The Geological History of the Chang’e-5 Sample Return Region

Round 1

Reviewer 1 Report

Anticipating the results of the analysis of the first lunar samples to be returned in over 40 years is hugely exciting and thus a paper describing the geologic history of the CE-5 landing site region is very much warranted. Overall the paper does a nice job of analyzing a range of remote sensing datasets to characterize the landing site region. However, it not immediately clear how much of an advance the work described in this manuscript is over other published papers that have explored the landing site region. I also believe the analysis of the data presented could go further in providing more significant context to the samples collected by CE-5. I therefore believe this paper should only be published after major revisions. 

Major comments:

I understand the rationale about having a unit name and also a stratigraphic sequence tag, but using the two interchangeably is confusing. I would suggest that when a stratigraphic tag is referred to the unit name should also be quoted (e.g. U7/Em5 - as is used in table 3). Also the abstract mentions Em5 without defining the unit. 

The geological units presented in the paper are very similar to those of Zhao et al [2017] and Qian et al [2017; 2020]. Indeed some of the units have the same name/stratigraphic tags, while others are switched. For example IR1 - 3 are the same where as Im1 - Em5 are different. This is acknowledge in the paper, but as new analysis is presented, the manuscript needs to justify why some boundaries remain the same and other differ, and what this means for the geologic history of the region. A figure comparing the previous and new map boundaries would be very helpful   

A major challenge associated with the returned CE-5 samples will be establishing which represent the geology of the landing site and which have been transported from afar by impact processes. I feel more analysis could be presented in this paper to aid these efforts. For example, in the discussion it states that unit U7/Em5 (the unit CE-5 landed on) contain multiple flow units with varying amounts of TiO2. The range in TiO2 quoted could be due to contamination from ejecta (both from the multiple internal large craters - which are likely sampling underlying basalts - and from rays that cross the unit). More analysis is required to distinguish ejecta from uncontaminated portions of the unit. The TiO2 map is only sensitive to the upper few nanometers and so much of the range of TiO2 values quoted will be due to sub-pixel mixing and will not reflect the actual chemistry of returned basaltic samples. I suggest that ejecta blankets and rays are mapped out across the unit to better constrain possible sources of contamination. 

The age of the youngest wrinkle ridge is misleading. In the manuscript the age estimated is only 320 Ma, whereas in the supplementary material it is revealed the age could be between 320 Ma and 3.2 Ga. This needs to be explained in the manuscript.

Minor comments. 

Fig 1 - How was the boundary of Oceanus Procellarum mapped? If it was from the LROC site please cite: D. M. Nelson, S. D. Koeber, K. Daud, M. S. Robinson, T. R. Watters, M. E. Banks, N. R. Williams (2014) Mapping Lunar Maria Extents and Lobate Scarps Using LROC Image Products 45th Lunar and Planetary Science Conference, Abstract #2861

Introduction:

Last sentence of paragraph 3, I assume you mean not consistent, rather than not inconsistent?

First sentence of paragraph 4, magmatic activities should be volcanic activities 

Results
3.1 - I don't believe the the 5km D crater can be identified as a secondary crater by its d/D ratio alone. It could simply be a degraded crater. Are other similarly shallow craters in the vicinity that would lend weight to this interpretation?

3.2.1 the text states: "the main components of lunar basalt are iron and titanium" This is an odd phrasing and not really accurate. I suggest you rephrase it to lunar basalts can be "characterized" by their iron and titanium content. 

As iron and titanium are used to characterize lunar basalts, it seems redundant to comment on the equivalent values for the Highland portions of the study region - or at least inform the reader that the lowest TiO2 and FeO values correspond to highland terrains. 

Fig 3. It would be helpful to state that map (a) is FeO abundance and map (b) is TiO2 abundance.

3.2.2 typo "res-yellow" should be red-yellow

3.3.1 Hiesinger et al needs corresponding years of publication.

The text states FeO and TiO2 concentrations are outlined in Table 2, but this only shows age and stratigraphy estimates of the mare units. 

4.1 "magamatic activities" should be volcanic activity 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

General remark:

Procellarum KREEP Terrane (PKT) is one of the most interesting areas of the Moon. It is a structure that has no terrestrial analogues. Its investigation can provide a better understanding of the Moon's origins and evolution. No wonder that PKT became the target of the mission. The authors use the results of Chang’e 5 mission extensively, but of course they also benefit from the results of other missions, including recent Chandrayaan-1. Focusing on recent missions is an important benefit of the work. Their results are interesting.

