Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars

Domain: earth_science
Published: 2025-02-25
Editorial outcome: minor_revision

Abstract
Iron oxide-hydroxide minerals in Martian dust provide crucial insights into Mars’ past climate and habitability. Previous studies attributed Mars’ red color to anhydrous hematite formed through recent weathering. Here, we show that poorly crystalline ferrihydrite (Fe5O8H·nH2O) is the dominant iron oxide-bearing phase in Martian dust, based on combined analyses of orbital, in-situ, and laboratory visible near-infrared spectra. Spectroscopic analyses indicate that a hyperfine mixture of ferrihydrite, basalt and sulfate best matches Martian dust observations. Through laboratory experiments and kinetic calculations, we demonstrate that ferrihydrite remains stable under present-day Martian conditions, preserving its poorly crystalline structure. The persistence of ferrihydrite suggests it formed during a cold, wet period on early Mars under oxidative conditions, followed by a transition to the current hyper-arid environment. This finding challenges previous models of continuous dry oxidation and indicates that ancient Mars experienced aqueous alteration before transitioning to its current desert state.

Introduction
Identifying the dominant iron oxide phases in Martian dust can provide quantitative constraints on the planet’s past and present chemical environments, climate conditions and habitability1. On Earth, poorly crystalline ferrihydrite (Fe5O8H · nH2O) forms through rapid Fe2+ oxidation in aqueous environments at circumneutral pH and low temperatures, transforming to crystalline hematite (α-Fe2O3) in warm conditions or goethite (α-FeOOH) under sustained water presence2–4. These formation and transformation pathways can therefore constrain past environmental conditions including pH, temperature, redox state, and water availability. The reddish coloration of the dust-covered Martian surface has been investigated since the early telescopic observations that hinted at the presence of impure iron ore known as limonite, which contains the iron hydroxide goethite5–8. Meanwhile subsequent ground-based telescopic and laboratory observations attributed the reddish hue to the presence of pigmentary anhydrous hematite (termed “nanophase NpOx”) dispersed in the surface regolith and/or coating of rocks9,10. Based on the lack of water absorption features at near-infrared (NIR) wavelengths (1–2.5 μm) as determined by European Space Agency’s (ESA) Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) spectrometer11, it was argued that the anhydrous and dusty regions contain ferric oxides, possibly hematite or maghemite (γ-Fe2O3). Furthermore, a widely used mineralogical model11 proposed that these anhydrous ferric oxides in Martian dust formed by continuous oxidation and weathering under water-poor surface conditions during the Amazonian period, which spans from approximately 3 billion years ago to the present.

Early spacecraft observations revealed a distinctive 3-μm hydration feature in the Martian dust spectrum12,13 well before the weaker NIR spectral features associated with alteration minerals were identified11. Later evaluation of the OMEGA data noted that the large 3-μm absorption band is deeper in the observations of bright, dusty regions when compared to dark, less dusty terrains14,15. The increased strength of this absorption band in dusty regions was attributed to either higher abundance of water adsorbed on grain surfaces due to the large surface-to-volume ratio of the dust particles16 or H2O bound in hydrated minerals in the dust. Using ten years’ worth of OMEGA data, it was shown that the 3-µm band can be attributed to tightly bound H2O and/or hydroxyl groups in the mineral structure of the dust17. NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also indicated a deep absorption centered at 3 µm in bright, dusty regions18. Finally, laboratory reflectance investigations of Martian meteorite ALH84001 revealed a 3-μm hydration band (due to H2O stretching vibrations), although no bands were observed at 1.4 µm (combination of OH stretching and H-O-H bending) or at 1.9 µm (combination of H2O stretching and bending modes)19. Basaltic volcanic glasses also typically include a broad 3-µm band due to H2O without the weaker 1.4 or 1.9 µm features (e.g. ref. 20). In addition, the hydrogen signature in the Martian meteorite “Black Beauty” was attributed to hydroxylated iron oxide minerals21.

Data collected by the MIMOS II Mössbauer instrument (MB) on the Mars Exploration Rovers (MER) showed the existence of mm-sized hematite spherules (also known as ‘blueberries’) and goethite in specific rock outcrops as well as the ubiquitous presence of undetermined iron oxide phase (“nanophase NpOx”) in the fine dust (e.g. refs. 22,23). While MER MB data can be used to determine the Fe oxidation state (Fe3+/FeT), it is difficult to distinguish the mineralogy of ferric iron present in the Martian dust24. This difficulty arises because in the nanocrystal (<10 nm) range, the distinct characteristics of different iron oxides gradually disappear as particle size and crystallinity decrease, resulting in broad and diffuse spectral lines25,26. Further, characterization of nanophase components is difficult in mixtures. However, data from the MERs showed that the iron concentration in the fine dust is positively correlated with sulfur and chlorine abundances, whereas dark olivine-rich soils contained lower abundances of these elements, suggesting that iron in the dust is a product of chemical alteration23,27,28. The MERs were also equipped with a series of magnet arrays designed to analyze airfall dust. The analysis of the magnetic targets using MB spectra and imaging systems identified two distinct ferric iron endmembers in the dust: one comprising strongly magnetic and dark-colored magnetite, and the other an unidentified bright-colored (oxy)hydroxide exhibiting weak magnetic properties29,30. Earlier results from the Mars Pathfinder mission31, which utilized five magnets of varying strengths, indicated that the magnetic properties of Martian soil are likely due to small amounts of maghemite present in intimate association with silicate particles, suggesting that the dust particles are composites containing both magnetic and non-magnetic components.

NASA’s Mars Science Laboratory (MSL) rover provided several key chemistry and mineralogy measurements of Martian dust and soils. The Chemistry and Camera (ChemCam) instrument utilized its laser-induced breakdown spectroscopy (LIBS) capability to analyze the composition of airfall dust. In each of the initial laser shots from a series of 50 shots on dusty rock surfaces and calibration targets that collected dust over the years, ChemCam consistently detected a hydrogen signal that exhibited no diurnal variation, suggesting that hydrogen is chemically bound within the dust particles32,33. ChemCam also detected sulfates in the rocks34 and dust35 in Gale Crater indicating an ancient evaporitic and acidic environment. Furthermore, samples from the dust-covered sand shadow known as “Rocknest” were measured with the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument. These measurements revealed that up to ~50 wt.% of the material comprising the “Rocknest” scooped soil is X-ray amorphous, with a significant portion (~20 wt. %) of the amorphous component containing iron, though its precise chemical form (whether as oxides, oxyhydroxides, or sulfates) remains unconstrained36–38. In addition, the Alpha Particle X-ray Spectrometer (APXS) instrument analyzed airfall dust on the science observation tray. These measurements39 indicated that the dust is compositionally similar to the bulk basaltic Mars crust40,41, but is enriched in SO3, Cl and Fe, which is in agreement with MER observations30. Both APXS and ChemCam measurements suggested that the amorphous iron oxide component observed in the “Rocknest” soil may be linked to dust33,39. The Sample Analysis at Mars (SAM) instrument, which includes a gas chromatograph and a quadrupole mass spectrometer, detected volatile species (H2O, SO2, CO2 & O2) when the ‘Rocknest’ sample was heated to ~835 °C42. This finding suggested that H2O is bound to the amorphous component of the sample, as the CheMin instrument did not detect any hydrated or hydroxylated minerals in this sample42. In conclusion, while MSL’s measurements established that iron oxides comprise ~20 wt.% of the X-ray amorphous component in Martian soils, the specific mineralogical nature of these phases has remained elusive.

