The Temporal Variation of Optical Depth in the Candidate Landing Area of China’s Mars Mission (Tianwen-1)

Round 1

Reviewer 1 Report

Uncharacteristically, I don't have comments to this manuscript.

Author Response

Thank you for the constructive and insightful criticism and advice. We have the responses to your comments in the attached word file.

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

A really interesting report. There are a few places where the English could be improved. Here and there an "in addition" or similar inserts. Also, you like the word "respectively". Please check once if it was always used correctly. Furthermore there are one/two repetitions. Please check this as well.

For additional proposed changes, please see the attached pdf file.

Comments for author File: Comments.pdf

Author Response

Thank you for the constructive and insightful criticism and advice. We have the responses to your comments in the attached word file.

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript describes the application of an existing method to retrieve the dust optical depth (OD) in the candidate landing site of the Tianwen-1 mission. The authors apply consolidated techniques to derive atmospheric OD on a case study and on the landing area, comparing in the first case the retrieved OD to satellite products. 
Overall, the manuscript may be of some interest as an application for the Tianwen-1 mission, however there are several issues that needs to be addressed; among them, the lack of clarity in some of the passages to explain methods and results, and the lack of proper references to past works to explain the state of the art of the knowledge of dust climatology on Mars. 

Here a series of recommendations to the authors:

1 - Line 28: Martian wind is only the last element in a chain of causation that generates storms (especially global dust storms). The authors may also want to reference previous papers that explain this issue in detail, such as the [21] already in the references.

2 - Line 34: The spatial resolution of an image per se depends on the instrument itself. The information content of an image, and the ability to discern surface characteristics, depend on dust opacity. The authors should rephrase this sentence.

3 - Line 42: There is a great deal of dust assimilation studies conducted in the last 10 years. Some of them are here, and the authors shall consider expanding the cited literature to give proper acknowledgement to the extensive work done in this sense:
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2012JE004097
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017EA000274
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2011JE003815

4 - Lines 49-52: This sentence is vague and confusing, and looks like is a repetition of what has been said in the previous one. Without referencing, it does not really serve the purpose of what the authors want to explain.

5 - Line 59: Here, as for previous comments, the authors should expand the cited literature. There are countless works that show how to calculate dust and water ice optical depth from satellite, especially in recent years. Here some suggestions:
https://www.sciencedirect.com/science/article/pii/S0019103515000044
https://www.sciencedirect.com/science/article/pii/S0019103503002872
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006250

6 - Line 65-66: The authors should explain why this is considered an issue with respect to the present work.

7 - Line 67: besides [7], more recently there is also Smith [2019] https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006107

8 - Line 74-75: supposing that this is the other approach, a newline here might make the text clearer.

9 - Line 77: Is there any reference for MER retrievals of optical depth?

10 - Line 82: This correction factor is referenced correctly, however it would be best to explain briefly where this number comes from and what is the applicability limit of it.

11 - Line 84: what is the reference for HiRISE shadow method? And for Spirit data?

12 - Lines 127-133: Is the calibration done by the authors? If so, this paragraph does not report the necessary quantitative information to evaluate the quality of the calibration.

13 - Line 136: the link reported here does not exist. 

14 - Lines 138-141: Are the images exactly colocated in time and space? Their footprints should be plotted in Figure 1.

15 - Line 159: better "radiation extinction" rather than "attenuation of solar radiation" (also atmospheric and surface-emitted when observing from satellite must be considered).

16 - Line 175-176: this sentence is not clear.

17 - Lines 182-183: In [8] it is clearly explained that this approach has many caveats. Some of them are correctly mentioned by the authors, however, Ts calculated as shown is only related to the true optical depth T. Therefore, a correction factor must be applied to reckon the actual T from Eq.(1).

18 - Lines 201-205: This is too qualitative and "several km" and "hundreds of meters" must be better quantified, eventually with references (perhaps [8])?

19 - Figure 2: The lines are poorly visible, especially the green ones.

20 - Lines 220-221: It is not clear what "statistically" means here. Is it intended to mean that it is calculated pixel-by-pixel and then on the distribution of values obtained by each of them?

21 - Figure 3 is missing a legend box with the meaning of the colors. It seems like, in addition, the red error bars have not been rescaled with the values. The correction coefficient (0.7?) is never clearly mentioned in this section of the text. Finally, the "number of dust storm activity" is never introduced in the text, therefore its relation with the OD is not clear.

22 - Line 244: There are countless studies about the seasonality of dust acrivity, yet the manuscript does not mention any of them. The authors should consider to better acknowledge the work done with specialized sensors such as TES (Smith, 2004), MCS (McCleese et al., 2010), CRISM (Smith et al., 2013), and more recently NOMAD (Liuzzi et al., 2020) and the 8-year climatology by Montabone et al. (2015).

23 - Line 247: In reality this interval extends more from Ls 180 to 360, where Global dust storms occasionally occur, and type-C regional storms happen (around Ls 310 to 330 usually).

24 - Line 248: saying "less frequent dust storm sequences" is misleading and approximative. Usually "dust storms" do not occur in this season due to seasonality (Southern Hemisphere winter), and only occasionally dust is lifted from the equator and Hellas regions on a regional scale. This should be better rephrased to properly reflect the current knowledge.

25 - Lines 253-254: This is an interesting point, although it can be seen that no retrieval has been attempted to quantify the OD of dust at the peaks of dust activity in Fig. 3, which are of the most interest for a rover mission. The authors should include at least some retrievals in this temporal instances, and verify whether or not the correction factor holds in these more extreme cases.

26 - Lines 273-275: This is repeated from previous text.

27 - The conclusion lacks of proper discussion of connections with the introduction, and how this work will effectively impact the science and operations carried out by the Tianwen-1 mission.

Author Response

Thank you for the constructive and insightful criticism and advice. We have the responses to your comments in the attached word file.

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors have responded to all the questions and comments raised in the first round of reviews.

The manuscript still requires some English editing (e.g. there are several sentences that are the continuation of the previous and that start with "And" after a full stop) before the manuscript can be published.