Peer review of: Longitudinal ridges imparted by high-speed granular flow mechanisms in martian landslides (2019)

Reviewers' comments:  
 
Reviewer #2 (Remarks to the Author):  
 
This short paper uses the prominent longitudinal ridges visible on two lobes of a large landslide 
deposit in Valles Marineris, Mars to test several small-scale models of such ridge formation. While 
the authors of the present paper did not propose these models [ref. 16], they measured the ridge 
spacing on the Martian examples, and estimated the thickness of the landslide lobe to conclude 
that the principal control on the ridge spacing is the lobe thickness. They further argue from theory 
and laboratory experiments that a low-friction substrate, such as ice, is not required for ridge 
formation.  
 
This is an interesting and important conclusion and certainly merits publication in Nature 
Communications. Such longitudinal ridges on landslide deposits have long intrigued many 
observers and this paper will serve to focus discussion on the meaning of these features. The 
paper also notes morphological evidence (S-shaped cross-ridges) that support the inference of 
differential velocities between the crests and troughs of the ridges—another important observation 
and conclusion.  
 
I do have some small suggestions and concerns outlined below:  
 
Introduction, end of paragraph 2, Ref. 19 is cited for longitudinal ridges on the Moon, however the 
longitudinal ridges in the Tsilokovsky ejecta have long been known and described by many 
authors, starting with Guest in 1971. A mention and citation would be appropriate here (Boyce et 
al, LPSC 47, Abstract #2471 would be OK, but perhaps a full publication would be better).  
 
The authors cite a grain size of ca. 1-20 m for “grains” in the landslide, mainly derived from an 
image in Extended Data Figure 3. It is by no means clear what this size really represents. Because 
it is close to the limit of resolution of the images, this may merely be a largest grain size, rather 
than a representative size. My own experience with large landslide deposits indicates that the 
largest clasts usually work their way to the top of the deposit, with finer grained material below 
(this is especially evident in the deposit of the Frank, Alberta sturzstrom), in which case using the 
observed surface boulder size as a representative size may exaggerate the most effective grain 
size and, through their formulas for velocity, overestimate the inferred emplacement velocity. I 
would, in a revised discussion, like to see more acknowledgement that the slide lobe may contain 
a variety of grain sizes and that the surface boulder size may be a biased sample. While the 
theoretical and laboratory studies cited by the authors may have assumed a single grain size, it is 
most unlikely that a natural landslide would be so well sorted. Yet the readers of the present paper 
could easily get the impression that the authors believe that the slide exhibits only a narrow range 
of fragment sizes.  
 
Methods section: While the authors do describe how the thickness of the landslide deposits was 
measured, they do not provide any estimates on how accurately they could reconstruct this 
parameter. Given that the relation between ridge spacing and lobe thickness is one of the principal 
conclusions of the paper, I would have liked to see an error estimate of this surface, as well as a 
plot of ridge spacing vs. slide thickness (thus illustrating the data reported in Extended Data 
Tables 2 and 3). As it is, the reconstructed valley floor topography is shown as green lines in 
Extended Data Figures 1 and 2, but no error estimation or uncertainties are indicated.  
 
I have no objection to the authors knowing my identity.  
 
Reviewer #4 (Remarks to the Author):  
 
The paper by Magnarini et al presents an analysis of the topography of a Martian landslide, which 
exhibits distinctive parallel ridges. The authors have developed image analysis to measure the 
typical wavelength of the pattern and they show that additional fine structures exist on top of the 
ridges, revealing some transverse velocities.  
The wavelength of the longitudinal ridges is found to be between 2 and 3 times the thickness of 
the deposit. The authors discuss these observations in the light of previously published laboratory 
experiments, which have shown that such longitudinal pattern can be observed in rapid granular 
flows, due to an instability induced by the development of a strong basal shear and a dilute basal 
region.  
 
I have found the paper interesting, suggesting an alternative and new explanation for the ridges 
observed in large landslides.  
 
However, I think that the paper should be revised before publication, my comments being 
described in details below  
 
1) The paper lacks some information about the measurements.  
- First it would be interesting to mention in the main text and not only in the method, that the 
thickness is computed from the extrapolation of the basal topography from the measurement of 
the free surface outside the landslide. How precise is this estimate? especially for the profiles far 
from the edges (1 and 2 in Fig. 1) ?  
-Secondly, It would be interesting to add in Fig. 1 a plot showing two or three thickness profiles 
(the red lines). This will provide in a glance information about the scale of the wavelength, about 
the thickness, about the amplitude of the deformation, and will also show the degree of periodicity 
of the ridges and the disorder of the pattern along the profiles.  
- Thirdly, It would be important to show the correlation between the wavelength and the thickness 
in a plot, and not only to discuss it in the text. The authors should plot the wavelength versus the 
estimated thickness for all the points along the selected profiles. The reader would then have 
information about the range of wavelengths, about the range of thicknesses, about the dispersion 
of the data around a linear fit. Perhaps the authors can put in different colours the data 
corresponding to the central region and the one corresponding to the east area, which seems to 
show different linear correlation.  
 