Here, we present evidence that ferrihydrite (Fe5O8H·nH2O) – a poorly crystalline, hydrated iron oxide mineral – is the dominant iron-bearing phase in Martian dust. Through systematic visible near-infrared (VNIR) spectral analysis, combining orbital and in-situ observations with laboratory studies of submicron ferrihydrite-basalt mixtures, we demonstrate that ferrihydrite provides the best spectral match to the color of Mars. Our spectral analyses show that the addition of Mg-sulfates to ferrihydrite-basalt mixtures reproduces the distinctive 3-micron hydration band observed in Martian dust spectra. Further, to investigate the particle size of Martian dust, we analyzed multi-angular observations from the Colour and Stereo Surface Imaging System (CaSSIS43) onboard ESA’s Trace Gas Orbiter, which, combined with laboratory spectrogoniometric measurements of ferrihydrite, suggests that dust particles are predominantly submicron (<1 µm) in size. In addition, we show that ferrihydrite maintains its mineralogical structure and does not transform into other iron oxide-hydroxide phases when exposed to simulated present-day Martian conditions (UV irradiation, 6 mbar pressure, CO2 atmosphere). We then discuss its importance and implications for the past climate and habitability on Mars.

Results
Spectral match of Martian dust and ferrihydrite-basalt mixtures
The spectrum of Martian dust acquired by CRISM exhibits key spectral features that fit with our laboratory mixture of hyperfine ferrihydrite-basalt powder in a 1:2 weight ratio, measured under ambient conditions (Fig. 1). Both spectra exhibit the following characteristics: (1) a strong absorption at visible wavelengths, (2) a downturn in reflectance near 1 µm, (3) a featureless continuum lacking hydration bands from ~1 to 2.7 µm, and (4) a deep absorption centered near 3 µm (Fig. 1c). Thus, the spectral properties across the 0.4–4 µm visible/near-infrared (VNIR) range of our hyperfine ferrihydrite-basalt mixture provide strong evidence for the presence of ferrihydrite in Martian dust.

Our synthetic ferrihydrite-basalt mixture (Supplementary Fig. 1) shows good agreement in the extended visible range (0.4–1.2 µm) with Martian dust spectra acquired by orbiters and rovers (Fig. 2a). Previous studies44,45 have demonstrated that the reflectance spectra of ferrihydrite-clay assemblages in the NIR range are consistent with the spectral properties of Martian dust. However, our ferrihydrite-basalt mixtures provide a superior model for the featureless portion of the Martian NIR spectrum, particularly in regions where OH and H2O bands are typically observed in smectite clay spectra. Our current study provides a comparison of Martian dust spectra from multiple instruments that are all consistent with our ferrihydrite-basalt mixture model. The Martian spectra analyzed for this study include orbital data from CRISM18 and OMEGA (Supplementary Fig. 2), as well as in-situ measurements from Mars Pathfinder’s IMP camera46, MER Opportunity’s Pancam29, and MSL’s ChemCam47. Especially telling is the multi-band spectrum of Pancam, which is an observation of the bright and weakly magnetic airfall dust component captured by the Opportunity rover’s sweep magnet. The sweep magnet was designed to study weakly magnetic/paramagnetic particles29, which is particularly relevant as ferrihydrite is paramagnetic under the Martian diurnal temperature range observed at the MER landing sites (~180–300 K)24. The Pancam spectrum, in agreement with the CRISM observation, suggests a weak absorption feature around 1 µm. This spectral feature is likely due to overlapping absorptions from ferrihydrite and mafic minerals present in the Martian dust, as suggested by the ferrihydrite-basalt mixture spectrum (Fig. 2a, see also Supplementary Fig. 3). Finally, we note that the dust spectra acquired at five different sites on the planet (Fig. 2a) are spectrally very similar in the visible wavelengths, supporting the hypothesis that Martian dust is well mixed on a global scale48.

In the extended visible region spectral range (0.4–1.2 µm) iron oxides and (oxy)hydroxides exhibit distinct and diagnostic spectral features44,49–52 (Fig. 2b, Supplementary Fig. 4). For instance, consider: 1) the deep UV-VIS absorption edge extending from 0.4 to 0.55 µm in the spectrum of hematite, caused by the paired electronic transition 6A1 → 4T1, 2) the broad 6A1 → 4T1 band at ~0.64 µm in goethite’s spectrum and to a lesser extent in spectra of hematite and akaganeite, and 3) the short wavelength ~0.86 µm of the 6A1 → 4T1 band in hematite’s spectrum. The specific band position and shape51,53 is a function of the mineral’s crystal structure hosting the Fe3+. The shape and position of these three bands in hematite-, akaganeite-, schwertmannite- and goethite-basalt mixture spectra are not consistent with the Martian dust spectrum (Fig. 2b). Note that a wider range of weight ratios of hematite-to-basalt cannot reproduce the Martian dust spectrum (Supplementary Fig. 5). In contrast, the spectrum of our synthetic ferrihydrite-basalt mixture replicates the shape and position of the three electronic bands observed in the Martian spectra (Fig. 2a), similar to previous studies of ferrihydrite-smectite assemblages (e.g. ref. 44). This suggests that ferrihydrite is a more suitable candidate than other iron oxides and (oxy)hydroxides to explain the ochre hue of Mars (Fig. 1a, b; Supplementary Fig. 6).

