Peer review of: The varied sources of faculae-forming brines in Ceres’ Occator crater emplaced via hydrothermal (2020)

 
REVIEWERS' COMMENTS: 
 
Reviewer #1 (Remarks to the Author): 
 
 
This was an interesting and generally well written paper to read. Minor comments are 
provided in the attached annotated pdf while some more major comments are below. 
My two most critical comments are highlighted in bold below and relate to the very short 
presentation of the impact-generated hydrothermal system case and the talk of an 
impact melt chamber, which is not consistent with what we know about impact craters 
on Earth and other planetary bodies. 
 
L47: The authors use the term “central pit”. This term has been used for craters on 
Mars and other bodies with various widely differing explanations for why there is a pit 
present versus a central peak. To avoid confusion I’d suggest using the term “central 
depression” as you haven’t really justified as to why this morphological feature is a “pit”. 
 
L58 and elsewhere: The authors make quite a few very unequivocal statements 
using “was”. Given everything remains an interpretation or a hypothesis until ground-
truth from surface missions happen, I would urge the authors to tone down some of 
these statements and use phrasing such as “have been interpreted to have formed 
by X” etc. 
 
L78-81: Given the main conclusion of this paper is that Occator Crater’s faculae 
formed from impact-generated hydrothermal activity, I think this sentence 
represents a huge oversimplification of the topic. I also think that this is putting 
the cart before the horse and I would strongly urge the authors to lay out their 
observations first and then explain why they think impact-generated hydrothermal 
activity was the process responsible for forming the faculae. 
 
With respect to this sentence, the hottest locations in impact craters (by far) 
are not central uplifts, but rather the crater-fill impact melt-bearing breccias and 
melt rocks, so at the very least, this needs to be reworded. In the Osinski et al. 
paper you reference, the various locations within and around craters where 
hydrothermal deposits are located is presented. I think a short summary of this 
is key here. 
 
L96-99: As a suggestion, reactivation need not be from some disparate post-impact 
process. It is known from studies of craters on Earth that faults generated during 
the few seconds to minutes of the crater forming process, will gravitationally 
readjust over thousands to millions of years. 
 
P102-103: I am not convinced that this is a vent feature. Do you have topographic data 
to  show  that  this  is  comprised  of  a  central  depression  surrounded  by  a  rim,  which 
would constitute a vent? 
 
L108-109: Here and elsewhere the authors refer to reference #24. I read this 
manuscript and I find considerable overlap in terms of the conclusions with this 
current manuscript. In their abstract the write “Our results suggest that impact heating 
from the Occator forming impact provides a viable mechanism for the creation of the 
observed faculae”. I think the authors need to be clearer about how their work differs 
from this previous/recently published study, given the similar conclusions. 
 
L206-212: I am having a hard time understanding this text and the Figure 5 and 
the suggestion of a “melt chamber formed by the impact”. I do not have access to 
reference #19, but nowhere in reference #31 do I see mention of this and in 
reference #24, those authors talk about “a ‘plug’ of hot material” in the central 
uplift. This is not the same as a melt chamber. Indeed, impact melting is 
completed different from endogenic igneous activity. The melting occurs during 
decompression from high shock pressures. This melting occurs within a zone 
around the point of impact and that melt is then driven along the base of the 
transient cavity. After crater formation ceases, the melt ends up as crater-fill 
deposits (i.e., forming a layer or as patches in breccias) lining the crater interior, 
in ejecta deposits, and in small dykes intruded into the crater floor. Even in 
craters as large as 250 km on Earth, there is no evidence whatsoever for melt 
chambers within which is essentially the central uplift. See this review article for 
an overview of the processes and products of impact melting: Osinski G. R., 
Grieve R. A. F., Bleacher J. E., Neish C. D., Pilles E. A. and Tornabene L. L. 2018. 
Igneous rocks formed by hypervelocity impact. Journal of Volcanological and 
Geothermal Research 353:25– 54. 
 
Central uplift rocks can be hot and any water within them can certainly be 
above the boiling point, but this does not constitute a melt chamber. This 
must be revisited. 
 
L215-216: Be very careful with statements like this. Hydrocode models have a hard 
time modelling where melt and ejecta end up. Much of this is resolution. Indeed, 
there should be melt-bearing crater-fill deposits within and throughout Occator 
Crater’s floor. 
 
