Aeolian sediment transport on Io from lava–frost interactions

Surface modification on Jupiter’s volcanically active moon, Io, has to date been attributed almost exclusively to lava emplacement and volcanic plume deposits. Here we demonstrate that wind-blown transport of sediment may also be altering the Ionian surface. Specifically, shallow subsurface interactions between lava and Io’s widespread sulfur dioxide (SO2) frost can produce localized sublimation vapor flows with sufficient gas densities to enable particle saltation. We calculate anticipated outgassing velocities from lava–SO2 frost interactions, and compare these to the saltation thresholds predicted when accounting for the tenuous nature of the sublimated vapor. We find that saltation may occur if frost temperatures surpass 155 K. Finally we make the first measurements of the dimensions of linear features in images from the Galileo probe, previously termed “ridges”, which demonstrate certain similarities to dunes on other planetary bodies. Io joins a growing list of bodies with tenuous and transient atmospheres where aeolian sediment transport may be an important control on the landscape.

(b) The summary here is also a bit unclear. So some are dunes but others may be yardangs, but they all have density to crest spacing ratios consistent with dunes? Is such a spacing also consistent with yardangs, then? Again, this is probably best left to later sections.
(c) The final sentence of the introduction again feels like a reprise of the abstract and the text seems more suited to the discussion section, not the introduction.
(3) Figure 1 is quite hard to puzzle out and I think could be described and presented far more clearly.

Plot (a) is very small and it's not immediately clear that the x-axis label is shared with plot b due
to the placement of the other labels below it. The caption for (a) only talks about Re being shown as a function of delta, but should also mention rho. rho is never defined in the caption. I infer it's the outflowing vapor density, but this should be stated more carefully. In addition, I feel that words rather than symbols should be used in the x-and y-axis labels.

Overall, I suggest clearly showing (in the figure) and stating (in the caption) what each plot contains.
(4) The bigger issue, though, is that the key results of Figure 1 are not well explained in the text. Figure 1 is very complicated and consists of multiple plots but is only referred to in passing early in section 2.1 and with none of the explanation that would be needed for anyone but an expert dynamicist to interpret the meaning. I think this could be improved by setting the scene a little for why the Reynolds number is important and explaining what the critical Reynolds number denotes, either here or in the Figure caption (see comment 1).
As an aside, this is why I dislike the placing of a 'Methods' section at the end of an article when the information contained therein is typically required to make sense of what's written earlier. [I really need to stop agreeing to review articles in Nature because this is nearly always an issue. At the same time, authors who choose to submit to Nature should consider how to summarize their methods better in the main text, even if the details must be placed at the end.] (5) First paragraph of section 2.1: (a) Perhaps unintentionally, this paragraph has the effect of implying that laminar flows cannot produce saltation. That's clearly incorrect -strong winds do not need to be turbulent to raise particles into saltation on Earth, for example -so the authors are really saying something different here, relating to the various types of thresholds for grain motion that exist (and which are a major topic of research in planetary aeolian studies at present, making this an admittedly tricky thing to delve into). However, I think this needs clarification. See also my previous comment.
(b) See also the sentence beginning on line 195: Again, it is really crucial to (earlier) explain the significance of turbulent flow for initiating saltation in these conditions.
(6) General focus of the paper should be more clearly on the new hypothesis: (a) The key idea -that the aeolian processes occur due to lava interactions occurring *beneath* a 'snow pack' -is almost buried. Not until Line 113 is this described clearly. Effectively, all of this happening under a sort of 'dome' of SO2, hence the pressures under discussion are not the pressure of the exposed-to-space atmosphere, but rather the pressure in a 'cave' of sorts. I think this needs to be made clear in the abstract and introduction. (11) Line 90 etc. talk about "volcanic plumes" as a source of SO2 frost. Are these the same "plumes" labeled in Figure 2b, which I believe are referred to as "SO2 vapor flows" or "outgassing" in the main text? Or are the latter a different phenomenon? I just want to make sure there are consistent definitions throughout.
(12) Line 188: This again jumps the reader straight into some range of a non-dimensional number for which we have no sense of its importance. Please explain the relevance of the Stokes number *here in the main text*, rather than simply giving numbers. (15) Methods section: Too much in this section goes unexplained in the main text, or even here. Even if the gory details need to go here, it is *crucial* that the reader be able to follow the basic concepts without needing to refer to this section (or seek out the references provided).

MINOR COMMENTS:
See annotated PDF.

