Strong nickel enrichment co-located with redox-organic interactions in Neretva Vallis, Mars

Domain: physics
Published: 2026-03-31
Editorial outcome: major_revision

Abstract
In 2024, NASA’s Perseverance rover explored Neretva Vallis, an ancient river channel that once transported water into Jezero crater. There, the rover encountered Mg-poor mudstones with diverse alteration features. In 32 rock targets in Neretva Vallis, nickel (Ni) was detected by the SuperCam instrument with concentrations in individual rocks as high as ~1.1 weight percent – the highest abundance ever seen in bedrock on Mars. In this work, we describe and contextualize these Ni enrichments using outcrop-scale imagery and petrographic-scale elemental maps provided by the PIXL instrument. We find Ni enrichment in Fe-sulfides and their weathering products. The geochemistry and morphology of Neretva Vallis Fe-sulfides are similar to pyrite present in terrestrial Archean and Paleoproterozoic sedimentary rocks. As an essential element for terrestrial microbial life, the proximity of Ni enrichment to reduced sulfur and organic matter adds to the interest in bringing back to Earth the rock sample collected by Perseverance at this location, which could provide key insights into complex redox chemistry on early Mars.

Introduction
The Mars 2020 Perseverance rover landed in Jezero crater, Mars, in February 2021 with the goal of searching for ancient habitable environments and collecting core samples for eventual return to Earth as part of a future Mars Sample Return mission1. Jezero is a 45 km-diameter Noachian (~3.8–4.0 Ga) impact crater that once hosted a lake, as evidenced by two inlet valleys, their associated fluvio-deltaic fans, and an outlet valley on the eastern edge of the crater2. Since landing, the rover has traversed the igneous crater floor3,4, ascended onto the western fan deposit5, crossed the olivine- and carbonate-rich Margin Unit (MU)6, and entered into the western inlet valley known as Neretva Vallis.

Investigation of Neretva Vallis began to the north, where Perseverance discovered a light-toned bedrock exposure named Bright Angel (BA) (Fig. 1c). Here, the rover identified clay-bearing mudstones overprinted by diagenetic features such as nodules and mineral veins7. At BA the rover also found ‘leopard spots’: mm- to sub-mm scale rounded reaction fronts with cores of Fe-sulfide, alongside detections of organic carbon7. South of BA lies the Masonic Temple (MT) exposure, where outcrops exhibit similar expressions but also contain poorly sorted, unstratified conglomerates with diverse, mm- to cm-scale clasts, including highly oxidized clasts of mudstone. These lithologies within Neretva Vallis collectively comprise the BA formation.

The features and textures of the BA formation indicate deposition in a low-energy, likely lacustrine, environment and while the structure of the unit may support that it is a younger deposit, the age relationship to the MU exposed in the channel walls remains poorly constrained7. As such, these rocks may represent a young, channel-fill deposit, or a much older unit exposed by the incision of Neretva Vallis8.

Geochemically, BA formation rocks are distinct from rocks encountered elsewhere in Jezero crater – including in the MU, western fan, crater floor, and Jezero watershed materials inferred from orbital remote sensing9,10. The BA formation is exceptionally Mg-poor (averaging ~2 wt% MgO) and contains higher abundances of SiO2, Al2O3, and FeO relative to the mafic-ultramafic rocks investigated elsewhere in Jezero crater. This pronounced geochemical difference suggests a unique provenance or paleoweathering history.

Results
Throughout rock exposures in Neretva Vallis, enrichments in nickel (Ni) were detected by the SuperCam instrument via the remote Laser Induced Breakdown Spectroscopy (LIBS) technique. This marked the first detection of Ni with SuperCam, apart from an enigmatic class of highly altered Al-rich float rocks found scattered across the Perseverance rover’s traverse11. SuperCam is a mast-mounted instrument with a suite of remote spectroscopic techniques including LIBS, visible/near-infrared (VISIR), and Raman spectroscopy12,13. LIBS samples ~100–450 µm in diameter, meaning most observations sample a mixture of minerals when grain size is smaller than the LIBS footprint14. This is particularly true of Neretva Vallis sedimentary rocks, which have a grain size of ~40–60 µm according to X-ray diffraction7. SuperCam LIBS observations are arranged in rasters and are sensitive to various major and trace elements including Ni13,15. SuperCam’s Ni calibration model has a limit of detection of ~0.12 wt.% and a root mean squared error of prediction (RMSEP) of 0.12 wt. % (see Methods for more details). Ni was detected (>0.12 wt. %) in 32 of the 126 distinct rock targets analyzed by SuperCam LIBS in Neretva Vallis (Fig. 1). These Ni-rich targets are concentrated in two distinct localities: Beaver Falls and Wallace Butte (Fig. 1).