L264-266: See previous comments. I think the authors have good evidence for the 
formation of the faculae by impact-generated hydrothermal activity (but see above 
comment about this) but there is no evidence from over a century of study of craters 
on Earth that melt chambers at depth exist. 
 
L308: The authors are not very explicit about this, but these lobate materials on their 
geological map and cross sections are exactly what one would expect to see in a 
complex crater like this – i.e., crater-fill impact melt rocks and melt-bearing breccias. 
See the review article cited in a previous comment. 
 
L391: Pits resembling these are common in craters on Mars, and have also been 
recognised on Vesta and Ceres, where they are interpreted as being formed from 
volatile release from volatile-rich impact melt-bearing breccias. Such a mechanism 
should be discussed. E.g., see: 
 
Denevi, B.W., Blewett, D.T., Buczkowski, D.L., Capaccioni, F., Capria, M.T., 
De Sanctis, M.C., Garry, W.B., Gaskell, R.W., Le Corre, L., Li, J.-Y., Marchi, S., 
McCoy, T.J., Nathues, A., O’Brien, 
 
D.P., Petro, N.E., Pieters, C.M., Preusker, F., Raymond, C.A., Reddy, V., 
Russell, C.T., Schenk, P., Scully, J.E.C., Sunshine, J.M., Tosi, F., Williams, D.A., 
Wyrick, D., 2012. Pitted terrain on Vesta and implications for the presence of 
volatiles. Science 338, 246–249. 
 
Hibbard, S. M., Osinski, G. R., and Tornabene, L. L., 2018. Crater-Related Surface 
Morphologies of the Ikapati Crater, Ceres (abstract #2761). 49th Lunar and 
Planetary Science Conference. 
 
Woodlands, Texas, March 2018. 
 
Tornabene, L.L., Osinski, G.R., McEwen, A.S., Boyce, J.M., Bray, V.J., Caudill, C.M., 
Grant, J.A., Hamilton, C.W., Mattson, S., Mouginis-Mark, P.J., 2012. Widespread 
crater-related pitted materials on Mars: Further evidence for the role of target volatiles during 
the impact process. Icarus 220, 348–368. 


Reviewer #2 (Remarks to the Author): 
 
This is a good and useful paper, and I hope it finds its way to publication— after major revisions. It is 
stratightforward— geologic mapping and photo interpretation, with aid of topography and 
modeling. I have struggled with it, firstly in that it is a very exciting topic, and there are many 
fascinating observations presented in the paper, but then there are some key weaknesses as 
presently developed. The images are spectacular (even if as I say below, I wonder whether there are 
the highest resolution versions available— if so they would help a lot). If published, it will represent 
what will likely be the definitive analysis of Dawn’s imaging of the fascinating bright deposits 
(faculae and also associated dark terrain features) of Ceres’ Occator Crater. I have three basic 
categories of major comments, as summarized here and then elaborated upon: (1) The figures can 
be improved in a few ways. (2) The topography we see today would likely have been profoundly 
affected by dehydration if the whole deposits have dehydrated (rather than just a surface veneer). 
And (3) there seems to be am ambiguity about plausible interpretations of craters— are they 
endogenic, exogenic, and which are which? The compelling evidence presented here and also in the 
companion paper by Schenk et al. for a rich variety of cryovolcanic features would lend some 
support to an alternative cryovolcanic interpretation of some crater interpreted here to be due to 
impact. The improvements to the figures are probably easily done. The ambiguities raised by points 
2 and 3 run deeper in terms of the analysis and interpretation. At a minimum, the ambiguities and 
implications of alternative interpretations and processes should be clearly specified. 
 

 
Comment details: 
 
1. Some figures could be much improved (but easily done): 

 
I am unsure whether the review copy’s figures are the same as what will ultimately be published, so 
maybe the fix is already in the pipeline (I hope. Otherwise there is a small effort that needs to be 
made)… But if the higher res figures are already available, I would like to see them; it may 
qualitatively affect the photointerpretation and my comments! 
 
 
 
 
— Image resolution in the highest res figures appears to be about 6 or even 8 m/pixel (even if they are 
reported out at an oversampled 3 m/pixel; or if data processing, e.g., mosaic development, has killed 
some of the original resolution). The text says that 3 m/pixel scenes exist. I’m thinking that especially 
in the supplement some higher resolution images could be provided if the data exist at higher res. This 
would help with photo interpretation, for example, impact versus endogenic craters, and fractures 
versus fracture-controlled eruptive deposits. Are higher resolution images available? 
 