Reviewer #1 (Remarks to the Author):
This is a harmless paper that explores in quantitative detail the previously-proposed hypothesis that SO2 volatilization might drive gas flows which could transport surface particulates on Io. The suggestion that aeolian processes driven by SO2 sublimation may have formed the ridges was advanced in Lorenz and Zimbelman 'Dune Worlds: How Wind-blown Sand Shapes Planetary Landscapes', Springer 2014. p.178-180 of that work shows exactly the same Prometheus ridges as in the present paper fig.4b , and suggests they may be aeolian, and documents their wavelength at 100-200m. So the core thesis of this paper is hardly novel, and at the very least, that book should be referenced here. That said, the present paper explores the relevant phenomena with some useful analysis, so it still has incremental value. Fig 3 -this looks puzzling to me. Most saltation threshold curves are smooth with a single minimum, but there is a 'waviness' to some of the curves here suggestive of a sort of suppressed second minimum around 1E-3m. Is this some sort of curve-fitting artifact of the plotting software? Or If this is real, it should be explained. FIg 6 -this does not look to me like a uniform-composition area with tilted facets, but rather more like slabs of one material partly buried in another. If so, then the uniform-optical-property assumption behind photoclinometry does not hold. A striking omission from the paper is the prospect that strong electric fields could modify aeolian transport (noting the high Jovian trapped particle radiation at Io that can even cause aurorae, and can also deposit charge into surface materials). These same authors have noted the possibility of triboelectric charging and the modification of saltation threshold on Titan by electrical effects -the interesting possibilities here surely deserve a mention in this paper too.

Reviewer #2 (Remarks to the Author):
Review by Claire Newman [claire@aeolisresearch.com] -please note that I am happy to clarify any of my comments via email if that would be helpful. This is a very interesting paper that examines whether flows due to the interaction between lava and SO2 frost could produce potentially aeolian features (ridges) seen on the surface of Io, rather than the weaker sublimation-condensation flows associated with Io's atmosphere freezing out regularly. The paper provides theoretical estimates of the outgassing velocities and compares these to estimates of the threshold for saltation to occur. It also notes that most ridges are directly adjacent to volcanic craters, and examines the ridges for evidence of an aeolian nature. I highly recommend this paper for publication in the near future -HOWEVER, I also have several major comments that I feel must be addressed first.
My primary concerns are that: -The paper is laid out somewhat oddly, with early emphasis on providing results over context and background in the introduction. - Figure 1 is not well-explained and may confuse more than it helps. -There is insufficient explanation of where the saltation thresholds come from and the conceptual differences between flows and thresholds under different situations. -Non-dimensional numbers are thrown in too often without providing enough context as to their meaning.
-The references to yardangs seem to be red herrings.

MAJOR COMMENTS:
(1) Lines 42-47: (a) Odd phrasing here. Suggest e.g. "either dismissal of this possibility due to the anticipated low gas densities and hence wind stresses available in Io's mean sublimation atmosphere" (b) But more importantly, what is meant by the "mean sublimation atmosphere" needs to be defined.
(c) In fact, I think (b) is part of a deeper issue, in that nowhere in the text is the sublimationcondensation flow on Io described. I assume that by "mean sublimation atmosphere" the authors are referring to the flows associated with 42-hour periodicity of Io moving in and out of Jupiter's shadow, causing the SO2 frost to freeze and sublime, creating air flow? OVERALL: Given that this flow is not widely described -and that it has previously been suggested to potentially be responsible for aeolian features, whereas this paper present an alternate hypothesis -I think spending at least a sentence or two on describing what it consists of is needed to set the scene.
(2) The final paragraph of the introduction: (a) This paragraph attempts to summarize both the methods and the key results. The latter especially belong in the results/discussion sections and the abstract, not the introduction. Removing the results from this paragraph, and making it more of a short 'roadmap' for the rest of the paper, would make valuable room for additional text.
(b) The summary here is also a bit unclear. So some are dunes but others may be yardangs, but they all have density to crest spacing ratios consistent with dunes? Is such a spacing also consistent with yardangs, then? Again, this is probably best left to later sections.
(c) The final sentence of the introduction again feels like a reprise of the abstract and the text seems more suited to the discussion section, not the introduction.
(3) Figure 1 is quite hard to puzzle out and I think could be described and presented far more clearly.

Plot (a) is very small and it's not immediately clear that the x-axis label is shared with plot b due
to the placement of the other labels below it. The caption for (a) only talks about Re being shown as a function of delta, but should also mention rho. rho is never defined in the caption. I infer it's the outflowing vapor density, but this should be stated more carefully. In addition, I feel that words rather than symbols should be used in the x-and y-axis labels.