2) I think the authors should be more careful and more precise in their physical interpretation 
when discussing the comparison with the laboratory experiments. To my opinion they have made 
some mistakes in the interpretation.  
In the lab, two kind of patterns have been observed. For very rapid flows, the surface deformation 
is sinusoidally deformed, and the transverse flow goes from the crest to the troughs, the troughs 
flowing faster than the crests and being much more dense (ref 14 and 16). For less rapid flows, a 
more complex pattern arises, with a secondary modulation and additional crests forming in the 
rapid regions (ref 16 , fig 2b).  
However none of the pattern observed experimentally is the one sketched in Fig4 of the present 
paper, where the authors show a sinusoidal deformation with crests moving faster and attracting 
the lateral particles.  
Another question not discussed by the authors concerns the possible difference between the 
pattern developed during the rapid flow and the pattern observed when the flow stops. From ref. 
14, it has been shown by measurements of the light transmission, that the total mass of grains in 
the crests is less than the total mass of grains in the troughs. This means that if the flow stops 
suddenly, one expects the crests to collapse when flow ceases, implying that what was a trough 
during the flow becomes a ridge in the deposit. I am not aware of any experiments showing this, 
but the observation of the density variation suggests that the interpretation of the deposit 
analysed by the authors should be take with great care.  
This point should be discussed in the paper.  

 
3) The estimate of the velocity is to my opinion irrelevant. The velocity of the landslide can not be 
estimated by \sqrt{gd}. In steady granular flows, the mean velocity of a Bagnold profile scales like 
\sqrt{g} h^3/2/d times a function of the inclination. But even this scaling is to my opinion no 
relevant for highly unsteady flow like landlsides. The velocity certainly mostly depends on the 
initial height of the cliff and not the particle size.  
 
 
In conclusion, I think that although the analysis of the Martian landslide is nice and interesting, 
although the connection with the longitudinal vortex instability is relevant and fruitful, I think the 
paper needs to be modified by taking into account the above comments before being published.  
 
 
Reviewer #5 (Remarks to the Author):  
 
This paper studies the origin of the parallel ridges often observed on the surface of gravity flows in 
planetary bodies. By reconstructing the 3D topography of the deposit of a Martian landslide, 
authors suggest that the ridge pattern results from a dynamic flow instability already reported at 
the scale of the laboratory with dry grains, rather than from the presence of a basal layer of ice as 
currently interpreted. 
 
Being not an expert in planetary sciences or geomorphology, I have found the paper stimulating in 
that it links some morphology of large-scale planetary flows to granular flow instabilities observed 
in the laboratory, raising possible new mechanisms for their formation. However, for the paper to 
be fully convincing, several issues must be addressed.  
 
1) My main comment is about the data processing and the morphological characterization 
methods. Authors write: “Distances between ridges were measured manually with the ruler tool. 
Number or ridges and their density along each profile were obtained using a custom Python script 
(See Code Availability)”. Have authors tried to use a more robust and systematic method (Fourier 
transformation) to identify a typical wavelength? How did the authors manually identify a ridge? I 
ask these questions because, based on the raw profiles given in the Extended Data Figures, it is 
not clear how the data provided in in Extended Tables 2 and 3 are extracted. As most of the paper 
is based on these data, wavelength extraction should be better explained and confronted with a 
non-manual and more objective measurement. The density must also be precisely defined.  

2) My second comment is about the instability mechanism and the flow pattern shown in Figure 4. 
Authors should be cautious when interpreting the flow structure from the deposit pattern, which is 
static. The static deposit reported by the authors actually matches both the “dilute” flow structure 
proposed by Forterre and Pouliquen (as sketched in Fig. 2c Borzsonyi et al, PRL 2009) and the 
“dense” flow structure sketched in Figure 4. In the “dilute” flow structure, the troughs flows faster 
than the crests but are much denser, such that the integral of mass across a trough is higher than 
across a crest (Fig. 4 Forterre and Pouliquen PRL 2001). As a result, when the flow stops, there is 
an inversion: the troughs of the flow become static ridges while the crests of the flow become 
static furrows, as in the Martian deposit. I therefore suggest that both mechanisms (the dilute and 
dense) be sketched in Figure 4, since the deposit alone (thickness modulation and en-echelon 
structures) cannot discriminate between them.  

3) My last comment is more on the form but important as well. I think Figure 2 is not very 
informative in the present version while the most important information is given in Extended Table 
2 and 3. I suggest that authors add a graph in Figure 2 plotting the local thickness as function of 
the local ridge spacing for all the measurements given in Ext. Table 2 and 3.