In northern Neretva Vallis (the Beaver Falls workspace, Figs. 1a, c, and 2), Ni is observed to be elevated in targets located within ~5 meters of both sides of the contact between the MU and BA sedimentary rocks. Detections of Ni are found within the dominant mudstone/siltstone lithology of BA at this location (e.g., Apollo Temple abrasion, Fig. 1a and “Soap Creek”, Fig. 2). Concentrations of 0.19–0.24 wt.% Ni were found in the most Fe-rich points of each SuperCam raster – suggesting it is present in a Fe-rich phase (Supplementary Data 1). Ni was also detected in a series of Ca-sulfate veins (e.g., Seiber Point, Fig. 2) which cross-cuts the primary mudstone lithology. In these veins (4 distinct SuperCam targets), 0.33–0.56 wt.% Ni was observed (Supplementary Data 1), but only in visibly dark Fe-rich points – suggesting Ni is present in a Fe-rich phase mixed with Ca-sulfates. SuperCam also detected up to 0.83 wt.% Ni (Supplementary Data 1) across the contact in the adjacent olivine-rich MU to the north as well as the Fern Glen Rapids member (Fig. 1a), a moderately sorted granule conglomerate interpreted as a subaqueous debris flow with clasts potentially derived from the MU. Notably, Ni-rich rocks in the putative MU exhibit high Fe and S abundances with LIBS, suggesting the presence of Fe-sulfide or Fe-sulfate.

Following the exploration of the BA-MU contact, Perseverance drove south into MT (Fig. 1c), a bedrock exposure near the southern wall of the Neretva Vallis channel. First observed there were the poorly sorted and highly oxidized conglomerates of the Mount Spoonhead workspace, where no significant Ni concentrations were detected. A traverse further south revealed the Wallace Butte workspace (Figs. 1b, c), characterized by a highly-oxidized (red) flat-lying conglomerate facies, with abundant mudstone matrix (e.g., Dragon Creek and Wallace Butte, Fig. 1b) as well as higher standing, dark, Fe-rich rocks (e.g., Tower of Ra, Fig. 1b) – both of which had strong Ni detections with SuperCam. Ni enrichments in the bulk mudstone (eight distinct observations) ranged from 0.14–1.21 wt.% (Supplementary Data 1) - with the highest values seen in Fe-rich LIBS observations, again indicating the presence of Ni in a Fe-bearing phase.

It is unclear whether the higher standing Ni-rich rocks (0.39–0.88 wt.%, Supplementary Data 1), are float rocks or, instead, represent poorly exposed and weathered bedrock. Notably, SuperCam infrared spectra (co-located with LIBS) of the Tower of Ra and Elaine Castle targets (Fig. 1b) exhibit a weak absorption band at 2.27 µm and weaker features near 2.42, 2.46, and 2.53 µm (Fig. S1) – consistent with jarosite16,17. An nearby high standing Fe-rich float rock, Phantom Creek, exhibits a strong spectral signature of the chlorine bearing iron oxide akaganeite but shows no Ni detection with SuperCam LIBS16,18.

Ni detections with SuperCam LIBS are highly localized within Neretva Vallis and have not been observed in bedrock targets elsewhere in Jezero Crater. In the dominant mudstone lithology of Beaver Falls, less than half of individual LIBS observations show elevated Ni detections (>0.12 wt.%), which suggests Ni is present in a minor, possibly dispersed, host phase. In contrast, within the Fern Glen Rapids facies, Ni abundances are consistently elevated across LIBS rasters, indicating either enhanced concentration or preservation. Fern Glen Rapids contains ~0.18 wt.% Ni in bulk (Supplementary Data 1), similar to enrichments seen in low-grade Ni ores on Earth19. In the Wallace Butte workspace, the average Ni in the mudstone was significantly higher, at ~0.29 wt.% (Supplementary Data 1). This includes Dragon Creek, a homogenous fine-grained target which averages ~1.1 wt.% Ni – the strongest enrichment in Ni ever seen in bedrock on Mars (Fig. 3b, Supplementary Data 1). Comparison with LIBS-derived major element composition shows that, excluding Dragon Creek (which has an unusually low Fe:Ni ratio), Ni abundance correlates with FeO (R2=0.66), suggesting that Ni is primarily hosted in a Fe-bearing phase (Fig. 3a).

