Peer review of: Iron whiskers on asteroid Itokawa indicate sulfide destruction by space weathering (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The authors present interesting new observations of and interpretations for iron whiskers on 
regolith particles returned from the asteroid Itokawa. No previous observations of iron whiskers on 
planetary materials have been reported. The finding of iron whiskers and their related impact on 
deciphering environmental conditions on asteroid surfaces and big-picture connection to solid-gas 
interactions in the interstellar medium are novel and will be of interest to a broad community. 
 
The major claims of the paper are that troilite grains in regolith particles from the asteroid Itokawa 
have lost sulfur due to energetic solar wind ion irradiation (e.g., space weathering). The loss of 
sulfur has caused decomposition of FeS to Fe, where Fe grows to form whiskers. Moreover, this 
process might play an important role in the distribution of Fe and S in the interstellar medium 
(ISM). 
 
The compelling data and science interpretation presented in the paper warrant publication in 
<i>Nature Communications</i>. 
 
 
There are a few edits and comments that I recommend the authors and editor consider making 
before publication, all of which are minor. 
 
1) It is my recommendation is that the paper should focus more on the discovery of these iron 
whiskers on planetary materials. The discovery of these unique features (and the value of 
planetary sample return) gets lost because the paper puts more attention on the inference of what 
these iron whiskers can tell us about the ISM. I would also ask that the authors take greater 
lengths to show that the whiskers are not some kind of terrestrial alteration product. My request 
stems from the idea that many Itokawa grains have been previously studied and none have shown 
these whiskers, and now this study finds 8 out of 10 grains show these whiskers? While there 
might not be much room in the main manuscript, perhaps the supporting information could discuss 
why whiskers haven't been seen before, what is special about these samples, sampling site 
location on the asteroid, and/or compositional evidence that these whiskers aren't some kind of 
post sampling alteration product. 
 
2) Introduction: The paper reports the finding of iron whiskers on Itokawa regolith particles and 
then infers that the process that makes these Fe-whiskers could apply to processes in the ISM. 
Hence, the Introduction should start with a background on asteroid space weathering and 
information regarding Hayabusa sampling of Itokawa because it serves as the core of the paper. 
Once the reader is given the details surrounding the focus of the paper, only then should the 
authors discuss the bigger implications inferred from conclusions based on investigating the Fe-
whiskers. 
 
3) line 60, the paper uses two references by Christoffersen & Keller (2011) and Keller et al. (2010) 
to conclude that iron sulfides are highly radiation resistant minerals. However, it is shown in 
irradiation experiments that the relative sputtering rate of FeS is much higher than olivine 
or enstatite in a more recent abstract by Keller et al. 2013 (Keller, L., Rahman, Z., Hiroi, T., 
Sasaki, S., Noble, S., Horz, F., Cintala, M., 2013. Asteroidal space weathering: The major role of 
FeS (abstract# 2404), 44th Lunar and Planetary Science Conference. CD-ROM.). These 
experiments also showed that irradiation of FeS with 5kV Ga+ resulted in preferential sputtering of 
S and the formation of a 5-8-nm thick surface layer of nanophase Fe metal. Hence, the authors 
need to reconsider the statement that iron sulfides are highly radiation resistant. The authors also 
state that (lines 184-186) sulfides are easily destroyed under such conditions, conditions in the 
ISM that are similar to the solar wind. The authors need to reconcile this apparent discrepancy 
about whether FeS is radiation resistant or relatively easy alter. 
 
