Peer review of: Effect of sulfur on sound velocity of liquid iron under Martian core conditions (2020)

Reviewers' Comments: 
 
Reviewer #1: Remarks to the Author: 


The paper "Effect of sulfur on sound velocity of liquid iron under Martian core conditions" describes 
the results of high-pressure-high-temperature laboratory experiments on the velocity of seismic P-
waves in liquid Fe-S mixtures with different Sulphur concentrations. Experiments are carried out at 
pressures relevant for the martian core-mantle boundary and results extrapolated to the pressure 
at the center of Mars. The obtained velocities can be used to test the hypothesis that the martian 
core is a Fe-S mixture. As NASA's InSight mission is currently recording seismic events on Mars, it 
is timely and necessary to have this kind of result at hand when it comes to the interpertation of 
relevant seismic data. 
 
I should first make clear that I am not a specialist for the kind of experiment described in the 
paper, but a seismologist who is mainly interested in the applicabiltiy of the results. I can therefore 
not really judge the quality of the experiments themselves and trust that another reviewer will do 
so. 
 
It is rare, although highly desirable, to get a clear statement like the authors make: if the 
veolocities measured in the martian core are outside the experimentally determined range, then a 
Fe-S composition can be ruled out. It would be great to have more such results for other types of 
composition (as a number of light elements have been proposed to be present in the martian 
core). This is not intended as criticism, since the authors can obviously not do everything at once. 
 
Although the determination of the martian core radius is one of the main goals of InSight, this 
does not necessarily imply that the P-wave velocity directly underneath the CMB will be available 
at the same time. The core radius may be determined from the extent of the core shadow or from 
travel times of seismic phases like PcP and ScS - such results will not provide a look into the 
uppermost core. Also, since the core likely has lower P wave velocity as the lowermost mantle, it 
will not be possible to determine P wave velocity from the slope of travel time curves, as there will 
be a core shadow (and hence no travel time curve for the uppermost part of the core). This limits 
the applicability of velocity as criterion. Although it is not the purpose of this paper to describe an 
appropriate seismic experiment, it would certainly be strenghtened if the authors could include 
some comments on the type of seismic phases needed. I do not consider this mandatory. 
 
A more important point, which I do consider mandatory, concerns the interpretation of seismic 
velocities measured by the authors. The figure showing seismic mwaveforms and phase 
identifications is nice and clear, I was however missing a schematic view that explains the different 
recorded phases (termed R1, R2, R3 and so on) in terms of the propagation paths through the 
sample. I suggest the authors include a diagram showing the sample geometry, the locations of 
sources and sensors, and the ray paths associated to the different phases indicated in the current 
figure. Also the figure contains to labels ("L" and "2T") which are not explained in the caption nor 
in the text, such that it is unclear what they are and how they are related. This point is of utmost 
importance, since the velocity result depends critically on the correct identification of phases, and 
the associated propagation paths. 
 
The omission of error bars in a figure in the main text is, in my opinion, not acceptable. Not all 
readers will download and inspect the supplementary material upon first read, but they should 
nevertheless find the uncertainty information. I did not have the impression that the figures 
containing error bars in the supplement are unreadable. When I see a figure where the caption 
states that error bars are omitted for clarity, I immediately assume that errors are so big that 
fitting a curve is meaningless. 
 
An information I was also missing in the manuscript is the acoustic wavelength used for sounding 
the samples, and how it relates to sample size and path length. Please add a few numbers in this 
regard. 
 
In some cases, fitted curves shown in the supplement do not fit well to some of the data points, 
but are very close to others. May be the authors decided to fit only part of the data, but this needs 
to be indicated (and justified), and otherwise it needs to be discussed why some fits look poor. 
 
For the center of Mars, the authors could only extrapolate their experimental results. Although I 
understand that this might be due to technical limitations of the apparatus (which needs to be 
indicated), I recommend that the authors discuss if it can be expected that the assumed mixtures 
remain liquid at such pressures and temperatures, and what the liquidus and solidus conditions are 
expected to be. This must be seen in the light of the open question whether or not Mars has a solid 
core. 
 
In general, seismolgists prefer thinking in terms of depth rather than of pressure. The uncertainty 
of the martian core radius, based on current knowledge, is several hundred km (may be up to +/- 
500 km, when pessimistic). The authors should indicate what core radius (or range) is associated 
to the pressures used in their experiments. 
 