Across Neretva Vallis, PIXL performed 15 scans on 8 distinct rock targets. In the north of BA, PIXL explored the Beaver Falls workspace with observations of three rock targets: the Apollo Temple abrasion patch, Cheyava Falls natural surface (not inside abrasion patch), and the Steamboat Mountain abrasion patch (of the Fern Glen Rapids member). At Apollo Temple, a representative mudstone, three PIXL scans revealed Ni enrichments associated with Fe-sulfide phases. In one scan, Ni-enriched points were localized in a ~0.3 mm wide, dark linear vein composed of Fe-sulfide. In two other scans, Ni was concentrated in dark-toned ~1–1.5 mm domains of Fe-sulfide (Fig. 4). One of these zones has been interpreted as greigite, a metastable precursor to pyrite7.

The Cheyava Falls natural surface, located on the same block as the Apollo Temple abrasion patch, also exhibited a ~1 mm Ni-rich, Fe-sulfide domain. At the Steamboat Mountain abrasion patch – stratigraphically below Apollo Temple and Cheyava Falls – PIXL identified coarse, sand- to granule-sized detrital grains of olivine and Fe-Mg carbonate surrounded by Ca-sulfate and supported in a mudstone matrix. Ni enrichments were detected primarily along the edge of olivine grains. In contrast, no enrichments in Ni were noted in either of the two PIXL scans in Southern BA and northern MT.

PIXL observations at Wallace Butte and Malgosa Crest in southern MT revealed a different host environment for Ni. Both targets sampled the dominant mudstone lithology of this workspace. At Wallace Butte (Fig. 5b), the first scan showed Ni concentrated in a ~1 mm rounded, dark-toned nodule that was associated with moderate Mg and S enrichments. The second scan identified two distinct Ni-enriched zones: one ~1 mm sub-rounded Fe-Mg-S nodule and one ~0.2 mm wide linear vein rich in a phase interpreted as Mg-sulfate (Fig. 5). At Malgosa Crest, Ni was associated with irregular bright patches and small zones (~0.4–0.5 mm) of Mg-sulfate.

PIXL characterized Ni in Neretva Vallis sedimentary rocks, primarily in the form of Fe-sulfides and their sulfate alteration products, through an integrated analysis of acquired X-ray fluorescence and X-ray scatter data21. In the primary mudstone of Beaver Falls (Apollo Temple, Fig. 4), Ni is associated with domains of Fe-sulfide (identified by its Fe/S ratio, Fig. 6a), while stratigraphically lower conglomerates (Steamboat Mountain) exhibit Ni that is distributed adjacent to detrital olivine grains. Southward in Wallace Butte, Ni is preferentially found in irregular zones and veins of Mg-sulfate (Fig. 5b and Fig. 6b), consistent with authigenic precipitation during post-depositional alteration.

Methods
With SuperCam LIBS, a pulsed laser is focused on a target, typically 2–6.5 m away, generating a plasma12,13. The diameter of the irradiated spot ranges from 100–450 µm, increasing with distance13. As the plasma cools, light is emitted with a wavelength characteristic of the elements present in the sample. SuperCam collects light in three spectrometers covering the ranges 243.5–341.7 nm, 382.1–467.5 nm, and 535–853 nm12. Typical SuperCam operations involve taking 30 laser shots (the first 5 being used to remove dust cover) at a single location, which are then averaged13. The mast is then rotated to take subsequent observations in the same manner, usually completing a 10×1 raster across a rock surface13.

Various trace elements have been detected and quantified using LIBS data from SuperCam’s predecessor ChemCam, providing valuable insight into the diagenetic and aqueous history of Gale Crater58–61. To quantify Ni for this work, a suite of pressed powder samples was created from standards purchased from Brammer Standard Company, Inc with well characterized Ni contents. A total of 64 powder mixtures were made from several endmembers including gypsum (CaSO4·2H2O), kaolinite (Al2Si2O5(OH)4), and basalt – all doped with varying amounts of Ni-oxide powder (0–22 wt. %). Samples were created by carefully weighing and thoroughly mixing the end members in set proportions and pressing them at 15 tons for 3 minutes using a manual hydraulic press. The samples were then shipped to Los Alamos National Laboratory, where they were analyzed by LIBS using the SuperCam Engineering Qualification Model (EQM). Samples were shot in three separate locations to account for any heterogeneity in the material. In addition to the pellets created specifically for this work, LIBS spectra collected by the SuperCam EQM from 129 samples as part of the major element calibration effort were added to the dataset to improve performance at very low (or zero) wt.% Ni. All LIBS spectra were processed via the same pipeline used for Mars data as described in Anderson et al. 2021 (dark subtraction, denoising, instrument response correction, continuum removal, etc.)62.