 
 
 
— Figure 1: This map, especially the upper panel, should be made available at higher resolution, maybe 
in the supplementary materials. Blowing up the PDF to 400% size doesn’t help; I just see a lot 
of blurry and illegible details that would be nice to see. A further improvement is possible if the 
practices followed in terrestrial and most Mars mapping were followed: Give each unit an 
abbreviated two or three letter name and put a few unobtrusive but legible labels in the map on 
those various units. The color scheme should be improved. There aren’t an extremely large number 
of units, so we should not be forced into doing our own photoshop color matching to distinguish 
some of the purple units; and the same for some green units. 
 
 
2. Dehydration will have affected the topography and maybe contributed to “weathering” and 
mass wasting, but also could affect some basic process and geochronological interpretations. I need 
to explain in some detail, so sorry for this long explanation: 
 
 
These chemical systems— assuming they were aqueous and not anhydrous “molten salts,” would 
have involved large volume losses when the frozen system dehydrates, so that the original 
topography was not even nearly the same as the present topograhy— not by a long shot; and 
furthermore, the dehydration process likely would have driven disaggregation of the salty residue, 
and this could be a primary cause of the weathering that the authors infer. As the composition is 
thought to be ammonium chloride and sodium carbonate, and these are thought to be salts left by 
dehydration of a frozen aqueous brine, it would be worth pointing out that volume of the deposits 
would have decreased dramatically, and the residual salts would be completely broken up by the 
dehydration. The binary water-sodium carbonate eutectic (between ice and decahydrate) is near 6.1 
weight% sodium carbonate, -2.2 C, and roughly 5 millbars water vapor pressure. The binary system 
also exhibits a series of three peritectics at increasingly high temperatures and increasingly high 
water vapor pressures— between the deca- and heptahydrates, the hepta- and monohydrates, and 
the monohydrate and anhydrous sodium carbonate. All of these peritectics are in the vicinity of 30 
weight percent sodium carbonate, 70% water. For example, the ‘least aqueous’ peritectic in this 
system is between the monohydrate and anhydrous sodium carbonate, and that solution is 31.3 
weight % Na2CO3 + 68.7% H2O, and occurs at +107.5 C and about 1.1 bars vapor pressure. It is not 
necessary that the brine was at a eutectic or peritectic, but these are likely compositions. All are 
aqueous, but only the ice-decahydrate eutectic contain ice as a stable phase, so if we are talking 
about derivation from some large frozen icy ocean or hydrosphere, the ice-sodium carbonate 
eitectic is what we may be talking about. If this is the case, it should be recognized that dehydration 
of the frozen eutectic brine— if approximated as the binary water-sodium carbonate system— 
would involve about 94% mass loss in stages, with a sequence of loss first of ice, then conversion of 
decahydrate to lower hydrates and finally to anhydrous sodium carbonate; this process, if it goes to 
completion, would cause something approaching 90% volume loss, and what remains might crumble 
to powder, or at least would go through a series of disaggregation stages (but maybe there would be 
annealing at each dehydration stage if a liquid was involved, i.e., without direct dehydration, which 
can hapopen if the H2O vapor pressure is low in the dehydrating deposit). In any event, the 
topography now seen would not be the topography that first formed. Now, the system is not binary. 
There’s ammonium chloride, too. The binary ammonium chloride-water system lacks hydrates to the 
best of my knowledge but has two solid anhydrous chloride pahses— the alpha and beta phases. 

 
There is a eutectic between water ice and alpha ammonium chloride near 19% NH4Cl + 81% H2O at 
 
- 15.69 C; and an aqueous peritectic between the alpha and beta phases near 60.5% NH4Cl + 39.5% 
H2O at +184.5 C. I am not sure about the ternary system, but actually it cannot be ternary. There 
must also be ammonium carbonate, though it has not been reported, and very possibly— as most 
similar natural systems on Earth also contain— there is likely to be sodium chloride and both 
ammonium and sodium bicarbonate. Basically, I think the system is apt to be pretty complex, but 
just going with what has been reported, the actual system will have a series of eutectics and 
peritectics, a variety of both binary and ternary solids, a lot of hydrate phases, and in all cases the 
complete dehydration of the solid brines would involve a large volume loss. If dehydration has 
affected only the surface veneer, then the immense volume losses I mention would only pertain to 
that veneer. But if the whole deposit dehydration, topographic collapse by an order of magnitude or 
at least a factor of several is likely. In fact original domes can collapse to holes (the hydrated salt 
version of glacial kettles). It can be that extreme. 
 
