sci2010012
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Version 1, Original
Published: 13 September 2019
DOI: 10.3390/sci1020053
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A prime site of astrobiological interest within the Solar System is the interior ocean of Enceladus. This ocean has already been shown to contain organic molecules, and is thought to have the conditions necessary for more complex organic biomolecules to emerge and potentially even life itself. This sub-surface ocean has been accessed by Cassini, an unmanned spacecraft that interacted with the water plumes ejected naturally from Enceladus. The encounter speed with these plumes and their contents, was between 5 and 15 km s−1. Encounters at such speeds allow analysis of vapourised material from submicron-sized particles within the plume, but sampling micron-sized particles remains an open question. The latter particles can impact metal targets exposed on the exterior of future spacecraft, producing impact craters lined with impactor residue, which can then be analysed. Although there is considerable literature on how mineral grains behave in such high-speed impacts, and also on the relationship between the crater residue and the original grain composition, far less is known regarding the behaviour of organic particles. Here we consider a deceptively simple yet fundamental scientific question: for impacts at speeds of around 5–6 kms−1 would the impactor residue alone be sufficient to enable us to recognise the signature conferred by organic particles? Furthermore, would it be possible to identify the organic molecules involved, or at least distinguish between aromatic and aliphatic chemical structures? For polystyrene (aromatic-rich) and poly(methyl methacrylate) (solely aliphatic) latex particles impinging at around 5 km s−1 onto metal targets, we find that sufficient residue is retained at the impact site to permit identification of a carbon-rich projectile, but not of the particular molecules involved, nor is it currently possible to discriminate between aromatic-rich and solely aliphatic particles. This suggests that an alternative analytical method to simple impacts on metal targets is required to enable successful collection of organic samples in a fly-by Enceladus mission, or, alternatively, a lower encounter speed is required. View Full-Text
Keywords: Astrobiology; Enceladus; space-missions; organic; aromatic; aliphatic Astrobiology; Enceladus; space-missions; organic; aromatic; aliphatic
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Burchell, M.; Harriss, K. Organic Molecules: Is It Possible to Distinguish Aromatics from Aliphatics Collected by Space Missions in High-Speed Impacts? Sci 2020, 2, 41.

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1

Reviewer 1

Sent on 02 Dec 2019 by Vladimir Airapetian | Not approved
NASA GSFC and American University

A broad class of aromatic hydrocarbons made up of fused benzene rings have been suggested to be important in prebiotic chemistry. While exact pathways are debated, these molecules can be potentially relevant to the formation of broad range of molecular species. In contrast, aliphatic molecules contain carbon and hydrogen atom in straight chains representing a part of lipids and identified in a number of environments including asteroids and comets. The paper by M. Burchell and K. Harriss reports the results of laboratory experiments to investigate whether the current analytical methods can discriminate between aromatic and aliphatic molecules in impact experiments as the way not to look for signatures of life, but to learn about processes producing different carbon bonds in Enceladus and other extraterrestrial bodies in the Solar system.

The introduction discusses the Drake equation and it is not clear what is the relation between intelligence, B2FH model and aromatics and aliphatics. However, many details of the major objectives of the proposed investigation have not been described. For example, why these molecules should be formed in icy environments of Enceladus and how they will specify the pathways of prebiotic chemistry? 

I think that the idea to use high-speed impact of molecules emanating from this body with a spacecraft’s metal plate to discriminate between aromatic and aliphatic molecules is generally interesting. However, the paper lacks many technical details before it can be recommended for publication

1. The experiment’s setup and analysis lack details in the flux of PS (C8H8) molecules (as a proxy of aromatic molecules) versus PMMA, C5H9O2) (as a proxy for aliphatic molecules).

2. It is not clear why aluminum target has been used in the experiments?

3. Figure 3 shows the the impact craters, but it is unclear whether what scans of the crater were used for Raman spectra of crater residues?

4. It is not clear how peaks on oxygen can inform us about the type of molecules as the autoxidation of aromatic molecules as well as contamination as mentioned by the authors can introduce the spectral signature. Also, oxidation of an aluminum plate itself can affect the result of the experiment as water vapor get exposed during a mission to Europa or Enceladus.

5. How many experiments have been preformed?

Also, no quantitative analysis of shock pressure and its impact on chemical bonds of used molecules has been performed to predict possible outcomes of described experiments. Peaks in the spectra presented in Figure 3 are not properly discussed and it is not clear what they represent. Also, why not other supporting methods including gas chromatography (GC) and GC-MS (mass spectrometry) have been used in the analysis to resolve the structures of residues.

Thus, it is not clear whether the reported results are surprising and “disappointing”. This is also not clear how these experiments propose a methodology to discriminate between aromatics and aliphatics in future space missions.  Thus, I would recommend the authors to perform a more thorough analysis of possible outcomes before raising the question of discriminating between two types of molecules in laboratory experiment. Thus, this paper cannot be recommended for publication before all above-mentioned points are highlighted and paper is substantially revised.