Samples were then randomly divided into 5 folds, 1 for testing and 4 for training. Various multivariate regressors commonly used for LIBS elemental calibrations were implemented and compared using their root mean square error of prediction (RMSEP) and consistency over a range of different target compositions. Partial Least Squares (PLS), Elastic Net, Support Vector Regression, Random Forest, and Extremely Randomized Trees were all attempted, each being tuned using a cross-validation technique which optimized for various hyperparameters that modify regressor performance.

Of all the regressors tested, Extremely Randomized Trees (ExtraTrees)63 had the lowest RMSEP and most stability across target types. ExtraTrees is an ensemble machine learning method which builds a forest of binary decision trees and combines their results to make predictions63. ExtraTrees, and the related Random Forest algorithm, while often more challenging to interpret than traditional multivariate methods like PLS, has proven very effective in predicting both major and minor elements with SuperCam LIBS58,62.

The ExtraTrees model developed to predict Ni with SuperCam LIBS has a RMSEP of 0.12 wt. % and the correlation coefficient (R2) between the predicted and certified wt. % Ni in the test set is 0.9 (Fig. S2). This demonstrates that over the range of Ni tested (0–2 wt. %), the model can predict Ni content with an average error of 0.12 wt. %. In addition, the coefficients for the model can be calculated. The coefficients of an ExtraTrees model show how important each feature (spectral channel) is to the overall prediction based on its ability to split data across all decision trees. The coefficients of the chosen model are highly elevated at the Ni I emission lines at 301.288 nm, 305.170 nm, and 310.278 nm (Fig. S3). This indicates that Ni is being predicted successfully based on Ni peaks present in the training set, rather than other LIBS emission lines that are merely correlated with wt.% Ni. There are, however, other LIBS emission lines with positive loadings, meaning that virtually all SuperCam Mars data will have a predicted Ni wt.% greater than zero. The detection limit of Ni with SuperCam LIBS, based on qualitative analysis, is ~0.12 wt.% (Fig. S4) and therefore, as expected, the vast majority of SuperCam Mars spectra have no detectable Ni emission lines – this creates a number of false positives.

To prevent false positive detections of Ni in SuperCam Mars data, a line checker was implemented. Peak areas for each of the three major Ni emission lines in all SuperCam Mars data were calculated with a custom peak fitting code written in Python using the lmfit package64. Only Ni quantifications where all three emission lines have a peak area above a set threshold (4×10-5) were reported. This ensures that false positives are eliminated. All SuperCam Mars data with Ni detections were verified to have Ni lines. A selection of the highest Ni quantifications was compared to laboratory LIBS spectra with a similar concentration of Ni to verify there was a qualitative match (e.g. Fig. S5).

PIXL (Planetary Instrument for X-Ray Lithochemistry) is a micro-X-ray fluorescence (µXRF) instrument mounted on the robotic arm of the Perseverance rover20. PIXL produces mm-scale elemental maps by irradiating rock surfaces with highly focused X-rays and measuring the subsequent X-rays emitted that are diagnostic of specific electronic transitions20. PIXL’s interrogation spot size varies with X-ray energy and has a FWHM of ~120 µm at ~8 keV65. Thousands of analysis spots are included in a typical PIXL map scan. Elemental quantification for each analysis spot is accomplished using PIQUANT66, which utilizes fundamental parameters and a limited suite of calibration standards. PIXL can detect and quantify major and minor elements, as well as trace elements, including Ni20. Precise quantification of Ni can be complicated by several factors, including limited counting statistics, X-ray diffraction, bremsstrahlung background modeling, detector temperature, and Fe Kβ tailing (e.g., Van Bommel et al., 2019; 2025; Christian et al., 2023). Typical detection limits for Ni with PIXL are on the order of 100 µg/g. This study focuses on using PIXL to better understand what phases host Ni by assessing elemental maps of Ni pseudo-intensities, visualized using the PIXLISE software20.