 

 
Line 71: “… salts in solution and blocks of unmelted silicates and salts.…” It would take very little 
 
silicate mass to make mixed salts and silicates tend to sink through brines, in fact very rapidly, no 
matter how salt-concentrated they are. Salts (even without silicates) also would tend to sink very 
readily; the exception is ice mixed with salts, such as frozen ice-salt hydrate eutectics, which can 
float; or brine equivalents of debris flows, where fine grained mass suspended in liquid makes a 
viscous slurry in which denser stuff can be entrained like rocky boulders in Earth’s aqueous debris 
flows. If the flows are basically like debris flows, the authors can say something like “low albedo 
lobate deposits on the crater floor may be impact-melt of saturated aqueous saline brine mixed with 
fine-grained solid silicates and salts, providing a matrix in which boulders of unmelted salts and 
silicates were suspended, rather like debris flows.” Or whatever may please the authors and their 
interpretation. 
 
 
Lines 86-88: “Occator’s domes and mounds may originate from eruptive and/or frost-heave-like 
processes derived from the solidification and expansion of the lobate material18.” Expansion only 
occurs in aqueous saline solutions if they are water-rich and form abundant ice in addition to salts or 
salt hydrates. So if volume expansion is the cause of these mounds, then when dehydration occurs 
(if it occurs to deep levels), there would be volume reduction of the whole deposit. 

 
Lines 90-93: “Vinalia Faculae are associated with a prominent set of fractures, from which the 
faculae-forming brines were proposed to originate13,15,20-22. However, our XM2-based geologic 
mapping reveals that these fractures cut through the Vinalia Faculae, suggesting that the 
fractures postdate, and did not provide conduits for, the faculae-forming brines (Figs. 1 & 4).” 

 
Maybe there is more that could be discerned at 3 m/pixel that I cannot see at the figures’ apparent 
resolution of roughly 6 or 8 m/pixel. But what I see looks a lot like parts of fracture-controlled Lunar 
Crater Volcanic Field, Nevada. Those fractures are associated with Basin and Range faulting. What 
could look on Ceres like simple fractures may be some small amount of eruptive material that came 
from the fractures. And for what likely are shallow sources in the case of Ceres, eruptive activity 
could very easily reactivate faults. So I am fine with saying that maybe the fractures post date the 
deposits, but I do not see anything definitive that rules out fracture-controlled eruptions, with or 
without reactivation after or during the eruptions. The manuscrupt does not absolutely rule that 
out, but I don’t think the idea of post-eruptive fracturing should be a strong point— just balance out 
the ideas, in my view, unless the authors are able to see more in their images that I am not seeing in 
what was provided. 
 
 
 
 
Lines 164-168: “It is not possible to gain such fine-scale resolution in the model ages derived from 
crater size frequency distributions: statistical errors of Cerean model ages are sometimes as low as 
a few hundreds of thousands of years, but they are typically on the order of a few millions of years 
or more, and do not include the larger, unquantifiable, chronology calibration errors.” The accuracy 
of any crater-count-based geochronologic analysis is completely dependent on the craters being 
impact craters, not endogenic craters. At the given resolution, I don’t see that it is possible to be 
sure, especially since there has been partial infilling of some crater interiors and exteriors (leaving 
the rim exposed to define that there is a crater). I attach an example drawn from this paper, and 
then offer two potential terrestrial volcanic cater analogs. The Lunar Crater Volcanic Field has 
aligned craters, like some areas in Occator Crater (as the paper points out), and the Pinacate 
Volcanic Field has some partly infilled crater interiors and aggradation also outside the craters— 
sand instead of salts. The scales and image resolution that I show are not that different from the 
Occator Crater examples in the paper. It’s hard for me to be certain what is impact and what is 
endogenic inside Occator Crater. I do believe there are impact craters, but it seems likely that there 
are endogenic ones, too, and how to isolate the two populations of craters is problematic in my 
view. 
 