Analyses of reflectance spectra of our synthetic mixtures demonstrate that relatively large amounts of ferrihydrite are needed to produce the characteristic red slope (0.4–0.8 µm) of Mars (Fig. 3a, b). 1 wt.% of ferrihydrite in a mixture with basalt does not produce the steep visible slope, nor the prominent visible region absorption at 0.4–0.5 µm. At least 10 wt.% ferrihydrite is needed to exhibit the steep slope, and our analyses indicate that as much as 20-33 wt.% (i.e. 1:4, 1:3 and 1:2 fractions) is needed to provide a good fit with Martian dust spectra. Based on electron microprobe analyses of Fe and O abundances in our synthetic ferrihydrite (Supplementary Fig. 7) and assuming a stoichiometric formula of Fe5O8H·nH2O, we estimate that the poorly crystalline iron oxide fraction in our favored 1:2 and 1:3 ferrihydrite-basalt mixtures comprise approximately 21-29 wt.%. This aligns with MSL measurements of total FeO in the amorphous component of the Rocknest sample, which ranges from ~23 to 29 wt.% in Martian soils37,54. However, our estimate somewhat exceeds the ~17–20 wt.% Fe2O3 reported for Martian fines from Viking and Pathfinder missions55 and the ~18–22 wt.% FeO total in Martian aeolian dust observed by ChemCam33,35 and APXS39. In conclusion, the substantial amount of ferrihydrite in our best spectral analogs supports the hypothesis that part of the significant X-ray amorphous iron component detected in Martian soils by MSL36,37 may be ferrihydrite.

The featureless portion of the Martian spectrum from ~1.2 to 2.5 µm (Fig. 1c) is consistent with the spectrally neutral properties of the basalt used in the mixtures (Fig. 3a). Variations in slope and continuum in this range may indicate differences in the compaction or induration of the material56,57. While our laboratory powders are loose, observations of the Martian surface indicate that dust in some areas may be indurated58. In addition, the OH/water absorptions at 1.4 and 1.9 µm are not observed in the ferrihydrite-basalt mixtures spectrum. These features are effectively masked because basalt is more absorbent (or less reflective) than ferrihydrite at these wavelengths. The reflectance properties of light, including the degree of multiple scattering and its optical path length, are primarily controlled by the most absorbent material—in this case, basalt. Ferrihydrite, being much more scattering and much less absorbent, has a reduced influence on these properties.

Quantitative spectral analysis indicates ferrihydrite
A quantitative assessment based on the root-mean-square error (RMSE; see Methods) calculation suggests that the ferrihydrite-basalt mixture provides a better fit to the observations of Martian dust compared to the hematite-, akaganeite-, and goethite-basalt mixtures (Fig. 4a-d). This is evidenced by the lowest RMSE values of approximately 0.02 for the ferrihydrite-basalt mixture, in contrast to values greater than 0.1 for the other mixtures (Fig. 4e). RMSE estimation provides a single numeric value that summarizes the overall difference between spectra in the normalized reflectance space, with smaller values indicating a closer spectral match between the laboratory and Martian observations.

The small shoulder feature near 600 nm in the Martian dust spectra (Fig. 4a), particularly notable in the ChemCam spectrum may be related to the electron 6A1 → 4T2 transition absorption, which is centered around 650 nm (see also Supplementary Fig. 8). This transition is affected by the strength and symmetry of the crystal field, which is dictated by the arrangement of the iron cations’ surrounding coordination environment51. Our synthetic ferrihydrite-basalt mixture spectrum exhibits less spectral contrast in this range. The contrast of this band is known to vary in natural and synthetic ferrihydrites59, with some synthetic ferrihydrites lacking this feature entirely. Consistent with these findings, natural ferrihydrite samples collected from a lava cave in Terceira Island, Portugal, and iron-rich springs in Block Island, USA, exhibit observable spectral differences in the 600–700 nm range (Fig. 5). This variability may be due to the sensitivity of ferrihydrite formation in nature to rates of oxidation and hydrolysis, resulting in a range of crystallinities from more disordered 2-line ferrihydrite to the more crystalline 6-line ferrihydrite and many intermediates60. X-ray diffraction patterns of our natural ferrihydrites indicate the presence of the 2-line variety (Fig. 5c). However, variations in peak widths (e.g., at ~1.5 Å) suggest some heterogeneity in the crystal structure of the ferrihydrite samples. An alternative explanation to the observed 600 nm feature is that Martian dust may contain small amounts of other (oxy)hydroxides such as goethite, which contain a ~600 nm spectral feature. Our mixtures of ferrihydrite, basalt, and goethite suggest that 1 wt.% goethite may explain the observed trends (Supplementary Fig. 9).

Dehydration experiments suggest ferrihydrite stability
A substantial part of the observed 3-µm band in the Martian dust spectrum (Fig. 1c) may be related to chemically bound water in ferrihydrite. This feature arises from the structurally bound water molecules (H2O) within ferrihydrite’s structure61. However, ferrihydrite alone cannot fully explain all aspects of the 3-µm band shape observed in the Martian dust spectrum, suggesting the presence of additional hydrated phases. We investigated how these additional hydrated phases might influence the Martian dust 3-μm band shape by creating mixtures with sulfate minerals. The addition of Mg-sulfate to our ferrihydrite-basalt mixture reproduces both the steep slope on the right wing of the 3 μm band and the rollover point at ~3.8 µm observed in the OMEGA dust spectrum (see Fig. 6). This suggests that both ferrihydrite and sulfates may contribute to the hydration signature in Martian dust, consistent with recent MSL ChemCam results that identify sulfates as possible carriers of hydration in Martian soils35.

Furthermore, under ambient conditions, ferrihydrite also contains some adsorbed water, but most of this adsorbed water is likely removed in the hyper-arid Martian environment. This is supported by a 40-day exposure of ferrihydrite powder to simulated present-day Martian conditions (6 mbar, UV, CO2 atmosphere), which showed a rapid reduction of the 1.9-µm H2O combination band (see Fig. 7a and Supplementary Figs. 10–12). In contrast, the 1.4 µm band changed little after dehydration. The observation that the 1.4-µm and 3.0-µm bands remain relatively unchanged, while the 1.9-µm band is significantly diminished under conditions of low water vapor relative pressure, is consistent with findings from previous studies62,63. Finally, X-ray diffraction (XRD) analyses conducted in our study before and after the experiment indicate that our 2-line ferrihydrite sample did not undergo a phase change due to dehydration (Fig. 7b).

Orbital and laboratory observations indicate submicron particle size
Our phase curve spectral ratios, derived from CaSSIS observations and laboratory spectrogoniometry, indicate that surface fines including ferrihydrite on Mars may have particle sizes of <1 µm (Fig. 8). Our observations reveal an arch-like curve when reflectance color ratios are plotted against phase angle, a photometric phenomenon previously documented in studies of Martian soils using data from the Viking64 and MER missions65. The roll-over point of both orbital and laboratory curves occurs at ~40° phase angle, suggesting a particle size dependency, as the <11 µm fraction of ferrihydrite exhibits a roll-over point at larger phase angles (Fig. 8b). Alternatively, smooth, semi-transparent, and forward-scattering particles may exhibit an arch-like color ratio curve, while rough particles and their structures may produce a more monotonic reddening66. The particle size range derived from these observations agrees with our laboratory submicron mixture samples (Supplementary Fig. 1). Moreover, the particle sizes inferred from our observations and laboratory work are consistent with those derived from atmospheric models67 and direct microscopic observations of surface fines by the NASA’s Phoenix lander68.