As discussed in Allwood et al. 20 and Lawson et al. 67, pseudo-intensities represent low data volume estimates of XRF line-intensities that are generated by the PIXL instrument on board the Perseverance rover. The onboard analysis involves summing the two X-ray histograms (represented as count rates binned across 4096 detector energy channels) collected at each location in a PIXL scan. The summed histogram is then smoothed, the histogram background is estimated and subtracted, and element line intensities are estimated by integrating over channel ranges specific to elements of interest, in our case, nickel. Because the pseudo-intensity values are calculated from the sum of the signal from both of PIXL’s detectors, it is possible for pseudo-intensity values to be overestimated if the measured signal from one of the two detectors contains contributions from the elastic scattering of X-rays from the PIXL X-ray source when they encounter the lattice structure of minerals in the scan area68,69. To ensure that our Ni-pseudo-intensity data is not affected by these diffraction phenomena, we have constructed maps of diffraction intensity in the vicinity of the Ni XRF line energy corresponding to the Ni-pseudo-intensity maps shown on Figs. 2 and 4. These diffraction intensity maps are presented in Figs. S6 and S7, and demonstrate that the pseudo-intensity of Ni shown in Figs. 4 and 5 are not the result of X-ray diffraction.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have fully addressed my comments.\nOn a different note, regarding Ni-rich sulfide in martian meteorites, they should look at work done by JP Lorand et al. (2015)\non the Ni-rich Fe-sulfides in the breccia, NWA 7533:\nNickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccias: a study in NWA 7533. Meteoritics &\nPlanetary Science 50, 2099-2120.", "reviewer_2": "General Comments\nThe authors have greatly improved this manuscript in several ways and the concerns I\nraised previously were addressed well. The reorganization of the “Discussion” section\nwith subheadings has clarified the analyses and I find the manuscript much easier to\nfollow in the current form. I especially like the way the authors have tied in their Ni\ndiscovery with the meteorite record and early evolution of life on Earth. This is an\nintriguing study of Ni in several rocks at the Neretva Vallis region in Jezero crater and\nprovides an important advancement in our understanding of these unique rocks that\nlikely experienced redox chemistry and potentially biogeochemistry. I find the presence\nand importance of Ni in the regions described compelling and I think this paper is nearly\nready for acceptance.\nI have a few relatively minor comments that I hope the authors will find useful in\nimproving this paper. Should the authors have questions regarding my review, they are\nwelcome to reach out to me directly: Janice Bishop – jbishop@seti.org.\nSpecific Comments\nLine 31 I suggest changing “We find nickel enriched in Fe-sulfides” to “We find Ni\nenrichment in Fe-sulfides”. 1) nickel already defined as Ni. 2) Ni enriched in Fe-sulfides\nsounds to me as if the Ni has an enrichment of Fe-sulfides, whereas the Fe-sulfides are\nenriched in Ni.\nLines 31 & 32 use same format for Fe-sulfides (9 occurrences) or iron sulfides (2\noccurrences) here and throughout the paper\nLines 57-58 If I'm understanding correctly that the “BA formation” terminology includes\nboth the BA and MT sites, it would be helpful to say that explicitly, e.g. “The BA and MT\nlithologies…” And if this is accurate, then correct the wording of “BA and MT formation”\nin the caption of Fig.1. Conversely, if the Fig. 1 caption is accurate, then correct the\nwording on line 58.\nLine 147 this would be a good place to cite Bishop & Murad (2005) for the spectral\nproperties of jarosite\nLine 149 this would be a good place to cite Bishop et al. (2015) for the spectral\nproperties of akaganeite\nLine 337 I believe this should be Fig. 1b rather than Fig. 2\nLines 339 & 354 2 misspellings of akaganeite are still present (it seems that Microsoft\nhas this spelled wrong in their dictionary, which could be the culprit)\nLine 370 I suggest adding “spectroscopy” following “Raman”\nLine 373 I suggest changing “in any case” to “either way”. To me that better represents\nthe idea that regardless of which of the 2 scenarios is true, the discovery is still\nimportant.\nED Fig. 1 I still find these plots difficult to read. Thicker lines for each spectrum and\nthicker lines for the axes and larger and bold font for the labels would be helpful. Please\nalso add tic marks on the upper X axis. I also suggest moving the BKR1 etc IDs from\nthe plot to the figure caption so that the mineral names can be made larger on the plot.