 
Lines 184-191: The interpretive inferences drawn in this paragraph depend on the observations of 
the topographic relief seen today being the same or similar to that when the deposits were 
emplaced. As I noted above, the volume losses of these frozen saline brines may have been 
enormous, enough to affect the topography significantly. The methodology used to draw the 
inferences as stated in this paragraph are dependent on the devolatilization not driving latrge 
volume changes; for instance, if volatile loss is confined to the upper surface veneer. Pending 
adequate time dependent thermal models and thermodynamic stability and vapor diffusion models, 
we won’t really know how deep the devolatilization goes; hence, we can’t know whether the 
implicit assumption needed for this paragraph is fulfilled or is implausible. Therefore, it should be 
explicitly stated somewhere that this inference is dependent on there not having been much volatile 
loss and differential topographic deflation. (except a surface veneer— fine, it can dehydrate, so long 
as resulting dehydrated salt powder does not slough off and expose new material to dehydration) 
 
 
Line 225 and elsewhere: In discussions of mass wasting, consider that dehydration of salts would 
also likely disaggregate them, especially if it occurred by production of a vapor phase rather than 
a liquid phase (a liquid would possible anneal or cement the less hydrated solids). The enormous 
volume losses and changes of crystal structure require the breakdown of the solid framework. This 
is well known from dehydration of hydrated salts: if it occurs under hyperarid conditions, a 
powder is formed. If liquids form at peritectics, then the dehydrated or less hydrated residue can 
be re-cemented. 
 
 
 
Figure S2. Beautiful image! I am not so sure that the mapped impact craters can be reliably 
distinguished from endogenic craters. See for instance, Pinacate Volcanic Field, Mexico and Lunar 
Crater Volcanic Field, Nevada. I attach a PDF showing some examples (from Google Earth). On Ceres, 
the more distinctly or most plausibly endogenic craters and fractures, or fracture-aligned eruptive 
deposits, look plausibly like cinder cones (with infill and partial burial), maars, ash and basalt flow 
deposits controlled by tectonic fractures in these terrestrial volcanic fields. Obviously impact craters 
are also abundant on Ceres, so the question when dealing with these faculae is: what rimmed or 
ringed depressions or features are impact, and which are volcanic? 
 
 
Lines 267-276 (final main text paraggraph). This might be a reasonable place to cite some relevant 
papers: 
 
 
Regarding geomorphological evidence of cryovolcanism on icy satellites and comets: 

 
Greenberg, R.; Croft, S. K.; Janes, D. M.; Kargel, J. S.; Lebofsky, L. A.; Lunine, J. I.; Marcialis, R. L.; 
Melosh, H. J.; Ojakangas, G. W.; Strom, R. G., 1991, Miranda, in: Uranus, J.T. Bergstralh, E.D. Miner, 
and M.S. Matthews (Eds.), U. of Arizona Press, p. 410. 

 
Croft, S.K., J.S. Kargel, R.L. Kirk, J.M. Moore, P.M. Schenk, and R.G. Strom, 1995, The Geology 
of Triton, in Neptune and Triton, D.P. Cruikshank, ed., pp. 879-947, University of Arizona Press, 
Tucson. 
 
Lopes, R.M.C., R. L. Kirk, K. L. Mitchell, A. LeGall, J. W. Barnes, A. Hayes, J. Kargel, L. Wye, J. 
 
Radebaugh, E. R. Stofan, M. A. Janssen, C. D. Neish, S. D. Wall, C. A. Wood, J. I. Lunine, and M. 
 
Malaska, 2013, Cryovolcanism on Titan: New results from Cassini RADAR and VIMS, Jour. Geophys. 
 
Res.: Planets, 118, 1-20. 

 
Mousis, O., A. Guilbert-LePoutre, B. Brugger, L. Jorda, J.S. Kargel, A. Bouquet, A.-T. Auger, P. Lamy, P. 
Vernazza, N. Thomas, and H. Sierks, 2015, Pit formation from volatile outgassing on 67P/Churyumov-
Gerasimenko, Astrophys. J. Lett. 814 (1). https://doi.org/10.1088/2041-8205/814/1/L5 

 
Desch, S.J., and M. Neveu, 2017. Differentiation and cryovolcanism on Charon: A view before 
and after New Horizons, Icarus 287 (2017) 175–186. 
 
