Dear Authors and Editors,

I had the pleasure to review this very interesting work titled: “Long Runout Landslides with Associated Longitudinal Ridges in Iceland as Analogues of Martian Landforms “. The study compares long runout landslide deposits with longitudinal ridges on Mars and Iceland. Since these morphological complexes are common in non-glaciated setting in Iceland, they act as extraordinary analogues for their Martian counterparts. The morphology of one specific landslide in Dalvík, Iceland was therefore studied in detail and compared to remote sensing observations from Mars.

Two databases are presented in this study. One for Icelandic landslides that have developed longitudinal ridges and one for Martian landslides with a runout distance of <5 km that show longitudinal ridges. The continuous development of such databases is an important contribution for the understanding of landslides in general and is valuable for future investigations.

The paper is generally well written, follows a logical structure, and is well illustrated. The applied method of comparing landforms on Mars with analogues on Earth is a widely accepted approach to infer parallels between depositional and environmental conditions during the formation of the described morphological features.

I added several correction suggestions for the presented figures including a more concise combination of figures 1, 4 and 6.

Furthermore, I requested a clarification in the text regarding why the presented Martian database was limited to a seemingly arbitrary runout distance of 5 km and suggest expanding it to a complete database which would certainly increase the significance of this work. However, I am aware that this would require a significant time investment and might be out of the scope of this work. Thus, a clarification of the 5 km cut-off value within the methods will be sufficient.

The discussion and conclusion should be expanded based on the descriptions presented in this work. It would be great to add a paragraph discussing why Icelandic landslides commonly form longitudinal ridges compared to other places on Earth and what we can infer from that for conditions on Mars and vice versa? This was the central theme of the introduction, and it is reasonable that the authors outline their thoughts on this in an additional paragraph. Adding this paragraph will round up the manuscript by reconnecting with the subject outlined in the introduction. Furthermore, it will increase the significance of this work. The conclusion paragraph should then be updated.

I added numerous comments with improvement suggestions to the attached pdf documents. If the authors decide to implement them all, it will require a significant time investment, which is why I chose the major revision option which gives the authors more time. However, some of these suggestions might be easily clarified in the text and the review may therefore be completed much quicker.

Thank you again for giving me the opportunity to read through this manuscript. I genuinely enjoyed reading it. With some extra work this study should become an excellent piece of work. If there are specific questions regarding my comments the authors may contact me directly.

Sincerely, 

Daniel Ben-Yehoshua 

Overall, this is a very useful addition to the study of long runout landslides, and in particular, the improved understanding of these features on Mars. It is mostly very well written and clear. Some statements regarding the suitability of the landslides studied on Iceland as appropriate analogues for Mars seem to lack support, and discussion of important aspects appears to be absent. This includes consideration of the different gravity and atmospheric pressure on Mars, and also the inevitable ambiguity of the role of surface snow or ice in the formation of the Icelandic landslides whatever their age. But these are relatively minor considerations that require some thought, and then small additions and/or edits to the text. I certainly found the paper extremely engaging and it has prompted me to think again about landslide generation on Earth and Mars. I had some problem understanding the logical flow of the argument in Section 5, and I think some editing of this section to help “hesitant” readers; I have added comments on these issues below. Probably the most constructive way forward is for the authors to add some text to this section to explain their thinking on these matters, but retain their clearly expressed opinions.

Line 33. Saying “such as” suggests that there is a longer list of specific distinctive morphologies, and it seems odd not to provide a full definition here that includes a complete list (perhaps in grouped or truncated form if the list is long).

Line 44. Change “are a necessary” to “is a necessary” to agree with “the presence” which is singular.

Line 49. “in tectonic context” – modify to “in a tectonic context” or “in tectonic contexts”.

Line 55. The term “slope stratigraphy” here is a little hard to interpret. I think the authors are referring to the original stratigraphic layering which is often preserved within the landslide mass, and observable in the slope at the point where the landslide started. Perhaps clarify this point?

Line 61. The words “these landforms” refers to glaciers, but I’m not sure what “and associated morphologies” means here.

Lines 71-72. The phrase “that either longitudinal ridges are indeed common in long runout landslides on Earth” appears to me to have an additional required element missing to work as an explanation of the observation. If they are indeed common, why have they not been found commonly elsewhere, or is there a preservation bias on Iceland (for example, being young in age, or at lower elevation, or some other factor)?

Line 80. From a consideration of the different gravity on Mars, would a good analogue be expected to be the same length as on Earth? Does the fact that the authors consider similar sized landslides as comparable between Earth and Mars suggest that they reject any role of atmospheric pressure in providing a gaseous lubricant during landsliding, as this is very different between the two planets?

Lines 81-83. The authors say “long runout landslides share similar morphometric values and diagnostic structures, such as longitudinal ridges…” Again, why simply provide one example of a similar structure when it would be possible to be precise and provide a more complete list of values and features?

Lines 83-84. Is similarity of morphology sufficient to conclude that Earth examples are “good analogues of martian landforms”, unless the authors simply mean analogues in terms of morphology? That is, can we conclude that similar morphology implies similar formation mechanism? Geomorphologists coined the term “equifinality” specifically to highlight the problem with this interpretation which is often found not to be the case on Earth. I feel that some discussion of the theoretical or philosophical implications of this statement is required to justify it here.