The authors give, at the end of the paper, the names of some beam types used to "x-ray" their 
samples. To me, the given designations do not mean much. I would find it helpful if the names of 
the facilities and institutions running the facility, and the location of the facilities, could be added. 
 
 
 
Reviewer #2: 
Remarks to the Author: 

This paper by Nishida et al. on Effect of sulfur on sound velocity of liquid iron under Martian core 
conditions reported the sound velocity of liquid Fe and Fe-S alloys using the ultrasonic method 
combined with large volume press and synchrotron X-ray up to 20 GPa and 2320 K, corresponding 
to the Martian core conditions. The velocity and density data of liquid Fe core alloys with a 
reasonable accuracy are required to evaluate such planetary seismological observations. However, 
the pressure range of such data has been stacked at below 10 GPa in last decades. The authors 
broke the situation and succeeded to measure the velocity of liquid Fe-S alloys to 20 GPa, that is 
double pressure range previously reported and reaches to Martin core conditions. Based on the 
present results, the authors model the seismic wave speeds through the Martin core as a function 
of sulfur content and found that the seismic profile of Martian core is insensitive to its sulfur 
content, which can be useful to evaluate often-used sulfur-rich Martian core model with the 
seismological data from the ongoing NASA’s Insight mission. 
 
The manuscript is well written, and the presented materials adequately illustrate the methodology 
and results. The authors’ achievement stands on the technical development as well as very-careful 
experiments by the authors including the steady evaluations of the velocity data and recovered 
samples. The sound velocity data presented here therefore appear to be of really-good quality. The 
seismic observations are one of the main projects of the Insight mission and the Martian core 
composition is strongly related to the building blocks and core formation process, such that the 
present results will be a key role in the mission and be spread to a number of fields such as 
cosmochemistry, planetary sciences, astronomy sciences, and other communities involved in the 
planetary explore mission. The developed technique here will be also applied to the fundamental 
physics like disordered-material physics as well as to material-sciences related to metallic glass 
and steel making. Therefore, I recommend this paper to be published in Nature Communications 
after fixing several issues I commented below. 
 
Major comments: 
-The authors claim that the sound velocity of liquid Fe is insensitive to the sulfur content and the 
Fe-S liquid Martian core is ruled out if the Martian core velocity was deviated from that of pure Fe 
(L40, L65-66, L113-115). This statement sounds funny for me. The deviation from pure Fe or Fe-S 
liquids cannot rule out the sulfur-rich Martian core because both Fe and S are not distinguishable 
from the velocity only as presented by the authors (L62-63). This statement can be established if 
the total Fe content (or light elements) in the Martian core is fixed from other evidence such as 
density and geochemical observations. Please carefully reconsider this statement. 
 
-The authors claim the EoS parameters presented here are “not physical properties just fitting 
parameters” (L227). If so, the Birch-Murnaghan EoS applied in this study also loses its validity 
because the EoS formula is based on the potential energy change by the finite strain and its 
parameters are resulted from the thermodynamically derivatives of the energy. As the authors 
suggested, it can be hard by the only present data to investigate the physically-meaningful density 
for the high-pressure liquid Fe-S. However, it is possible to evaluate the obtained density or its 
temperature when using previously reported the density data of liquid Fe-S by the authors 
(Nishida et al. 2011AmeMine; 2013PCM; Terasaki et al. 2019JGR; Umemoto et al. 2014JGR) 
and/or by the others (Morard et al. 2013EPSL; Huang et al. 2018JGR). 
 
Minor comments: 
-“…, only data collected at ~13 GPa and higher…” (L99-100), how did the authors chose this 
value? Though the slope in Fig. S1 looks to change around ~10 GPa for Fe80S20 but at ~6 GPa, 
more quantitative evaluation for the critical pressure is required. 
 
- In “Method”. It is well known that it is difficult to keep the liquid state under high-pressure and -
temperature for long time. The authors may carefully check the liquid during measurements based 
on XRD from the samples. The textural image of recovered samples of Fe-S can be evident of the 
liquid state during ultrasonic measurements. However, it is hard to judge the liquid state of pure 
metal such as Fe from recovered samples. I recommend the authors to show the XRD patterns for 
the judgment during experiments. In addition, please describe the time scale of the ultrasonic 
measurements for the present liquid samples, in other words, how long the authors kept the liquid 
under high pressure and temperature for the measurements. 
 