 
Regarding predicted occurrence and chemical properties of brine cryovolcanism and cryolavas: 

 
Kargel, J.S., 1991, Brine volcanism and the interior structures of asteroids and icy 
satellites, Icarus, 94, 368-390. 

 
Kargel, J.S., 1992, Ammonia-water volcanism on icy satellites: phase relations at 1 atmosphere, 
Icarus, 100, 556-574. 

 
Porco, C., Helfenstein, P., Thomas, P. C., Ingersoll, A. P., Wisdom, J., West, R., Neukum, G., 
Denk, T., Wagner, R. 2006. Cassini Observes the Active South Pole of Enceladus. Science. 311 
(5766): 1393– 1401. 

 
Marion, G., J.S. Kargel, D.C. Catling, and J.I. Lunine, 2012, Modelling ammonia-ammonium aqueous 
chemistries in the Solar System’s icy bodies, Icarus 22, 932-946. 
 
 
 
The photointerpretation of craters on Ceres would be easier or more reliable if the reader had 
3m/pixel images rather than what look more like 6 or 8 m/pixel images. Are these figures, as 
reproduced, the highest resolution scenes available (and presented at full resolution?) If these 
images truly are 3 m/pixel, something in the data processing pipeline seems to have degraded the 
true resolution by roughly a factor of 2 (or worse) even if they are reported out at 3 m/pixel. 
 
 
 
 
Figures 4 and S2. The largest craters occur on fractures or fracture-controlled eruptive deposits. The 
craters are consistent wit the appearance of rimmed maars, like Lunar Crater, Nevada (see attached 
PDF). This could be noted. Alternatively, they could be impact craters, but I agree with the mapping 
that they are not included as impact features. My questioning of the endogenic vs exogenic 
interpretation then extends to smaller craters, also. What is the geometry of solar illumination 
(azimuth and elevation) in this area? 
 
 
 
 
About the crater distribution in Figure S2: See the attached PDF of Pinacate Volcanic Field. In that 
case, of course the craters and other volcanic features lie within the volcanic field (they define the 
volcanic field). So in Figure S2a, why is it that the authors are so certain that the mapped imoact 
craters really are impact craters? I don’t think the resolution is there to make a unique and 
confident determination. So what to do? I think at a minimum the authors need to state that 
there is a difficulty to make a definitive interpretive disambiguation of impact versus endogenic 
craters, and the geochronological analysis is dependent on having made a successful identification 
of the two types of craters. 
 
 
In Figure S2b (and S1d) I have a similar difficulty to discern clearly whether the linear features are 
fractures or fracture-controlled eruptive materials. To my eyes, they seem more likely the latter, 
rather than fractures that purely and simply cut across the deposits. Then the text discussion of the 
geochronological/stratigraphic sequence is not so clear to me, and the former interpretation that is 
cited may well be accurate. But it is not a clear situation. Again a true 3 m/pixel would help. 


Figure S3. There are four cyan-to-purple or violet color gradations. The cyan unit (bright material fresh) is 
easily distinguished. The other three purple or violet color units are very difficult to distinguish. Likewise, 
there are the green to olive colored units; they are very difficult to distinguish. 
 

 
Reviewer #3 (Remarks to the Author): 
 
Chloe Beddingfield’s review for Scully et al. “The Varied Sources of Faculae-Forming Brines in Ceres’ 
Occator Crater, Emplaced via Brine Effusion in a Hydrothermal System” 

 
General Comments: 

Overall, I enjoyed reading the manuscript. The manuscript is well written and presents important new 
information regarding the properties, emplacement mechanisms, and timing relationships of Occator 
Crater Faculae. The methodology for the manuscript is presented in clear detail and the results and 
interpretations are important and interesting. I greatly appreciate how much additional information 
and detail was provided in the supplementary material. 
 
 
Some more discussion should be included about what would cause Vinalia Faculae, and the brine 
reservoir, to be so much more brine-limited than the central faculae. You mention that Vinalia 
Faculae could be sourced from a deep, long-lived brine reservoir that existed prior to the Occator-
forming impact. However, it is unclear why this source of the brine would be so much more limited. 
 