Methods
Laboratory sample preparation
Nine different iron oxide phases were used in this study: magnetite, maghemite, goethite, hematite, ferrihydrite, lepidocrocite, akageneite, feroxyhyte, and schwertmannite (Supplementary Fig. 6). Maghemite, ferrihydrite, akageneite, feroxyhyte, and schwertmannite were synthesized at the University of Arkansas following the procedures of Schwertmann and Cornell53, and were kept under ambient conditions for 14 years before being investigated in this work. The samples were not altered in any way and their XRD patterns can be seen in Supplementary Fig. 14. For ferrihydrite specifically, the synthesis procedure is as follows: 40 g of Fe(NO3)3∙9H2O was dissolved in 500 mL distilled water, then we added 330 mL of 1 M KOH solution to bring the pH around 7-8, while stirring the solution. The last 20 mL of KOH solution were added drop by drop. The samples were then centrifuged, then rinsed several times with DI water and dried at 50 °C.

For akaganeite synthesis, 54.06 g of FeCl3∙6H2O was dissolved in 2 L distilled water (0.1 M solution) and held in a closed flask at 40 °C for 8 days, yielding approximately 5 g of product that was subsequently washed and dried. Schwertmannite was prepared by dissolving 10.8 g FeCl3∙6H2O and 3 g of Na2SO4 in 2 L of distilled water preheated to 60 °C, followed by 12 minutes of additional heating at the same temperature. After cooling to room temperature, the suspension was filtered, washed and dried. The presence of natrojarosite in the XRD spectra was noted as a natural co-precipitate during this reaction, possibly due to the omission of dialysis.

Feroxyhyte synthesis began with preparing 300 mL of 0.1 M FeII solution using FeCl2∙4H2O in distilled water, which was filtered to remove oxidized products. The solution’s pH was raised from 3 to 8 using dropwise addition of 5 M NaOH under stirring, resulting in green rust precipitation. Addition of 40 mL of 30% H2O2 solution caused instantaneous oxidation, forming a reddish brown precipitate. The pH was readjusted to 8, and the precipitate was filtered, washed and dried. Maghemite was synthesized by preparing a 0.064 M mixture of FeCl2-FeCl3 with a FeII/FeIII molar ratio of 9, which was then slowly oxidized with an airflow of approximately 10 mL/min in a closed flask at 20 °C. The magnetite powder is from the University of Bern geological collection and goethite is a natural sample from the mine of En Bournegade, Tarn, France. The lepidocrocite sample is from Siegen, Germany. The hematite powder (<1 µm particle size as shown in Supplementary Fig. 1), was purchased from Sigma-Aldrich Co. The basalt sample used in this study is from the University of Bern geological collection and comes from an unknown locality in Virginia, USA. However, we include mineralogical analysis of this sample as shown in Supplementary Fig. 15. The natural ferrihydrite samples shown in Fig. 5 are from a lava cave in Gruta Dos Balcões, Terceira island, Azores, Portugal and were collected under permit CCIR-RAA/2023/43 issued by Direção Regional da Ciência e Tecnologia, Região Autónoma dos Açores. The ferrihydrite from the iron-rich spring was collected in Block Island, Rhode Island, USA. Both natural ferrihydrite samples were collected as aqueous suspensions and subsequently air-dried under ambient conditions. The Azores 2-line ferrihydrite remained poorly crystalline, showing no phase transformation even after 6 months in solution. pH measurements of the source water yielded circumneutral values (~7) for both Block Island and Azores samples.

The sample preparation procedures differed between samples. For seven of the nine iron oxide phases (magnetite, maghemite, goethite, lepidocrocite, akaganeite, feroxyhyte, and schwertmannite), preparation consisted of a single size-separation step. To remove large grains and homogenize our samples using a dry sieving procedure, we used a sonic separator VariSifter™ (Advantech Manufacturing, Inc.) at the University of Bern. Because cohesiveness properties varied across all samples it was not always possible to sieve to the same particle size, so the mesh size had to be varied and experimentally determined for each case. We used mesh sizes of 40-, 30-, 20- and 10-µm. Particle sizes were: goethite (<11 µm), lepidocrocite (<40 µm), akaganeite (<30 µm), maghemite (<20 µm), schwertmannite (<30 µm), feroxyhyte (<40 µm), and magnetite (<10 µm).

The remaining two samples, ferrihydrite and basalt, underwent additional grinding to simulate Martian dust particle size. This process was carried out at the University of Grenoble using a Planetary Grinder Retsch© PM100, following similar procedures to the ones defined by ref. 109. The grinder uses a 25 mL zirconium oxide (ZrO2) bowl filled with equal volume fractions of ZrO2 grinding balls, sample powder, and free space. The initial sample powders made of grains smaller than 50 µm were first dry-ground for 20 min with 2-mm grinding balls. Then, the sample powder was recovered and wet-ground for 150 min in ethanol with 500-µm grinding balls. After grinding, the balls were separated from the sub-micrometer-sized sample powder by sieving using additional ethanol. The suspension of sample powder in ethanol was centrifugated and the ethanol evaporated in ambient room temperature under a hood to obtain the final dry powder. The resulting sub-micron particle size (<1 µm) was confirmed by scanning electron microscope (SEM) images (Supplementary Fig. 1). Given the extensive time required for the specialized grinding procedure and the primary focus on comparing ferrihydrite and hematite as potential major components of Martian dust, we prioritized these two phases for submicron size preparation. The other iron (oxy)hydroxides with particle sizes ranging from <40 µm to <10 µm were included to demonstrate their distinct spectral properties and rule out their dominance in the Martian dust, which is why we compared them with the ferrihydrite-basalt preparations.

We prepared mixtures of basalt with various iron oxides including ferrihydrite, hematite, goethite, and akaganeite at a constant 1:2 weight ratio. For ferrihydrite and hematite specifically, we explored additional mixing ratios ranging from 1:99 to 1:1 with basalt. The specific mixing ratios used in this study are given in Supplementary Table 1. To investigate the 3-µm band shape and potential presence of sulfates in Martian dust, we created mixtures of ferrihydrite, basalt, and Mg-sulfate. While anhydrous MgSO4 from Sigma-Aldrich Co. was used, spectral analysis revealed multiple hydration features characteristic of hexahydrite (MgSO4·6H2O). These hydration features reflect structurally bound water molecules within the crystal structure, rather than adsorbed water, indicating hydration has occurred. All sample powders were homogenized through gentle disaggregation and mixing using a pestle and mortar.