4) The idea that "metallic iron may form when the troilite comes into contact with H2 gas may not 
be the complete scenario. As discussed in Guo et al. 2019 (Guo, L., Zhong, S., Bao, Q., Gao, J., & 
Guo, Z. (2019). Nucleation and Growth of Iron Whiskers during Gaseous Reduction of Hematite 
Iron Ore Fines. Metals, 9(7), 750.) heating and reduction must occur for the formation of iron 
whiskers to occur. The authors of this paper appear too quickly to discount the need for heating to 
catalyze the reaction. For instance a pulse heating event would allow a "rapid iron diffusion" event, 
however, I do not envision constant ion bombardment providing a means for rapid iron diffusion. 
In addition, Guo et al. explain that substrate composition influenced iron whisker formation. The 
presence of SiO2 decreased the total porosity, which stopped the near-surface nucleation of iron 
particles and thereby prevented the formation of iron whiskers. Could the process be that iron 
sulfides are heated/vaporized, S is lost as a vapor, and Fe recondenses preferentially on FeS 
grains? This would mean that the iron whiskers are not from the FeS grain they are in contact 
with. Such a scenario would circumvent apparent problems with micrometeorite heating because 
the FeS vapor forms in one location but is deposited in a different location, which might only be 
mm or 100 mm away from the impact or sputtered source. This situation would be similar to the 
Hapke 2001 formation of nanophase iron in the lunar regolith. Hapke, B., 2001. Space weathering 
from Mercury to the asteroid belt. J. Geophys. Res. 106, 10039-10073. 
 
5) The Sulfur may not leave as S2 but as H2S and H2S instead. Gillis-Davis, J.J., Zhu, C., Góbi, S., 
Abplanalp, M.J., Frigge, R., Kaiser, R.I., 2019. Laboratory Space Weathering Induced Formation of 
Sulfides in the Murchison (CM2) Meteorite, Lunar & Planetary Science Conference, abstract 1213. 
 
6) line 162: The authors say that the degree of whisker growth (which to me implies whisker size 
and/or population) might be a novel proxy for the duration of regolith space exposure.... If, as the 
authors say and I agree, that whisker growth occurs by rapid growth, then it would seem that it is 
not the size of whisker growth but more specifically the population or number of iron whiskers that 
could be used as a proxy for the duration of space exposure, which is loosely analogous to lunar 
nanophase iron and its associated ferromagnetic measurement (FMR), which yields an 
accumulated exposure age as quantified by Is/FeO. 
 
7) Could the 7-15 nm layer of silicate be used to estimate the timing of formation of the Fe-
whiskers? i.g., silicate rims on mineral grains are used to estimate exposure to space. Lunar grains 
in more mature soils have rims that are > 100 nm while in less mature soils they are <60 nm or 
even incomplete. 
 
Fig 2f in the caption is as a darkfield image but high Z elements id darkfield images are bright. 
Both the Fe and the Pt are dark. 
 
Fig 3c the caption says TEM-BF image but the image label states correctly that it is a HAADF 
image. Also in this image, the label for the scale bar is either too light to see or is not there. And 
last, the caption for image 3d says the SAED pattern from the circled area in 3c but there is no 
area in 3c circled that I can see. 
 
Suggestions for different word choices: iron monosulphide can just be called iron sulfide 
(abstract); relicted is a better word for heirloom (introduction line 50), which is used to describe 
grains leftover from the early solar system; (line 106) the authors use the word segregation but I 
think they mean coalescence; lines 113-114 the authors use the terms vesicles and cavities as if 
they refer to different physical properties. If the two terms mean the same thing (which I think 
they do) I would recommend the authors not say vesicular regions and cavities or vesicles and 
cavities but just say vesicles; I would recommend not using modifiers like very and heavily. 
 
It would also be good to make Supplementary Fig. 1. much bigger. I could barely see the features. 
 
Supplementary Fig. 4., I do not see where troilite is coexisting with olivine in the figure 4a, it 

would be great if the image were annotated with rim and host olivine, also the context for the 
compositional should be given (i.e., where do those rectangular composition maps overlay on the 
sample). Because I can't tell where the x-ray data are from I don't know whether the C map shows 
a high C amorphous rim? or just the carbon coat? 
 

 
Reviewer #2 (Remarks to the Author): 
 
Review of Iron whiskers on asteroid Itokawa indicate sulphide destruction by space weathering – 
Matsumoto et al. 
 
This manuscript presents important observations of the effects of space weathering on sulfide 
minerals returned by the JAXA Hayabusa spacecraft. These observations are important as they 
relate to the long-standing puzzle in planetary science about the depletion of sulfur observed on S-
type asteroid surfaces, most notably the near-Earth asteroid (433) Eros characterized by the NASA 
NEAR-Shoemaker mission. The authors present a thorough and meticulous data set, including 
many outstanding images of the asteroid grain surfaces. The inferences about sulfur loss are 
important not only for understanding asteroid surfaces but also for understanding sulfide mineral 
evolution in the interstellar medium and protoplanetary disks. As such, this works has applicability 
to a broad range of space scientists and is suitable for publication in Nature Communications. It is 
well-written and organized. I have a few minor comments to help improve the manuscript. 
 