 

Response to Reviewer 1

Sent on 15 Jul 2020 by Mark Burchell, Kathryn Harriss

We thank the referees for their work General comments: All the referees noted and objected to a somewhat imbalanced paper with an overly long introduction. This was due to some confusion. The paper was based upon an hour-long open lecture, and (as one referee seems to have spotted) was originally written up for publication as a book chapter. The introduction was thus a deliberately broad discussion of astrobiology. Publication plans changed and the paper was then to appear in a special issue of a journal, but unfortunately the original style was retained. Change: The broad introduction has been removed and replaced by a more focussed introduction on the specific question addressed in the paper. Specific comments of referee 1 1. The experiment’s setup and analysis lack details in the flux of PS (C8H8) molecules (as a proxy of aromatic molecules) versus PMMA, C5H9O2) (as a proxy for aliphatic molecules). Response: We have now explained that we looked at 7 PS craters and 10 PMMA craters in this study 2. It is not clear why aluminum target has been used in the experiments? Response This has been widely used for impact residue studies and was the sub-strate used in the NASA Stardust mission to collect cometary dust grains. This is now stated in the text. 3. Figure 3 shows the the impact craters, but it is unclear whether what scans of the crater were used for Raman spectra of crater residues? Response; We have added text to make it clear we took spot spectra from various craters where the SEM work showed there was residue. 4. It is not clear how peaks on oxygen can inform us about the type of molecules as the autoxidation of aromatic molecules as well as contamination as mentioned by the authors can introduce the spectral signature. Also, oxidation of an aluminum plate itself can affect the result of the experiment as water vapor get exposed during a mission to Europa or Enceladus. Response: Agreed. That is why we pointed out that although O is expected in PMMA and not PS, the appearance of O as a contaminant means it is hard to build up an elemental composition of the impactor. Thus the ideal solution would be an analysis technique based on structure. 5. How many experiments have been preformed? Response; We have now explicity said now many craters were examined in each shot. Also, no quantitative analysis of shock pressure and its impact on chemical bonds of used molecules has been performed to predict possible outcomes of described experiments. Response: We have added an estimate of peak shock pressure. We have also added a calculation of temperature increase in the samples, and compared it to the melting point of the samples. Peaks in the spectra presented in Figure 3 are not properly discussed and it is not clear what they represent. Response: We have improved the notation on the spectra. Also, why not other supporting methods including gas chromatography (GC) and GC-MS (mass spectrometry) have been used in the analysis to resolve the structures of residues. Response: Given that we have found melt and observed that the original structure has been lost we did not go further with the current samples. However, when we repeat the work at lower speeds, and hopefully find a speed where some material is not melted, we will then explore how to remove reside from the craters and present it for a wider chain of analysis. This comment has been added to the ms.

Reviewer 2

Sent on 02 Dec 2019 by Marta Filipa Simões | Not approved
State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau.

Dear authors,

 

Your manuscript is interesting and easy to read. Nevertheless, I feel that your introduction is too broad, sometimes far from your research question.

In my opinion, you should re-write this manuscript in order to submit it as a short paper, in a more assertive way.

Your introduction should focus more on what you write in the first couple of paragraphs of your discussion. And your discussion should clarify the relevance of your assays in a more direct way and refer to the results obtained.

Your introduction would read very well as a book chapter on a more general topic.

I hope my comments are useful.

Response to Reviewer 2

Sent on 15 Jul 2020 by Mark Burchell, Kathryn Harriss

We thank the referees for their work General comments: All the referees noted and objected to a somewhat imbalanced paper with an overly long introduction. This was due to some confusion. The paper was based upon an hour-long open lecture, and (as one referee seems to have spotted) was originally written up for publication as a book chapter. The introduction was thus a deliberately broad discussion of astrobiology. Publication plans changed and the paper was then to appear in a special issue of a journal, but unfortunately the original style was retained. Change: The broad introduction has been removed and replaced by a more focussed introduction on the specific question addressed in the paper.

Reviewer 3

Sent on 27 Nov 2019 by Frédéric Foucher | Not approved
CNRS, Centre de Biophysique Moléculaire

Dear authors,

I have read with great attention your manuscript entitled “Organic Molecules: Is It Possible to Distinguish Aromatics from Aliphatics Collected by Space Missions in High Speed Impacts?” submitted for publication in Sci.

Unfortunately, I think this article is not suitable for publication for several reasons, the most important among which are:

- The relevance of the samples used – 20 µm diameter spheres of polystyrene and poly(methyl methacrylate) – is very poor. The fact that these samples are destroyed during the impact does not demonstrate anything regarding the potential degradation of other organic molecules entrapped within Enceladus icy particles. The physical properties and the nature the samples are crucial. The sample mass is also of primary importance, not only the speed, since the kinetic energy is a key parameter. 

- The way of writing is not corresponding to a scientific publication; it is rather a spoken style.

- The article is badly structured: the introduction is 4 pages (approx. 200 lines), the materials and methods are less than 20 lines, results are only 15 lines and the discussion is only 1 page (approx. 50 lines).

- The first two pages (first part of the introduction) are totally out of the scope of the article, with no link to the experiments.

This study requires further investigations, with complementary experiments and analyses, and the manuscript must be fully reorganised prior resubmission, therefore I do not provide a detailed review including line by line comments.

Best regards

Response to Reviewer 3

Sent on 15 Jul 2020 by Mark Burchell, Kathryn Harriss

We thank the referees for their work General comments: All the referees noted and objected to a somewhat imbalanced paper with an overly long introduction. This was due to some confusion. The paper was based upon an hour-long open lecture, and (as one referee seems to have spotted) was originally written up for publication as a book chapter. The introduction was thus a deliberately broad discussion of astrobiology. Publication plans changed and the paper was then to appear in a special issue of a journal, but unfortunately the original style was retained. Change: The broad introduction has been removed and replaced by a more focussed introduction on the specific question addressed in the paper. Regarding the comment that energy is the key, this is not quite right. The energy density is what is important. Further, the mass is constrained, as the particles which may be present in the plumes are limited in size, so we can't just imagine large particles.

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