VNIR and FTIR spectroscopy
The spectra analyzed in this work were acquired under ambient conditions for all samples. The VNIR range of each spectrum was acquired with MoHIS110, which is a high-resolution imaging spectrometer that covers from 0.4 to 2.4 µm. The near infrared (NIR) part up to 4 µm was acquired at IPAG Grenoble with a Brucker Vertex 70V FTIR spectrometer equipped with a biconical reflectance kit A513/QA (i=0°, e=30°). To obtain a complete spectrum of each iron oxide phase, we spliced the VNIR and FTIR datasets at around 1.8 µm, resulting in the composite spectra shown in this work. Spectra of ferrihydrite-basalt mixtures in the VNIR and NIR range were acquired first at IPAG using SHADOWS spectro-gonio-radiometer. A more in-depth analysis spanning also other (oxy)hydroxide mixtures such as akaganeite, schwertmannite and goethite was performed at Brown University’s NASA RELAB facility. In addition, 3 µm observations of the sulfate-bearing mixtures were also acquired at RELAB. Measurements were acquired in dry air to remove potential atmospheric effects. The instruments used were the UV-Vis-NIR Bidirectional Spectrometer and the Thermo Nexus 870 FT-IR Spectrometer. The spectra were spliced in a similar way to provide high resolution spectra up to 4 µm. RELAB’s nominal resolution is 1.5, 3, and 6 nm for the gratings 1200, 600, and 300, covering 300-880, 860-1800, and 1780-2600 nm in the standard operation.

Root mean square error (RMSE) calculation
To compare the goodness of fit of our lab mixture VNIR reflectance spectra and the VNIR reflectance spectra acquired by orbiters and rovers, we employed a root mean square error calculation. For each observed spectrum Sobs, the original spectrum data Sobs were retrieved and normalized at 800 nm. The observed normalization value Nobs,800nm was obtained from the data at 800 nm. The normalized observed spectrum Sobs,norm was calculated by dividing the original observed spectrum by its normalization value:

Sobs,norm = Sobs / Nobs,800nm (1)

The root mean square error (RMSE) was then calculated by comparing the normalized observed spectrum Sobs, norm to the laboratory normalized spectrum S lab in the same way. This involved computing the differences ΔS between the two spectra, squaring these differences, averaging them, and finally taking the square root of the mean value:

ΔS = S lab,norm - S obs,norm (2)

RMSE = sqrt(1/n * Σ(ΔS)^2) (3)

where n is the number of data points in the spectra. Data gaps in the CRISM and OMEGA datasets were linearly interpolated.

CaSSIS photometry and lab spectrogoniometry
We analyzed approximately 200 unique observations from CaSSIS, utilizing the latest absolute and radiometric calibration111,112. Previous studies have demonstrated the CaSSIS capability to detect phase angle-dependent phenomena related to surface roughness and particle size properties113, as well as ices under low signal-to-noise ratio conditions114, underscoring the instrument’s great performance and suitability for our spectral analysis of Martian dust.

To collect the individual dust spectra from each CaSSIS cube, we used the Environment for Visualizing Images (ENVI) software to draw regions of interest (ROIs) over relatively flat, spectrally homogenous and stable surfaces. Each spectrum was the result of averaging 100–1000 pixels in size (at 4.6 m/px scale). Images obviously contaminated by clouds or other atmospheric phenomena were discarded. Following past analyses115 we also excluded observations above 70° phase because of increasingly dominant aerosol effects. Additionally, for each CaSSIS observation we modeled optical depth (τ) values, using the climatological assimilation of dust opacities derived from the Mars Climate Sounder (MCS) experiment116. This model serves as a proxy for the atmospheric conditions during the time of image acquisition and provides confidence when interpreting surface reflectance trends.

Phase curve ratios of ferrihydrite were acquired using PHysikalisches Institut Radiometric Experiment-2 (PHIRE-2117), which is a lab spectrogoniometer at the University of Bern. In order to mimic CaSSIS observations, PHIRE-2 was set to observe only in the principal plane and in the phase angle range of 5–70° (when i=0°, phase=emission). PHIRE-2 was also equipped with the flight spare CaSSIS filters (NIR, RED, PAN & BLU), which allows direct spectral comparisons between the lab and orbital data. PHIRE-2 reflectance data was radiometrically calibrated using a standard diffuse reflector (Spectralon™) and outputted in units of REFF.

Before each measurement, the samples were flattened using a smooth metal spatula118,119. After flattening, each surface was sprinkled with the remaining sample material using very fine nylon sieves of various mesh sizes (SEFAR NITEX®). The sprinkling procedure was performed to simulate the surface texture effects of dust resulting from airfall settling.

Ferrihydrite dehydration experiment
The Mini-ME vacuum chamber in the University of Winnipeg was used to simulate Mars surface conditions (e.g. ref. 120). A commercial-grade, dry CO2 atmosphere was maintained by allowing a constant flow through the chamber, unless otherwise noted below. The powders were packed in aluminum wells that are held under a 9.4-mm-thick sapphire window that allows both UV-irradiance and reflectance spectroscopy.

The ferrihydrite-basalt samples were initially exposed to a ~5–7 mbar CO2 atmosphere at 22 °C, and reflectance spectra were measured after 10 minutes without irradiation. Following this, the samples were irradiated with a Heraeus DO802 deuterium lamp, with spectral radiance assumed to be identical to that of the DO902 measured by ref. 121, which has near-identical specifications. The methods used here were previously used to quantify the stability of the MastCam-Z calibration targets when exposed to UV-irradiation122. A solarmeter 8.0 UVC radiometer quantified the irradiance of the DO802 at 50 mm from the aperture through a 9.4 mm sapphire window, yielding 0.25 Wm−2. The radiance spectra were normalized to the measured irradiance at 258 nm and adjusted for sapphire window transmittance. Dual-pass transmittance measurements using a halon reflectance standard showed no significant atmospheric absorption (180–2500 nm) between air and 400 Pa CO2, likely due to the short 1-2 mm path length, so atmospheric absorption was ignored. The samples were subjected to both vacuum and UV irradiation for 678 h until UV bulb failure, with a final measurement taken at 966 hours under vacuum conditions only. All spectral measurements were conducted at room temperature (22 °C).