Some minor (but important) problems:

1. Different ways of citation, i.e.:  [12], Author{2011], Author [12]. I suggest unification.
2. Use proper sizes of parentheses in formulas, please.  
3. R in formulas (5, 6) is not explained. Are references to these formulas correct? 
4. Minor spelling/grammar  problems, i.e.: "spectra of Fresh pyroxene" (in Fig. 5 caption), "res-yellow", "mthod", etc. 
5. The white bar (100 km) is usually invisible in figures. Please check the manuscript carefully. There are probably more similar mistakes.

Please check the manuscript carefully. There are probably more similar mistakes.

 

I have found a few groups of  problems in the manuscript.

1. The first group deals with a number of details such as editorial, linguistic, grammar, and spelling mistakes (see above).  The removing of them shouldn't be a problem. The work should be just carefully reviewed by the authors. Generally, the manuscript is written in a good language, but it might be useful to consult a person whose first language is English.
2. Description of missions and their results in Introduction. I suggest authors put here more references to the works where one can find the most important results of the recent lunar missions. It would be an important help for scientists interested in the Moon.
3.  There are several interesting results in the paper. The problem is with the way the results are presented. It makes some problems to review the manuscript. It especially concerns the chapters Results and Conclusions. A number of results are given for the age of the areas, mineralogy and age of rocks, etc. However, in some cases it is difficult to see (from the text) which results are the first published results, and which ones are those that have already been published. With already published results, references to the sources must be provided. With the results obtained by the authors and not yet published, it is worth noting this fact explicitly.
4. I suggest some expansion of the last chapter "Conclusions". I believe that the number of results justifies such expansion.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

A nicely written manuscript with important results relative to dating of the lunar surface. I have few edits other than to note that the use of "And" to start a sentence is not proper English usage. Those sentences appear to be added since the version of the manuscript I received for review appears to be an edited version of an earlier manuscript. So it appears that the authors quickly added material suggested by earlier reviewers by simply inserting sentences without thinking about how they actually fit into the text.

For example: lines 172-177 currently read:

"From the profile (Figure 2b), it can be seen that the depth-to-diameter ratio is less than 0.2 (D = 5 km, depth = 0.7 km), which indicating that the rim-to-floor depth of crater A shallower than the primary impact crater. In addition, 
the impact crater is elliptical and there are similar shallow craters in the eastsouthern of the crater. Therefore, it is reasonable to infer that the crater near the mare unit may be the secondary crater."

This is poorly written and should be corrected to:

"The profile (Figure 2b), shows that the depth-to-diameter ratio is less than 0.2 (D = 5 km, depth = 0.7 km), which indicates that the rim-to-floor depth of crater A is shallower than the primary impact crater.  This impact crater is elliptical and there are many similar shallow craters in the southeastern portion of the primary crater. Therefore, it is reasonable to infer that the crater near the mare unit may be a secondary crater."

Every new insertion in the text (blue underlined on my copy) must be checked and corrected for proper English usage. This must be corrected before final acceptance since as the manuscript currently stands there is a great story that is muddled by poor English usage in the inserted text.  

The Supplemental materials are fine but could use a bit of text for Figure S2. There are 10 parts of this figure but no detail is provided other than the text which reads;

Figure S2: Kaguya TC images of distorted craters and their respective topographic profile using TC DTM data.

And as far as I can ascertain from searching the main text only Supplemental Figure 4 (S4) is actually called out in the text. S1, S2 and S3 need to be referenced from the main text. for instance Figure S3 makes reference to Moore et al. crater morphologies yet with the exception of a reference to Moore et al. on line 399 there is no direct connection between the main text and Figure S3.

In the text for Figure S4 (lines 32-33) there is another example of poor English usage, namely;

"Therefore, the youngest age of young ridges must be <320 Ma."

should be;

"Therefore, the minimum age of young ridges must be <320 Ma."

In the supplemental text the parts are initially listed as sections, but then "sections" disappear. Is there some valid reason for the use of the term "Section". If not remove it.

Another example (one of many) of poor wording that must be corrected from lines 409-410:

"And the age of the oldest wrinkle ridges is between 1 Ga and 3.2 Ga and the exact age cannot be figured out."   Once again a sentence starting with "And" that is poorly written.

Should be something like "While the age of the oldest wrinkle ridges is between 1 Ga and 3.2 Ga their exact age cannot be determined precisely." 

Many more similar corrections necessary.

My recommendation is "Accept after minor revision" with final acceptance only after these issues have been corrected.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.