Under the “Mechanisms of whisker formation” section, the authors quickly rule out formation by 
condensation. I suggest that this mechanism needs a more thorough treatment. The work cited in 
references 28-30 provides a solid foundation for their preferred mechanism. However, whisker 
growth via condensation has been discussed in the meteorite literature (e.g., Bradley, John P., 
Ralph P. Harvey, and Harry Y. McSween Jr. "Magnetite whiskers and platelets in the ALH84001 
Martian meteorite: Evidence of vapor phase growth." Geochimica et Cosmochimica Acta 60.24 
(1996): 5149-5155.; Fries, Marc, and Andrew Steele. "Graphite whiskers in CV3 meteorites." 
Science 320.5872 (2008): 91-93.; Bradley, John P., Donald E. Brownlee, and D. R. Veblen. 
"Pyroxene whiskers and platelets in interplanetary dust: evidence of vapour phase growth." Nature 
301.5900 (1983): 473.). Furthermore, micrometeorite impacts into Itokawa grains are known to 
deposit energy into a small volume of the asteroid surface, causing melting, vaporization, 
ionization, and recondensation of the target and impactor material (M. S. Thompson et al., 
Microchemical and structural evidence for space weathering in soils from asteroid Itokawa. Earth, 
Planets and Space 66.1, 89 (2014). Thus, it is not so straightforward to dismiss vapor growth as a 
potential source mechanism. The authors should at least reference this other work and compare it 
to the preferred whisker growth mechanism more thoroughly. 
 
For the whisker composition, it would be useful to have the quantitative Ni data for all the whisker 
compositions. Also, I request that the authors include the Ni profiles in the whiskers that showed 
enrichment in this element. Other elements that are commonly associated with troilite in ordinary 
chondrites are Cr, P, Co, and Cu (e.g., Lauretta, D. S. "Sulfidation of an Iron–Nickel–Chromium–
Cobalt–Phosphorus Alloy in1% H 2 S–H 2 Gas Mixtures at 400–1000° C." Oxidation of Metals 64.1-
2 (2005): 1-22.; Rubin, Alan E. "Metallic copper in ordinary chondrites." Meteoritics 29.1 (1994): 
93-98.). For these other elements, it would be important to know if they were analyzed for, as well 
as their detection limits. These elements could provide important clues to the formation 
mechanism. 
 
The authors discuss both solar-wind irradiation and solar heating as potential mechanisms for the 
production of the iron whiskers. These two mechanisms do not seem to be mutually exclusive. In 
particular, the sulfur evaporation rate is determined to assume a vacuum environment. However, 
this mechanism is likely to be enhanced by the presence of solar-wind implanted hydrogen. If it is 
not feasible to include the effect of hydrogen in the evaporation model, then this interaction should 
at least be discussed in the supplement section on this model. 
 
 
Reviewer #3 (Remarks to the Author): 
 
Lines 23-24 (abstract) state "Whisker growth is a novel and unexpected aspect of space 
weathering." This is incorrect, whisker growth associated with space weathering has been widely 
reported, mostly in the context of space weathering on the lunar regolith, for the past 50+ years. 
Many papers are published on the topic yet none of them are cited in this manuscript. Perhaps the 
most relevant paper is by Carter 1973 who describes metallic iron whiskers on the moon, 
interpreted to result from micrometeoroid impacts. 
 
Hibbs, A. R. A hypothesis that the surface of the moon is covered with needle crystals. Icarus, 2, 
181-186 (1963). 
 
Brownlee et al., Whiskers on the moon. In Analysis of Surveyor 3 material and photographs 
returned by Apollo 12. NASA SP-284, 295 pages, published by NASA, Washington, D.C., 1972, 
p.236 
 
Carter, J. L. VLS (Vapor-Liquid-Solid): Newly Discovered Growth 
Mechanism on the Lunar Surface? Science 181, 841-842 (1973). 
 
Steel, A. et al., Graphite in an Apollo 17 Impact Melt Breccia. Science 329, 51 
(2010).