Reflectance spectra over the 350- to 2500-nm range were measured with an Analytical Spectral Devices FieldSpec spectrometer with a spectral resolution of between 2 and 7 nm and spectral sampling interval of 1.4 nm for the 350–1000 nm interval and 2 nm for the 1000- to 2500-nm interval. The data are internally resampled by the instrument to output data ultimately at 1-nm intervals with a cubic spline interpolation. Spectra were measured at a viewing geometry of i=40° and e=0° with incident light being provided by an in-house quartz-tungsten-halogen collimated light source (<1.5° divergence). Sample spectra were measured relative to halon that is held in an aluminum well inside the vacuum chamber, which was calibrated to a Fluorilon® 99% diffuse reflectance standard and corrected for its absorption properties according to the ASTM-E1331 standard method. For the spectral data acquisition, 500 spectra of the dark current, standard, and sample were acquired and averaged, to provide high signal-to-noise data. Reflectance spectra over the 2500–5000 nm range were collected at Mars pressures before irradiation and after the experiment was completed with a Designs & Prototypes Model 102F emission spectrophotometer. Samples were illuminated with the same light source used for VNIR reflectance, but at a much lower power to avoid saturation of the interferograms. The sample spectra were collected relative to an aluminum powder held inside the vacuum chamber, and then were normalized relative to the data collected with the VNIR spectrometer at 2500 nm. Self-emission of the spectrophotometer was corrected by measuring a blackbody at 10 °C and 50 °C, and the correction was applied to all collected spectra. Reference spectra and sample spectra were acquired with the same viewing geometry of i=40° and e=0°, and 100 spectra were averaged for each collection.

