Salt Constructs in Paleo-Lake Basins as High-Priority Astrobiology Targets

Round 1

Reviewer 1 Report

Overall Evaluation: The study by Phillips and co-authors is a novel one that documents salt constructs for two Chilean salars, highlighting their form in aerial imaging and compositionally-related formation mechanisms.  The authors use these foundational ideas to evaluate candidate martian salars, ultimately identifying 16 locations and illustrating commonalities with the Chilean salar reference model.  It is a timely investigation that will be of interest especially to martian researchers and it adds to the criteria for site selection for future Mars landers.  Furthermore, the authors propose an alternate model for several common meter-scale textures observed on Mars and typically ascribed to ice-related processes, but plausibly could be salt morphologies as the authors convincingly demonstrate. The study is well written, suitably illustrated and is recommended for publication with very minor revisions.  Below are some questions and comments for the author’s to consider, several of which are largely targeted to future work. 

Detailed comments: 

With multiple locations featured in this work, please ensure that adequate geographic references are made in the text.  Here are examples from two areas of the manuscript where minor text revision is useful to the reader.

Line 462: Topographic constructs were observed in all but one of the **martian** scenes investigated in this study.

Line 545: Across two salt-encrusted environments **in northern Chile**, one in the Atacama and the other in the Altiplano, with distinct evaporite mineralogy….

Section 1.1 and/or section 1.3.2:  Text and references here could be augmented to document the habitable environments in gypsum domes.  The illustration in figure 2 are for halite and the citations are exclusively for halite even though there is abundant literature on gypsum as a habitable niche.  Can more be added regarding gypsum habitable niches in the scenario where gypsum forms abiotically?

The author’s emphasize the morphological differences based on composition for their two sites (halite and gypsum).  However, there is no discussion of what morphological constructs would be expected for the composition on Mars that they evaluate (chloride) if it were another mineralogy, such as calcium chloride.

Section 3.2 In future work, paleolake sites could also be used as a geographic criteria to identify sites that likely experienced fog or precipitation. It is interesting the authors use modern deliquesce potential as one of their selection critiera for evaluating martian salars. when one of their documented salt-encrusted models (gypsum) is not reliant on deliquesce. 

Clarify that clay refers to phyllosilicate mineralogy, rather than grain size. 

 Do the authors wish to comment on the relevance of their study for active Mars rover investigations which have identified abundant sulfates?  Are there morphologies in the existing rover images that are consistent with those observed at the Chilean salars in this work?  Are there diagnostic criteria at the rover scale that could be added to this paper as part of the search strategy for the Curiosity and/or Perseverance rovers?

This study used meter-scale HiRISE images to evaluate martian salars.  There is additional images available at comparable resolution (MOC) that could be exploited to identify additional candidates in future work. 

Add lat/lon coordinates to figure 1.

Although not an issue for reviewing this manuscript, some of the supplementary files were missing for me.  From the reviewer portal, the ‘download supplementary file(s)’ link only resulted in Table S1 (Excel file), and did not include the two figures.  Figures A1 and A2 are in the PDF manuscript for peer review, but the first one is supposed to be an animation. 

Figure 3C:  Text labels are small and may be unreadable in final publication if figure is further reduced in size.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors utilize information acquired from potential analog sites on Earth to better understand areas interpreted as salt playas/salars on Mars, particularly with regards to features that could be of potential exobiological interest.

The topic is clearly of interest to general and specialized readers and could potentially advance our understanding of salt-bearing regions on Mars. However, the current analysis suffers from unsubstantiated claims and conclusions that are not fully supported by the observations. General issues include:

1) The study does not provide sufficient data to support the conclusion that changes in radar properties of analog cites can be interpreted as seasonal changes. Looking at multiple years could be used to support such a conclusion.

2) The study dedicates a large portion of the paper to discussing regions that are thought to be areas where "deliquescence of calcium-perchlorate is possible", and conclusions are drawn based on this interpretation. However, no adequate discussion of criteria to identify such terrains or locations is provided, nor an explanation as to why certain regions would be identified as such, while others are not.

3) There are multiple descriptions of features in HiRISE images focusing on their false-colors in the IRB products, which is of limited scientific value (for example, referring to locations as being orange or lavender in color). A more scientific description would be to describe their tone/brightness in certain filters, texture, reflectance based on the brightness levels acquired from the three HiRISE filters, etc.

4) Certain hypotheses regarding the formation and evolution of salt bearing terrains are presented without adequate support from observed data, or potential analogs on Earth. Particularly, a hypothesis involving salt precipitation due to hydrothermal ground water or glacier-supplied water bodies submerging pre-existing periglacially modified terrain, is not supported by the data nor is such a sequence observed on Earth presented to explain how this would work on Mars.

5) There are numerous editorial issues, but they are most probably attributed to the process of exporting the manuscript into the specific journal template leading to captions getting mixed up with body text among other issues. One particular case is the table provided, which has no caption to describe its contents or the implications of the different colors used in the table cells.

 

Specific comments and additional edits can be found as annotations in the attached PDF of the submitted paper. The paper is currently in need of moderate to large revision to be considered for publication.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Other contributions that merit to be cited (from Puna and Atacama): papers of Manuel Contreras et al. (1998) Extremophiles and the origin of life in Atacama, Romina D’Almeida (2019) Novel nematode species in living stromatolites in the Andean Puna Новый вид нематод из пресноводных строматолитов Андского плоскогорья and the references cited therein. Canadian and Chinese places, I don’t know

Other contributions that merit to be cited (from Puna and Atacama): papers of Manuel Contreras et al. (1998) Extremophiles and the origin of life in Atacama, Romina D’Almeida (2019) Novel nematode species in living stromatolites in the Andean Puna Новый вид нематод из пресноводных строматолитов Андского плоскогорья and the references cited therein. Canadian and Chinese places, I don’t know.

Author Response

Thank you for the suggestions of additional citations. We have added these to the text in an appropriate location (section 1.3.2, citations 46 and 47).