 
 
 
Your geologic maps in Figure 1 are very well detailed. Based on looking at the geologic maps, I can 
clearly see the relationship between the fractures and Vinalia Faculae. Something I’m wondering about 
is, why are there not more faculae in other locations that are severely fractured. For example, the 
region to the southwest of Pasola Facula is even more fractured than the region of Vinalia Faculae, and 
yet faculae are not present here. Why? Is there some implication here for the extent of the pre-existing 
brine reservoir? What other possibilities are there? Discussion about this is needed. 
 
 
 
 
In the abstract, it would be useful to mention the interpretation that Pasola Facula was not emplaced 
simultaneously with and was not originally connected to Cerealia Facula, and that Cerealia Tholus has an 
intrusive origin. Also, you say in the abstract that the faculae were emplaced from numerous localized 
origins. However, in the manuscript you only discuss two origins, so wouldn’t it be better to specify two 
localized origins than use the word numerous? Also, I didn’t get the impression from reading the 
manuscript that the pre-existing brine reservoir is considered to be localized, although it is described 
this way in the abstract. In addition, I think it would be useful to point out in the abstract that you are 
providing a new geologic map. 
 
 
 
Figure 5 is a very nice figure that really helps to visualize your interpretations. However, most of the 
faults and fractures do not reach the deep brine reservoir in the illustration. If evidence for this fault and 
fracture depth geometry has been tested, then I suggest mentioning that reference in the caption. It 
seems that the fractures sourcing the impact induced melt chamber should reach similar depths as the 
surrounding fractures and faults, especially since you mention in the manuscript that there may be 
some additional contribution from the deep brine reservoir in this location. Some short explanation is 
needed here regarding why the depths of these faults vary spatially, or just a reference if one exists. 
From this figure I see that the illustrated fractures underlying Vinalia Faculae are the only ones that 
reach the deep brine reservoir, which could be a possible explanation for why faculae are not present in 
other fractured regions. However, this information is not discussed in the manuscript results or 
discussion. Additionally, I’m skeptical that this is the best explanation based on the similar plan view 
lengths of fractures/faults in both the Vinalia Faculae region compared to fractured regions elsewhere 
where faculae are not present. It seems to me that these fractures/faults likely reach similar depths. 
This possibility should be mentioned and other possibilities should also be discussed to explain the lack 
of faculae in these locations. For example, perhaps the deep brine reservoir is not present, or is deeper 
in the subsurface in this region than elsewhere, etc. One other very minor comment is that one of the 
normal faults appears listric (third fault from the right), and I suggest changing the geometry of this fault 
to match its neighbors. 
 
 
 
Regarding the data availability, it would be so useful to the community if the digital and georeferenced 
version of the map provided in Figure 1 was made available for import into a GIS 
 
software so others could build off the great work you’ve done and so that it’s preserved long term. 
Although the detail of the map shown in Figure 1 is fantastic, some of the details are lost due to the 
restricted size that the figure can be on the page. A GIS version of the map would be very useful for 
those wanting to see a bit more detail and to use your geologic map to investigate specific features. I 
recommend providing a georeferenced and GIS importable version of this map. 
 
 

 
Minor Comments: 
 
 
 
 
LINE 23: I suggest providing more information on what specific questions remained about the faculae 
that you address in your manuscript. Perhaps the source, timing relationships, etc? Providing more 
information here will help the reader immediately see how important your work and results are. 
 
 

 
LINE 24-25: This sentence can be removed if you need additional space in the abstract. 
 
 

 
LINE 42: Add the word “the” before “largest” 
 
 
 
 
LINE 48: It’s unclear from this sentence what is meant by a ledge deposit. I suggest defining a ledge 
deposit here. 
 
 
 
 
LINE  49:  What  is  visual  normal  albedo?  Do  you  mean  visible?  Although  it  may  be  discussed  in 
Reference  9, I suggest rephrasing the  sentence  here to specify what is meant  by visual normal 
albedo. 
 
 

 
LINE 50: I suggest adding the words “composed of” before “sodium carbonate” 
 
 

 
LINE 76: “allows” should be “allow” 
 
 

 
LINE 105-107: A subject is needed after the word “this” 
 
 
 
 
LINE 119: You state that flow fronts are not visible within any of the faculae, and that this can be 
explained by multiple overlapping/intermingling flows. However, this argument is not entirely 
convincing since I can think of examples of overlapping flows that still have clearly visible flow fronts. 
 
I think this sentence could use a reference. 