X-ray diffraction patterns were acquired with a continuous 2-theta scan from 5 to 80° 2θ on a Bruker D8 Advance. A Bragg-Brentano goniometer with a two theta/theta setup was equipped with a 2.5° incident Soller slit, a 1.0 mm divergence slit, a 2.0 mm scatter slit, a 1.0 mm receiving slit, a curved secondary graphite monochromator, and a scintillation counter collecting at an increment of 0.02° and integration time of 1 s per step. The line focus Co X-ray tube was operated at 40 kV and 40 mA, using a take-off angle of 6°. Samples that had not been exposed to Mars-like atmospheric pressure and radiation were front-loaded into an X-ray amorphous sample cup. Samples that had been exposed to Mars-like conditions were sprinkled onto a zero-background plate due to limited sample volume. Phase identification was made using the Bruker EVA software package linked to the International Center for Diffraction Data Powder Diffraction File (ICDD PDF-2) database.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review report of the manuscript “Why Mars is Red - Ferrihydrite Detection in Martian Dust and Implications for Early Mars”\nsubmitted to Nature Communications, NCOMMS-24-49227.\nDear Authors,\nThank you for the opportunity to review your manuscript. I enjoyed reading the manuscript and seeing the number of\nevidence that sub-micron ferrihydrite explains the observations. Overall, the manuscript proposes an interesting hypothesis\nsupported by reasonable data and thoroughly describes the implications of the finding. Thus, I consider that there is no need\nfor major corrections. Several minor comments are listed below, line by line.\nGood luck and all the best\nMinor comments\nLine 247: Figure 7 appears before Figure 6. Please reconsider the order.\nLine 318: Could you add an explanation or reference why the basalt may inhibit the transformation of ferrihydrite?\nLine 357: Could you discuss the interpretation of the sub-micron size nature of ferrihydrite for the origin of the ferrihydrite? I\nsuppose it is also an important constraint suggested by this study.\nLine 371: A space after (atmospheric photolysis) is lacking.\nLine 377: Could you add a brief explanation of why such a reaction occurs?\nLine 494: Please reconsider the notation and explanation of Ferrihydrite-2L (2-line) in Fig. 8, as it is not reader-friendly.\nLine 518: Information on the origin of goethite seems to be lacking.\nLine 519: Although the reference is cited (Schwertmann and Cornell, 2000), I suggest briefly explaining the synthesis\nprocedure, especially for ferrihydrite, as it is the key material of this study and would be of interest to readers.\nLine 524: I suggest explicitly explaining that, from here, the description of the sample preparation begins, rather than just\n“First...” and “Second...”.\nLine 529: maghemeite => maghemite\nLine 530: Please add spaces between < and numbers.\nLine 530: Please reconsider the flow of paragraphs. For example, please state that this paragraph describes the preparation\nprocedure for “7 samples” out of the 9 mentioned above paragraph and that the preparation procedure for others, including\nferrihydrite, is written in the next paragraph so that readers can understand easily.\nLine 531: Please add a description of the origin and nature of the basalt used in this study.", "reviewer_2": "Mars is a red planet. Previously, the red color on the surface of Mars was thought to be due to the ubiquitous presence of the\nanhydrous iron oxide hematite, which is associated with dusts on the planet. However, this study shows that the ubiquitous\niron oxide on Mars is not hematite, but ferrihydrite. I think the finding is quite novel and has major implications for the past\naqueous environment on Mars, which is worth publishing in the journal Nature Communication.\nI agree with the authors that significant ferrihydrite with basalt can explain the red color of the dust on Mars. However, I am\nstill concerned that (1) the presence of hematite rather than ferrihydrite is not completely ruled out (2) the ferrihydrite used in\nthe study is the reasonable standard material for comparison with the spectra of Martian dust. In order to reach the authors'\nconclusion, these concerns need to be addressed.\nRegarding the first point, the author made the systematic study using ferrihydrite and basalt to reproduce the spectra of the\nMars spectra. However, they only showed the comparison with the 1:2 hematite-basalt mixture in Figure 2. Is it really\nimpossible to reproduce the Mars spectra with hematite not ferrihydrite? As the authors also mentioned, the MSL measured\nthe chemical compositions and mineralogy of the Rocknest site (dust sample) in Gale. The quantitative analysis of the XRD\npattern suggests the presence of 1% hematite in the sample. Does the 1% hematite never explain the color of the Martian\ndust?\nRegarding the second point, the color (or visible spectrum) of ferrihydrite must vary with conditions. The authors also\nmentioned that the spectrum of natural ferrihydrite is different from the synthesized material. Possible ranges of spectra of\nferrihydrite are characteristic of other iron oxides, especially for hematite.\nMinor comments:\nLine 123-128: The motivation and purpose of this study should be written in the end of the introduction.\nLine 126: What does it mean “stable”? What is the definition of the term of “stable” in this study?\nLine 173: The dust sample from the Rocknest site contains hematite. Ferrihydrite is a metastable phase and changes to\nhematite and/or goethite. I think the mixture of ferrihydrite and hematite is a reasonable scenario for Martian dust, although\nthe contribution of ferrihydrite may be much higher than hematite. The authors examined the inclusion of goethite with\nferrihydrite (Supplementary Figure 8). The inclusion of hematite with ferrihydrite should be investigated, which is related to\nthe main comment (1).\nLine 237-: However, the periods that ferrihydrite survives must be 8-9 orders of magnitude longer than the experimental\nduration. These experiments may not provide evidence that ferrihydrite is stable on Mars. This comment is related to the\nsecond comment in the Minor Comments.\nLine 247: Figure 7 should be Figure 6.", "reviewer_3": "The goal of this manuscript is to constrain the mineral assemblage of the martian dust primarily using visible/near-infrared\nspectroscopy. This manuscript presents visible/near-infrared reflectance data of Mars-relevant iron-bearing phases\nmeasured under modern martian atmospheric conditions (e.g., composition, radiation, pressure) and compares the\nlaboratory spectra to martian orbital and in-situ spectra. The manuscript finds that a mixture of ferrihydrite and basalt, along\nwith magnesium sulfate and goethite, best match the visible/near-infrared spectra from Mars. The manuscript concludes that\nthe presence of ferrihydrite in martian dust can help constrain the early aqueous history of Mars and that the ferrihydrite\nformed just before the martian surface became hyperarid.\nThe topic of the manuscript will be of broad interest to the Mars science community because it addresses a long-standing\nquestion: What is the martian dust made of? The manuscript will also be of broad interest to the planetary spectroscopy\ncommunity because it presents visible/near-infrared reflectance data of a variety of iron-bearing phases measured under\nMars-like conditions. The methods, results, and conclusions are generally sound, but I think the manuscript should be\nrearranged a bit to highlight the important results and conclusions more clearly. For example, some data in the\nsupplementary information should be moved to the main text, and a brief description of the methodology should be moved to\nthe main text. I have a few substantial comments below, followed by minor line-by-line comments (most of which are\nsuggested grammatical changes). If the authors have any questions about my review, they should feel free to email me, Liz\nRampe, at elizabeth.b.rampe@nasa.gov.\nBroad comments/suggestions\n(1) Abstract: I suggest mentioning the results of the irradiation experiments in the abstract. The spectral measurements of\ndifferent Fe phases under martian conditions (e.g., UV radiation, CO2-rich atmosphere) will be important data for the\nplanetary community to interpret spectra collected from a variety of planetary bodies.\n(2) Introduction:\na. The introduction to Fe-oxides/oxyhydroxides in the martian dust is thorough, but I suggest adding more about how\nferrihydrite and other Fe phases are used to interpret the past martian climate (e.g., under what conditions does ferrihydrite\nform and transform into other phases?).\nb. CaSSIS data are not described in the introduction. I suggest introducing these data, what they’re used for, and how\nthey’ve been previously interpreted to justify the analyses of CaSSIS data in the manuscript.\nc. Add a paragraph between the Introduction and Results sections that briefly describes the mixtures that were created so\nthat the reader knows what’s coming in the results.\n(3) Methods:\na. Describe the composition of the basalt in the mineral mixtures (i.e., mineral assemblage, geochemical composition) to\nhelp the reader better interpret the spectra.\nb. Further explain why Fe-oxides aside from hematite were not finely ground for spectral measurements.\nc. Add information about the natural samples that were measured (e.g., sites, collection procedures, previous publications) in\nthe supplementary information or methods.\n(4) Results: Spectra of basalt-ferrihydrite-Mg sulfate mixtures show a good match to the 3-micron feature on Mars, but these\ndata are in the supplementary information and sulfates are not mentioned in the introduction. I suggest briefly discussing the\nimportance of sulfates on Mars in the introduction and moving the basalt-ferrihydrite-Mg sulfate mixture methods and results\nto the main body of the manuscript. Explain how Mg sulfate fits into the timing of the formation of ferrihydrite if a ferrihydrite-\nbasalt-Mg sulfate-goethite mixture best explains the vis/NIR data.