Mechanical Properties of Cometary Surfaces

Round 1

Reviewer 1 Report

The authors report on the mechanical properties, especially the strength and porosity, of comet 67P/Churyumov-Gerasimenko based on the available data from Rosetta and Philae. The paper describes and discuss the strength of 67P materials and its correlation with size. The authors discussed a published data, and they did not present any new or relevant results compared to the published papers in the literature about this comet. In its current form, I do not recommend this work for publication in the MDPI Journal. I encourage the authors to develop the content of the article by presenting clear and new relevant results concerning the mechanical properties of cometary surfaces, and in particular for comet 67P which has been widely studied.
Specific comments:
The authors must put a lot of effort for improving the style and language of the paper by a deep revision of the text and try to explain the ideas with a clear and short sentence. Avoid repetitions in the text, especially between the method and results sections. The quality of the figures (resolution, no grid for figure 3…) must be improved and keeping the same format.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper offers a review of results found so far on mechanical properties of cometary surfaces, with particular focus on 67P and the Abydos site. This paper is a good reference for the future and gives a snapshot of the current knowledge about cometary materials strength.

The paper seems to be written very fast. More citations would be needed. Tables are sometimes not easy to read. Language is often too colloquial for a scientific paper.

While I do not have major comments on the scientific content, I encourage authors to improve the paper presentation.

I recommend publication after moderate revisions.

 

Line 18: Citations in the abstracts should follow a different style, see editorial guidelines

Line 27: “the material a comet is made of” Change in “cometary materials”

Lines 28-31: Review punctuation in this paragraph to make it more clear.

Line 36: Why capital letters?

Line 40: It is based on the current state of art, do not need to specify is incomplete.

Line 53: add space after bracket

Eq. (1): Since you are defining bulk porosity, the first equation to insert is the second one. Then you specify the relation between bulk, macro and microporosity, explaining the reason why it is necessary to consider the denominator term (1-e_micro).

Lines 60-62: add references

Line 67: clarify what reported in the brackets

Line 99: recall here these strength definitions

Line 105: needS to be given…

Line 109: add references

Line 115: change in “tensile and compressive strength are 10-20 times lower”

Line 117: add example

Line 188: “that” is repeated

Line 123: typo in “capacity”

Section 2.3.1. Highlight subsection titles

Lines 140-142: all the symbols in the equation are not defined.

Lines 149-150: add references

Line 157: “remove “of course!”

Line 168: define “failure envelope”

Line 187: change in “with uncertainty of about 10%”

Line 191: add reference

Line 193: remove “quasistatic, size scale…”

Table 1 is difficult to read, add horizontal delimitators

Line 202: “factor 4” -> “4 times”

Figure 4 is not referenced to in the text.

Lines 219-221. There are many references about the 67P nucleus porosity. Add a number of them.

Line 231-232: “that are something else”. I suggest rewording, saying that different definitions of strength are adopted in literature, and a particular care is needed when comparing results.

Line 232: includeS

Lines 240-241: language is too colloquial.

Line 249: cliff and spires are prominent features…

Line 266: add reference

Lines 275-277: private communication does not add information with respect to cited papers. I suggest removing it.

Lines 284-285: add references for both Rosetta and Deep Impact.

Line 288: reference missing

Line 314: reference missing

Line 323: I understand that it is beyond the scope of the paper, but a few words about possible explanations may be useful

Line 325: Is this a new subsection? Or part of the previous subsection?

Figure 5 is not referenced to in the text.

Introduce what is the MUPUS, CASSE and SESAME experiments before to mention its main results.

Line 355: by -> be

Lines 393-399: references are needed for the values given here.

Line 419-426: How is large the obtained thermal conductivity on Abydos with respect other comet terrains? It should be one order of magnitude larger. Then, how much does it depend on the assumptions made on dust-to-ice ratio and thermal inertia? How much would it change if we consider other values among those suggested in literature?

Line 430: explain what Digby theory is.

Figure 6 is not referenced to in the text.

Line 472: WE believe

Summarize in the conclusions the values found for different strengths (and reported in Figure 6).

 

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear editor and authors,

 

Please find below my review of the paper "Mechanical properties of cometary surfaces". I am not a comet expert nor deeply familiar with the concepts and discussion in this paper, so I read it from more of a general planetary scientist's perspective to try to provide feedback on the content of the paper that could offer additional clarity for a reader with a background similar to my own. My comments are all minor, so once they are addressed, I recommend this paper for publication.

 

Section 2.1:

 

Par. 1, lines 2-4 & 5-6: "In the physics literature, this is commonly referred to as the packing fraction or packing density φ, defined as the fraction of space occupied by particles." and "We will use ε for porosity = void fraction, η for packing density or packing fraction..."

I noticed that you first introduce the packing fraction or packing density as phi, but then in the next sentence choose a new symbol (eta) for packing fraction. Is there a reason for not either first introducing the packing fraction as defined by the symbol you choose to use in the paper (eta) or explaining why you choose a different symbol (eta) than is common in the physics literature (phi)?

 

Equation 2 & Par. 3, line 1: It appears that you use nu in equation 2 but a script "v" in the text for "the volume fractions of particles with microporosity ε_i"?

 

Par. 4, lines 1-3: "The (macro-)porosity of a random packing of equal-sized, non-cohesive spheres is within the loose and dense packing bounds, 36.5% and ~42-44%, the latter depending on friction and stress."

Where does the very specific value of 36.5% here come from? There's no reference or context given.