\nLine 770 Most spectra are from RELAB as mentioned here; however, the bassanite\nspectrum is from the USGS spectral library. Their library doesn’t seem to be working\nonline anymore (maybe there are issues since Roger Clark and Gregg Swayze\nretired?). Thus, instead I suggest citing Bishop et al. (2014) that includes Gregg Swayze\nas a co-author and describes the bassanite spectrum used here. Another option would\nbe to cite Jim Crowley, since he originally prepared this sample and published his\nspectrum in Crowley (1991), which is similar to the spectrum I provided (made from\nsplicing the USGS library spectra).", "reviewer_3": "The authors have thoroughly revised their manuscript, and my earlier concerns are now effectively addressed. My review\ncan therefore be rather short:\nI very much like the new Fig.2 which clarifies the Ni distribution. I earlier was wondering about the various Ni sources, and\nthis is now explained much better (particularly also that a Ni source from meteorites cannot be excluded).\nIn my previous review, my main comment was that high Ni concentrations cannot exclusively be interpreted as a smoking\ngun for prebiotic chemistry and the origin of life. The authors have now rewritten a large part of the Discussion, and in the\nsection ‘Astrobiological Potential of Ni-rich Mudstones’ provide a much more balanced interpretation.\nI therefore recommend publication without further revisions."}, "author_response": "Response to Reviewers\nResponse: The authors really appreciate your detailed comments and thorough\nevaluation of our manuscript – it has been significantly improved because of your\nfeedback! We have addressed the specific comments below.\nSpecific Comments\nLine 31 I suggest changing “We find nickel enriched in Fe-sulfides” to “We find Ni\nenrichment in Fe-sulfides”. 1) nickel already defined as Ni. 2) Ni enriched in Fe-sulfides\nsounds to me as if the Ni has an enrichment of Fe-sulfides, whereas the Fe-sulfides are\nenriched in Ni.\nResponse: Good suggestion! That change has been made now.\nLines 31 & 32 use same format for Fe-sulfides (9 occurrences) or iron sulfides (2\noccurrences) here and throughout the paper\nResponse: Good catch. I have changed the references to ‘iron sulfide’ and replaced\nthem with ‘Fe-sulfide’. (line 225, line 32).\nLines 57-58 If I'm understanding correctly that the “BA formation” terminology includes\nboth the BA and MT sites, it would be helpful to say that explicitly, e.g. “The BA and MT\nlithologies…” And if this is accurate, then correct the wording of “BA and MT formation”\nin the caption of Fig.1. Conversely, if the Fig. 1 caption is accurate, then correct the\nwording on line 58.\nResponse: You’re correct that the consensus is that the BA formation includes both the\nBA and MT sites. I have changed the wording to say ‘outcrop’ rather than ‘formation’ in\nCaption 1 (line 109-110).\nLine 147 this would be a good place to cite Bishop & Murad (2005) for the spectral\nproperties of jarosite\nResponse:  added! Thank you\nLine 149 this would be a good place to cite Bishop et al. (2015) for the spectral\nproperties of akaganeite\nResponse:  added!\nLine 337 I believe this should be Fig. 1b rather than Fig. 2\nResponse: Good catch! I’ve made that change.\nLines 339 & 354 2 misspellings of akaganeite are still present (it seems that Microsoft\nhas this spelled wrong in their dictionary, which could be the culprit)\nResponse: Thank you for finding this, I have now fixed those misspellings.\nLine 370 I suggest adding “spectroscopy” following “Raman”\nResponse:  added!\nLine 373 I suggest changing “in any case” to “either way”. To me that better represents\nthe idea that regardless of which of the 2 scenarios is true, the discovery is still\nimportant.\nResponse:  Good call, I’ve changed it to ‘either way’ in the last line of that\nparagraph.\nED Fig. 1 I still find these plots difficult to read. Thicker lines for each spectrum and\nthicker lines for the axes and larger and bold font for the labels would be helpful. Please\nalso add tic marks on the upper X axis. I also suggest moving the BKR1 etc IDs from\nthe plot to the figure caption so that the mineral names can be made larger on the plot.\nResponse: I have made those changes now, thank you for the suggestions!\nLine 770 Most spectra are from RELAB as mentioned here; however, the bassanite\nspectrum is from the USGS spectral library. Their library doesn’t seem to be working\nonline anymore (maybe there are issues since Roger Clark and Gregg Swayze\nretired?). Thus, instead I suggest citing Bishop et al. (2014) that includes Gregg Swayze\nas a co-author and describes the bassanite spectrum used here. Another option would\nbe to cite Jim Crowley, since he originally prepared this sample and published his\nspectrum in Crowley (1991), which is similar to the spectrum I provided (made from\nsplicing the USGS library spectra).\nResponse: Thank you for the suggestion! I’ve gone ahead and added citations to\nBishop et al. 2014 and Crowley et al. 1991."}]

Decision letter