Overall, I suggest clearly showing (in the figure) and stating (in the caption) what each plot contains.
(4) The bigger issue, though, is that the key results of Figure 1 are not well explained in the text. Figure 1 is very complicated and consists of multiple plots but is only referred to in passing early in section 2.1 and with none of the explanation that would be needed for anyone but an expert dynamicist to interpret the meaning. I think this could be improved by setting the scene a little for why the Reynolds number is important and explaining what the critical Reynolds number denotes, either here or in the Figure caption (see comment 1).
As an aside, this is why I dislike the placing of a 'Methods' section at the end of an article when the information contained therein is typically required to make sense of what's written earlier. [I really need to stop agreeing to review articles in Nature because this is nearly always an issue. At the same time, authors who choose to submit to Nature should consider how to summarize their methods better in the main text, even if the details must be placed at the end.] (5) First paragraph of section 2.1: (a) Perhaps unintentionally, this paragraph has the effect of implying that laminar flows cannot produce saltation. That's clearly incorrect -strong winds do not need to be turbulent to raise particles into saltation on Earth, for example -so the authors are really saying something different here, relating to the various types of thresholds for grain motion that exist (and which are a major topic of research in planetary aeolian studies at present, making this an admittedly tricky thing to delve into). However, I think this needs clarification. See also my previous comment.
(b) See also the sentence beginning on line 195: Again, it is really crucial to (earlier) explain the significance of turbulent flow for initiating saltation in these conditions.
(6) General focus of the paper should be more clearly on the new hypothesis: (a) The key idea -that the aeolian processes occur due to lava interactions occurring *beneath* a 'snow pack' -is almost buried. Not until Line 113 is this described clearly. Effectively, all of this happening under a sort of 'dome' of SO2, hence the pressures under discussion are not the pressure of the exposed-to-space atmosphere, but rather the pressure in a 'cave' of sorts. I think this needs to be made clear in the abstract and introduction. (11) Line 90 etc. talk about "volcanic plumes" as a source of SO2 frost. Are these the same "plumes" labeled in Figure 2b, which I believe are referred to as "SO2 vapor flows" or "outgassing" in the main text? Or are the latter a different phenomenon? I just want to make sure there are consistent definitions throughout.
(12) Line 188: This again jumps the reader straight into some range of a non-dimensional number for which we have no sense of its importance. Please explain the relevance of the Stokes number *here in the main text*, rather than simply giving numbers.
(13) Line 208: " Fig. 3 shows the saltation threshold as a function of grain diameter and temperature." (a) WHAT saltation threshold? This is introduced with zero background or references.
(b) Line 420 (buried in the Methods section) finally states "We utilize the saltation relations of Jia et al. 2017" -but this really needs to be referenced in the main text and more explanation is needed than "were derived with use in the dilute gas/high Knudsen number regime in mind." (c) Lines 462-465: "For the derivations of the above relations as well as additional context, the reader is referred to the "Transport Threshold" section of the Supporting Information for Jia et al. 2017 and equations S57 -S64 in that work." Referring to the supporting Information from another paper in your methods section is about three levels down from the explanation needed here. OVERALL: Rather than summarizing the results in the introduction section, I think this paper would benefit from including a 'primer' there about saltation thresholds under different types of conditions. The idea is not to explain everything or derive any equations, but simply to provide the reader with a clearer *conceptual* understanding of how the various situations differ.
(14) All mention of yardangs (including the abstract). (a) Line 261: "one steep face and one that is gradually sloped" This immediately made me think of transverse dunes, not yardangs.
(b) Also -as I noted in comment 2b -would you expect yardangs to have a similar density to height ratio as dunes?
(c) My impression is that yardangs are rather symmetric in the direction parallel to winds -i.e., parallel to the ridges. So if the 'steep' and 'gently' slopes here correspond to either side of the ridge line, is that really consistent with yardangs? Isn't Pelletier talking more about the slopes perpendicular to the wind? But perhaps I'm not clear on what you're talking about here.
(d) Lines 281-283: "At Chaac Patera, the steep face is parallel to, not perpendicular to the longest axis, as occurs with yardangs." Okay -yes, this is what my previous comment (c) is getting at.
OVERALL: I feel like mentioning yardangs is a bit of a red herring.
(15) Methods section: Too much in this section goes unexplained in the main text, or even here. Even if the gory details need to go here, it is *crucial* that the reader be able to follow the basic concepts without needing to refer to this section (or seek out the references provided).

MINOR COMMENTS:
See annotated PDF.