Peer review of: Impacts drive lunar rockfalls over billions of years (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review comments: 
 
The manuscript by Bickel et al., presents a new dataset of global distribution of rock falls 
throughout the manuscript, their spatial variation and their relationships to local geology, in terms 
of their occurrence on impact craters, tectonic and volcanic features. The authors used lakhs of 
high-resolution images that cover the entire lunar surface and automatic rock fall detection 
algorithms and come up with a new dataset that clearly documents the rock fall distribution on the 
Moon. The dataset shows new knowledge about the lunar rock falls and their spatial association 
with different geological features of different origins and formation times. I agree with the authors 
that the manuscript substantially improves our knowledge about the origins of lunar rock falls and 
recommend that the manuscript could be published by Nature Communications. 
 
The authors correctly relate the formation of boulders to impact cratering processes. In this 
regard, I would like to point out some more recent publications that may be useful to understand 
the roles of impact spallation processes on boulder origins. The publications are: 
https://www.sciencedirect.com/science/article/abs/pii/S0019103515004479 
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JE004543 
 
Formation of boulders is also related to impact fracturing that has been well established on a few 
terrestrial impact craters. Say, for example: 
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2005JB003662 
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008JE003115 
 
A few boulder falls have been dated on the Moon. The results can be found in ref [8]. 
Although I agree that the ages of boulder trails could be less than 20-30 Ma, there is no evidence 
to suggest that they can be as old as >100 Ma. Hence, the boulder trails are related to erosional 
processes that occurred in recent rimes but not older than 20-30 Ma. Therefore, the observed rock 
fall distribution reflects the lunar erosional processes that occurred in the last few million years. I 
request the authors to add more discussion on the recent geological processes. Of course, the 
boulders are produced by impacts that occurred over a span of billion years and those sitting on 
the lunar surface also are also destroyed subsequently. Recent studies also highlighted the survival 
times of boulders. The authors discussed these aspects but recent processes should be 
emphasised with more details and clearly link them to the rock falls. 
 
Overall, I enjoyed reading the manuscript. I congratulate the authors for their exciting research 
work. 
 

 
 
Reviewer #2 (Remarks to the Author): 
 
Comment 1. Line 19-46. It is said in line 19 that rockfall is a process. But in line 46 it is said that 
authors identified 136,610 rockfalls, probably meaning in this case the traces of this process. It is 
not clear if authors use term "rockfall" for an individual boulder with visible track or two and more 
nearby located boulders with tracks are considered as individual rockfall too. It is not clear if a 
boulder which already lost its track but obviously had it is considered as a rockfall too. This should 
be clarified in the very beginning of the paper. 
 
Comment 2. Line 60. This figure shows mean values. Statistical reliability of the conclusion that 
older lunar regions host boulders with systematically larger volumes should be provided. 
 
Comment 3. Lines 63-67. Figures 2e and 2g shows high concentration of rockfalls associated with 
Gibbs and S of Rowland R. But why the activity in these locations is considered as recent it is not 
clear. Some references (to geologic maps) may help and concrete situations should be shown in 
LROC NAC images as Supplementary figures. 
 
Comment 4. Lines 77-84. Authors write that that most mapped rockfalls are located in craters and 
that there is no of increase in rockfall density within or in the direct vicinity of tectonic features, 
such as graben and faults, except several listed cases. They conclude that impacts appear to be 
the dominant driver of rockfalls while tectonic and volcanism may play some role. But the fact of 
association of rockfalls with impact craters does not necessarily means that impacts are drivers of 
this process. Cited in the considered paper work of Kumar et al (2016) describes evidence of very 
recent tectonic/seismic activity which provoked numerous rockfalls in ancient impact crater 
Schroedinger. Abundant rockfalls in this crater are due to recent seismicity not to the ancient 
crater-forming impact. Impact craters are just more numerous on the Moon than volcanic and 
tectonic features and relatively steep slopes of any origin are the precondition and arena of 
development of rockfalls mostly caused by shakes of seismic or impact events. 
So this part of the paper should be reworked.