-Please give the uncertainties for the fitting parameters in “Supplementary Table 3”. 
 
-“assuming g is constant” (L243). It is unclear what values for which alloys the authors assumed 
here. The Grüneisen parameter has the volume and composition dependency. Also, several values 
for the Grüneisen parameters among Fe-S alloys are listed in the referred papers. Please clarify 
which values used here. 
 
-In Fig. S3, the symbols of "This study" and "I14” are hardly distinguished by my eyes. Please 
make clear diffidence among the symbols. 
 
-In Fig. S6, i of Ni in the legend should not be subscript. 
 
-Table S1. I am worry that the authors overestimate the oxygen contamination in the recovered 
sample. Usually, small amount of O can be detected even pure metal due to the surface oxidation 
of metallic samples during the sample preparation. Did you consider such an artificial oxygen in 
the EPMA analysis? 
 
 
 
Reviewer #3: 
Remarks to the Author: 
This manuscript reports new experimental data on the sound velocity of liquid Fe and two liquid 
Fe-S alloys (Fe80S20 and Fe57S43) to 20 GPa. These data add to existing data at relatively low 
pressures, but demonstrate a different pressure dependence. The manuscript claims that if these 
data can be extrapolated to to Martian core pressures (20-40 GPa), then there would be little 
effect of S on the velocity of liquid Fe. It this is the case, then future seismic data from planetary 
missions may not be used to constrain the S content in the core, but may suggest the presence of 
light elements other than S, depending on the actual seismic velocity observations. 
 
These are challenging experiments and the data are important to constraining the composition of 
the liquid outer core of Mars. However, there are some issues with the experiments as well as the 
data analysis as detailed below that could result in large uncertainties with the extrapolation, 
hence weaken the major point of the paper. These need to be clarified and additional 
measurements may be required to validate the results of the paper. 
 
1. I am concerned with the consistency of the data presented in this manuscript with some of the 
co-author’s previous data published in Terasaki et al, 2019, Journal of Geophysical Research, doi: 
10.1029/2019JE005936. This manuscript does not mention Terasaki et al. (2019) at all, but 
Terasaki et al. (2019) performed similar experiments on Fe-10Ni-17S and Fe-10Ni-30S up to 14 
GPa, and showed a different pressure dependence of Vp for the S-richer composition than 
observed in this study. Although there is some difference in composition that samples in Terasaki 
et al. contained Ni, but as discussed Terasaki et al, Ni has little effect on the velocity. The Terasaki 
et al. 2019 results need to be compared in this paper, and the discrepancy needs to be discussed 
and clarified in the manuscript. 

2. The sound velocity data for the Fe-S liquids were analyzed using Eqs (2-4) without the 
knowledge of density. The manuscript argued that although this would constrain the equation of 
state, but would affect the extrapolation for Vp. This needs to be demonstrated in more detail, for 
example different fittings with different assumed densities need to be performed to show the effect 
of the choice of density. It is potentially dangerous to leave one parameter unconstrained during 
the extrapolation (interpolation might be OK). Also, the calculation of adiabatic temperature (eq. 
(5)) requires the knowledge of density. The uncertainties in Ts due to the choice of density should 
also be discussed. 

3. The sound velocity of liquid Fe was determined using the type 2 assembly, in which the sample 
was fully surrounded by single crystal sapphire. Because of the hardness of single crystal sapphire, 
it is questionable that the sample inside would achieve the same pressure as indicated by the 
pressure markers that were partially surrounded by soft materials. This would result in some 
overestimation of the experimental pressure in these experiments, and hence could at least 
partially explain the discrepancy in the sound velocity of liquid Fe between this study and those 
from references 6, 7, and 27, 28. This need to be examined experimentally and discussed in the 
manuscript. 

4. In Figure 4, it can be seen that there is clearly some effect of S on Vp at 20 GPa and 30 GPa (as 
large as a few percent). It is not clear to me why this much effect cannot be distinguished from 
future seismic observations as claimed in the paper. Some more quantitative discussion may help 
explain this.