 
LINE 136: The word compressional should be replaced with contractional 
 
 
 
 
LINE 136: It should be mentioned that although contractional linear features are not observable, 
compressional stresses can be present without the presence of contractional features. Commonly, 
contractional structures are often not observed since they require larger differential stresses to form 
than extensional structures. Some good references for this are Gold (1977), Hobbs (1974), and Haynes 
(1978). 
 
 
 
 
LINE  148:  Since  the word correlation  implies  a  relationship  backed  up  by  regression  analysis  results, 
the word correlation here should be replaced with relationship. I have the same comment for Line 149. 
 
 

 
LINE 149 – 151: I think this is a great observation. Interesting! 

 
LINE 152 – The sentence here needs to be rephrased. The part about Cerealia Facula ranges in 
thickness from <3m, through ~5.5m or ~31 m, to >50 m is confusing. What does “through ~5.5m or 
~31 m” mean? Although this may be discussed in more detail in Methods 3-4, it needs to be made 
clear in this sentence. 
 
 

 
LINE 154: the word “illustrates” should be “illustrate” 
 
 
 
 
LINE 154-156: The word “Cerealia Tholus” is used twice in this sentence and just sounds a bit 
redundant. I suggest rephrasing the sentence to fix this issue. 
 
 
LINE 159-163: Very interesting result and interpretation! 
 
 
 
LINE 163: How was 10,000s -100,000s of years derived? More information for how these values were 
estimated need to be provided here. I realize that in the methodology you refer to some personal 
communication as a reference. However, this reference should also be included here and some 
explanation for how these values were derived should also be mentioned. 
 
LINE 175: You mention earlier in the manuscript that the lack of a clear termination may be due to 
overlapping flow fronts. I think this should also be mentioned and discussed here as a possibility along 
with the low viscosity possibility. Could multiple higher viscosity brines that overlap each other so that 
clear termination scarps are not observable? In this case, is an intrusive origin still the most likely 
explanation for Cerealia Tholus? 
 
 
 
LINE 192: This sentence is redundant with a sentence earlier in the manuscript on line 152. I suggest 
removing one of the sentences. If you keep this sentence, then I have the same comment here as 
mentioned above for line 152. 
 
 
LINE 201: A subject is needed after the word “this”. 
 
 
LINES 209-212: Very fascinating results! 
 
 
 
LINE 228: A subject is need after the word “this”. I have the same comment for Lines 251, 450, 
and 477. 
 
 
LINE 294: Include a reference for ESRI ArcMap/ArcScene 
 
 
 
LINE 298: Your technique using 3D view to more accurately place contacts and stratigraphic 
relations is neat! 
 

 
 
LINE 225: What indicates that the brine reservoir would be deeper than the Cerealia and Pasola-
Facula-forming brines. Although the Cerealia and Pasola Faculae are sourced from an impact 
induced melt chamber, couldn’t the Vinalia Faculae brine reservoir also be shallow? Some 
explanation is needed here and/or a reference. 
 
 
LINE 303-306: I don’t think the last sentence of this paragraph is needed. I suggest removing it. 
 
 
 
LINE 310: I suggest replacing “shortly” with the actual timing of when this event took place. By 
shortly do you mean days? Years? Thousands of years? 
 

 
LINE 311: The words “comparatively” is used, but it’s unclear what comparatively is referring to. Is the 
lobate material water ice rich compared to the surrounding terrain? Or, to the  compositions of the 
original flows around the crater interior? I suggest rephrasing the sentence to clarify this. 
 
 
LINE 616: I have one very minor suggestion for Figure 1b. The words “Material” and “here” are not 
needed. If you’d like to reduce the amount of annotation on your map, you could remove these words 
in each of the white boxes and include a short explanation in the figure caption. 
 
 

 
LINE 622: In Figure 2, elevation values are needed on the contour lines. 
 
 
 
LINE 626: The word correlation should be replaced with spatial relationship or something 
similar since statistical tests were not performed to quantify a correlation. 
 
 
 
LINE 628: Although it is mentioned in the figure caption, it would be so much easier to interpret the 
figure if a legend was provided for the yellow, orange, and white material on Figure 2b. 
 
 
 
LINE 650: Figure 4 looks great, but an arrow is needed that points to the landslide in the bottom 
right of the figure because it’s not immediately obvious. Also, this is very minor, but it’s hard to see 
the white box around the vent-like structure. I suggestion changing the box color.