Lines 101, 106 and elsewhere. Avoid double parentheses.

Lines 127-128. My understanding of the term “paraglacial” is that it refers to processes following local deglaciation. That is, the ice front may be a few meters away only. The concept of “paraglacial period” is therefore potentially confusing to readers unaware of this. Note that an exponential decrease in mass-wasting might be consistent with much of this movement having occurred while there is still glaciation in the area (depending on the relative exponential lifetime of mass-wasting and rate of glacial retreat).

Line 147. Quotation of calibrated radiocarbon ages should be presented as a 2 sigma age range (as for the earlier ages); the present authors are probably quoting Mercier et al. (2017) when they provide estimates with +/- uncertainties, but these are not typically meaningful for calibrated ages. Please can the authors carefully check the original data, and recalibrate the raw data if necessary, to derive a useful age range here. Note that lab codes for these age estimates should also be provided (possibly in supplementary data, but with a citation here).

Line 149. Presumably the authors mean 1 km northwest (not northeast) of Dalvik; 1 km NE is in the sea.

Line 153. For a landslide, the “accumulation zone” is at the bottom (presumably). Is this what is intended here, or does this mean the upper part (somehow analogous to a glacier accumulation zone)? Possibly clarify the wording here, as it is potentially confusing.

Line 177. Change “is “ to “are” in “the statistics… is summarised…”

Line 183. In what ways does this “inclined surface” differ from the higher elevation slope? Why consider this part as distinct from the higher slope? Maybe there are morphological differences such as slope angle or roughness or concavity? Please add if this information is available, or simply refer to this as the lower slope.

Line 211. See Line 177 comment above. Change “is “ to “are”.

Lines 221-224. After reading and rereading four times, I still cannot understand these sentences. Are the ridges visible in CTX and HiRISE images or not? OK, in simple images, yes, but there aren’t DEMs available to determine their elevation characteristics? Is this what is being said here? It is very hard to follow.

Line 232. Add “do” before “they share”.

Lines 238-246. It seems very strange to me to have this discussion of the differences in size of landslides without mentioning the differences in gravity between Earth and Mars, and without considering potential differences in the atmospheric involvement. Why expect a scaling factor of 1?

Line 249. “The former suggests…” Surely this should be “The latter suggests” as the removal hypothesis comes second in the previous sentence? But why the sudden jump to clear expectation of an exact match in shape and size between Earth and Mars? If in one population removal of some landslides biases the estimates, then surely the comparison will be confused by relatively recent glaciation in Iceland, that was extensive around the LGM and probably retreated in the late glacial. Most or all of the landslides studied in Iceland are likely to be post-LGM (as found by the small dating sample), so earlier landslides in the Iceland population were certainly removed. Are there some assumptions about removal that are not discussed here? I can’t understand how it is possible to propose that a comparison can be drawn between these two populations as they are so morphologically similar, but then infer some geologic process based on the observed differences.

Lines 253-255. “Therefore, we suggest that the different population numbers reflect the removal of the geomorphological records of martian long runout landslides.” Again, I cannot easily follow the logic here. Why do you expect the populations to be identical? I thought the point of the paper was to suggest that they might be similar by comparing the statistics. If they are not the same, are they simply not the same? Or am I missing something fundamental here? I’m happy to accept that both populations represent snapshots of landslide creation, weathering, erosion and/or burial, with some fraction of landslides still visible. We know and can infer something about the timescales on Earth for this population, but on Mars, so far no chronological considerations were presented (though these do appear below). So I find this section hard to understand. In comparing the statistics of the two datasets, don’t you have to normalize for the available vertical drop? This (the vertical drop) clearly can’t be greater than the height of the mountain hosting the landslide. What about the frequency of large magnitude earthquakes? This is likely greater in Iceland than Mars, and an important trigger for landslides, but not considered here.

Line 260. More recent than what? Than the last 20Ma? Doesn’t that time period extend to the present? How many recent deposits (and what is “recent” in Mars terms?) would one expect when the available time period is 3.5Gyr? 1 landslide per million years in the target area would equal 3,500 landslides. Again, perhaps I’m missing something here.

Line 267. Replace “significant” with “significance”.

Lines 278-9. The H/L ratio vs elevation drop (Fig. 13c – note the letters need adding to the figure) shows what appears to be a very significant lower boundary in both datasets. This boundary is parallel in both datasets. This suggest to me there might be something significant about this, so I’m a little surprised that attention isn’t drawn to this feature.

Line 295. Should “decrease” here be “increase”?

Line 326. I wonder whether “structure behaviours” would be better as “characteristics of structures” or Structure characteristics”?

Figure 1. Is the longitudinal profile an average of the slope profile or one particular line? Consider adding letters to distinguish the different panels (perhaps an editorial decision).

There are no references to Figs. 2 and 3 until after Fig. 4 is mentioned, so change numbers or add figure citations. Note Fig. 13 is mentioned in the text before Fig. 7 and later figures.

Figure 3. Top right panel (add letters to these?) has incomplete white line.

Figure 7. Add north arrow, and consider showing location on Fig. 6?

Figure 8 caption. I think the word “which” (line 2) should be “with”.