 

Section 2.2:

 

Par. 1, lines 4-7: "There are various mechanisms, like cold or hot pressing sintering involving solid diffusion, cold-welding, or, more important for cometary matter, recondensing volatiles forming sinter necks around particle contacts."

I think this is an incomplete sentence. What are the "various mechanisms" for/what do they do?

 

Figure 1 caption: The top and bottom left panels are described as "illustrations", but the right panel is described as a "two dimensional illustration". I don't think the "two dimensional" aspect of the right panel description is needed.

 

Section 2.3:

 

Par. 1, lines 14-15: "...strain rates are naturally order of magnitudes higher than for almost all other processed we regard here..."

Should "processed" be "processes" here?

 

Par. 3, line 1: "Note that that in this paper..."

An extra "that" appears here.

 

Par. 3, lines 4-5: "...between two plates of the same radius.in a compression-testing machine..."

Misplaced period.

 

Par. 4, line 1: "It is different from the '(ultimate) bearing cacacity’ used in soil mechanics..."

Two things:

1) Is "cacacity" meant to be "capacity" here?

2) Why is " '(ultimate) bearing cacacity' " underlined here? It seems unnecessary.

Par. 4, lines 2-3: "...(b) exists even in cohesionless granular matter with friction angle φ..."

Phi is now used again here for friction angle. Even though it wasn't chosen as the symbol to represent the packing fraction in this paper, it is a bit confusing to have the same symbol show up again in the paper now referring to a different quantity.

 

Par. 4: This paragraph is the most hard to follow for me in this section. The other paragraphs describe the terms being used a bit more and put then into a bit more context than in this paragraph. Can you elaborate slightly on what "a semi-confined situation (intruder on semi-infinite soil)" is?

 

Par. 5: Why is "Penetration resistance" underlined and "Penetration" capitalized. Both seem unnecessary.

 

Section 2.3.1:

 

Par. 1, line 1: "Strain rate dependence."

It seems a bit out of place to have an incomplete sentence here. Perhaps a colon after this phrase would make more sense?

 

Par. 1, lines 1-3: "It is known that the compressive strength of brittle materials (note that ice additionally flows plastically at very low strain rates), depends on the rate of deformation: the faster the deformation, the greater the measured strength values."

I found this sentence a bit hard to follow. Perhaps if the parenthetical phrase is moved to the end after a semi-colon it would flow a little better, e.g., "It is known that the compressive strength of brittle materials depends on the rate of deformation: the faster the deformation, the greater the measured strength values; note that ice additionally flows plastically at very low strain rates."?

 

Par. 1, lines 3-7 & Equation 3 (? unnumbered): "All brittle solids, including geophysical and engineering materials, appear to follow a universal dependence of the compressive strength on the strain rate...where ε_dot_0 is a material constant, of order 200 to 1000 s^-1 for terrestrial rocks, ordinary chondrites, and concrete (Ramesh, Hogan et al. 2015). Thus a rate effect of σ_c is expected..."

Which part of this equation is the "compressive strength" and which part is the "strain rate"? Is σ_c the strain rate? What is ε_dot and ε_dot_*? More explanation of the terms in this equation would be helpful.

 

Par. 2, line 1: "Size dependence."

It again seems a bit out of place to have an incomplete sentence here. I think a colon after this phrase might make more sense.

 

Par. 2, line 2: "...it generally decreases with increasing size..."

What generally decreases with increasing size? The strength or the volume?

 

Par. 3, lines 4-5: "Note that the 1/d^1/2 dependence (LEFM) is the asymptote at large sizes...."

What is LEFM? Please define.

 

Section 2.3.2:

 

Equation 4 (? unnumbered): sigma, phi, and "c" have each already been used to represent other things in this paper (sigma was (I think) the strain rate; phi was first packing fraction then friction angle; "c" was a cohesion term). I think perhaps phi is used the same way here as one of the previous instances, but sigma and "c" have already been used for different quantities.

 

Section 3:

 

Par. 1, lines 8-9: "Uncertainty: at least one order of magnitude."

This would flow better as a complete sentence.

 

Par. 3, line 2: "Quasistatic, size scale 1-10 cm"

This appears to be another incomplete sentence.

 

Table 1: Some of the comments in the right-hand column appear misaligned with the rows in the middle column. Is that because they relate to multiple rows in the middle column? It's a bit confusing to see what the comments are referring to with the current formatting.

 

Par. 5, lines 1-2: "The dependence of different strength measures for a temperature of about -15°C on the density of porous ice is shown in Fig. 1."

Do you mean Fig. 4 here?

 

Par. 5, line 16: "The ratio of Youngs module E and σ_c was found ~100."

It would be helpful to redefine σ_c here.

 

Par. 5, line 17: "...with a density of on 250 kg m -3."

"of on"

 

Section 4.2:

 

Par. 5, lines 2-5: "...(Groussin, Jorda et al. 2015) derived a compressive strength of the consolidated material that most likely is in the range 30 – 150 Pa (overhangs, scale 5 m), with an upper limit of 1.5 kPa (fine surface materials, 10 cm scale); collapsed structures <11.6 kPa (1 m scale)."

I don't fully understand the end of this sentence (after the semi-colon): "...collapsed structures <11.6 kPa (1 m scale)". It reads as another incomplete thought.

 

Par. 11, line 5: "...report a compressive strength of ≤ 12 Pa..."

Why is "compressive" underlined here?

 

Section 5:

 

Par. 3, line 2: "...ee believe..."

"ee" --> "we"?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear,

For the first time, I did not notice that it was a review paper as it was not clearly stated in the abstract. Thus, since the authors have improved the quality of the paper content, I recommend the paper for publication in the current format.

Best regards,