\n(5) Discussion: The discussion of how ferrihydrite formed on Mars could be better developed. There are some seemingly\ncontradictory statements in the discussion about the timing of the formation of ferrihydrite. Did ferrihydrite form early or late in\nMars’ history? Did it form within a rock and then become physically eroded to get incorporated into the dust? Or did it form\nfrom water-rock interaction of unconsolidated eolian materials? If it originally formed in a rock then was physically eroded,\nwhy aren’t there other secondary phases common on Mars in the martian dust (e.g., smectite)?\nLine-by-line comments\nMain text\nAbstract: define all acronyms in the abstract.\nThe manuscript uses “um” and “micron” interchangeably. I suggest using one term and being consistent throughout the\nmanuscript. I also suggest changing ampersands to “and”.\nLine 28: add an alpha symbol before “Fe2O3”\nLine 31: clarify that the mineralogy of the surface dust is unresolved from MSL data (e.g., “…, but the minerals present in the\ndust remains unresolved).\nLine 45: This introductory sentence indicates that the Fe-oxide assemblage only tells us about ancient climates. From your\nmeasurements of Fe-oxides/oxyhydroxides under modern martian conditions, the secondary Fe assemblage can also tell us\nabout modern climate and how long those conditions may have persisted.\nLine 46: add a comma after “Earth”\nLine 48: add “typically” after “The” because the martian surface isn’t red everywhere.\nLine 55: Hyphenate “near-infrared”\nLine 56: Define the acronym “ESA”\nLine 58: Add a space between “)(“\nLine 69: Hyphenate “surface-to-volume”\nLine 76: Explain what bond vibrations cause the 1.4 and 1.9 um bands\nLine 77: Add a comma after “e.g.”\nLine 79: Specify what “coarse-grained” means (e.g., what is the crystallite size?)\nLine 81: Add “e.g.,” before “Morris et al.”\nLine 82: add a comma after “(Fe3+/FeT)” and delete “more”\nLine 84: Define “microcrystal” size range\nLine 88: Change “while” to “whereas”\nLine 92: Change “spectral” to “spectra”\nLine 107: hyphenate “dust-covered” and change “dune” to “shadow”\nLine 110: The abundance of Fe oxides in the amorphous component in Rocknest is unconstrained. We have calculated the\nabundance of Fe in the amorphous component using CheMin mineral abundances and APXS elemental abundances, but\nwe don’t know the speciation of the Fe (e.g., oxides, oxyhydroxides, sulfates).\nLine 116: Change “at” to “in the” and change “soils” to “soil”\nLine 121: Change “crystalline phyllosilicate minerals” to “hydrated or hydroxylated minerals”\nLine 128: After reading the introduction, I suggest adding a paragraph on the methods before moving into the results to help\nprepare the reader for what data they will see in the results. For example, I was surprised to learn that you made\nmeasurements of mineral mixtures and not just individual minerals.\nLine 133: clarify whether the FH-basalt powder was measured under martian conditions\nLine 148: Delete the comma between CRISM and OMEGA\nLine 168: Remove the accent in “akaganeite”\nLine 181: Delete “VIS”\nLine 182: Add a comma after “slope”\nLine 186: Clarify what the amount of oxidized iron is smaller than\nLine 218: Is it possible that the shoulder feature at ~600 nm could be from chemical substitutions or impurities in ferrihydrite\n(e.g., Si)?\nLine 231: Add a comma after “e.g.”.\nLine 231: I suggest changing the angle to Angstroms.\nLine 234: At the end of the sentence, consider adding a clarifying clause like: “…may contain small amounts of other\n(oxy)hydroxides such as goethite, which contain a ~600 nm spectral feature.”\nLine 234: Add a comma after “basalt”\nLine 241: This is the first mention of mixtures with sulfate. I suggest adding some text about sulfates in the introduction to\nmotivate the measurement of mixtures with sulfate.\nLines 249 & 250: hyphenate “3.0-um” and “1.9-um”\nLine 280: hyphenate “iron-oxide”\nLine 284: Although there is very little H2O in the martian atmosphere, the relative humidity can be very high sometimes,\nright? Consider changing “extremely low humidity” to something like “extremely low water vapor concentration”.\nLine 289: The ferrihydrite structure did not change over the course of the 40-day experiments presented here. The\nferrihydrite in the martian dust has been exposed to these conditions for many thousands/millions of years. Would you\nexpect the ferrihydrite structure to change over those much longer timescales?\nLine 294: Add the calculations using the Arrhenius equation to the supplementary information\nLine 301: Add “e.g.,” before Fairen et al\nLine 305: Add “e.g.,” before Klingelhofer and consider adding references for the jarosite detection by CheMin (e.g., Rampe et\nal., 2017; Morrison et al., 2024)\nRampe, E. B., Ming, D. W., Blake, D. F., Bristow, T. F., Chipera, S. J., Grotzinger, J. P., et al. (2017). Mineralogy of an ancient\nlacustrine mudstone succession from the Murray formation, Gale crater, Mars. Earth and Planetary Science Letters, 471,\n172-185.\nMorrison, S. M., Blake, D. F., Bristow, T. F., Castle, N., Chipera, S. J., Craig, P. I., et al. (2024). Expanded insights into\nMartian mineralogy: Updated analysis of Gale crater’s mineral composition via CheMin crystal chemical investigations.\nMinerals, 14(8), 773.\nLine 309: Add “e.g.,” before Bishop & Murad\nLine 318: Please explain how basalt may inhibit the transformation of ferrihydrite to a more crystalline phase.\nLine 323: Add “e.g.,” before Childs & Parfitt\nLine 326: Add references that show ferrihydrite is more prevalent than allophane and imogolite in Fe-rich volcanic materials\nLine 328: In the discussion, further explain why the presence of ferrihydrite provides insight into early Mars history and not\nmore modern environments. Consider defining “early history” (e.g., number of billions of years ago).\nLine 337: Add “e.g.,” before Carr & Head\nLine 350: The statement that ferrihydrite may have formed from the last widespread aqueous episode seems to contradict\nthe statement that ferrihydrite formed during early Mars history.\nLine 360: Add a comma after “solutions”\nLine 366: Add “e.g.,” before Burns\nLine 385: Change “+2” to “2+”\nLine 398: Capitalize “Mars Sample Return”\nLine 398: Change “bring back regolith samples” to “deliver regolith samples to Earth”\nLine 400: Consider elaborating on what we can learn about ferrihydrite and how it formed in the returned samples. What\ntypes of measurements shows be performed on the regolith? Could the short-range order of ferrihydrite tell us if ferrihydrite is\npure or beginning to transform to another, more crystalline Fe phase? Could the physical relationship between ferrihydrite\nand other phases tell us what has been altered to form ferrihydrite?\nLine 403: Hyphenate “light-toned”\nFigure 2b: Include the spectrum of basalt for comparison to the mixtures. Show spectra of mixtures with other Fe phases\n(e.g., akaganeite) either here or in the supplementary information.\nFigure 5c: Label ferrihydrite peak positions in angstroms. Show the search-match results in the supplementary information.\nFigure 5d: Show the LIBS spectrum for comparison to the shoulder feature in Mars data.\nFigure 5: Could impurities in ferrihydrite contribute to the shoulder at ~650 nm?\nFigure 6: Explain the reason for the difference in the curves at high phase angles (e.g., there is a roll off in Mars data but not\nin the lab data)\nFigure 7a: Explain the reason for the shift in the 1.4-um band\nLine 488: Hyphenate “background-removed”\nLine 491: Hyphenate 1.9-um\nFigure 8: Explain how the figure was made in the methods section. Also explain what “Ferrihydrite-2L” means.\nLine 499: Move the RMSE calculation section after the VNIR and FTIR spectroscopy section.\nLine 500: Clarify that the spectra are VNIR reflectance spectra (e.g., “To compare the goodness of fit of our lab mixture VNIR\nreflectance spectra and the VNIR reflectance spectra acquired by orbiters…\")\nLine 501: Add a comma after “rovers”\nLine 518: Add a comma after “goethite”\nLine 520: Clarify whether iron oxide powders were analyzed via XRD before the experiments to confirm purity and that the\nminerals were not altered over the course of 14 years.\nLine 524: Add a comma after “samples”\nLine 524: Specify whether sieving was done dry or wet\nLine 526: Change “sample” to “samples”\nLine 528: Fix the spelling of goethite\nLine 531: Add a comma after “size”\nLine 531: Explain the source of the basalt in the mixtures\nLine 540: State the temperature under which the ethanol was evaporated\nLine 545: Explicitly state the proportions of minerals and basalt in the mixtures in a table in the methods or supplementary\ninformation\nLine 554: I suggest rearranging the sentence to “Spectra of ferrihydrite-basalt mixtures in the VNIR and NIR range were\nacquired…”\nLine 556: Specify which phases and mixtures were measured at RELAB\nLine 559: Delete “spectral”\nLine 560: Define the spectral resolution of the instruments.\nLine 576: Define “MCS”\nLine 576: Define what “This” refers to (e.g., “This model…”)\nLine 597: Hyphenate “mm-thick”\nLine 599: State for how long the samples were exposed to martian atmospheric and radiation conditions before spectral\nmeasurements. Also state the temperature of the measurements.\nLine 599: Add a comma after “atmosphere”\nLine 601: Delete “was used”\nLine 623: The sentence that starts on line 623 doesn’t read correctly to me. I think “data.” should be added after “signal-to-\nnoise”, and “reflectance” should be the start of a new sentence.\nLine 633: Add a comma after “0 deg”\nLine 640: Be more descriptive of “pre-“ and “post-exposed”. For example, “Samples that had not been exposed to Mars-like\natmospheric pressure and radiation were front-loaded…”\nLine 641: Change “sample was” to “samples were”\nSupplementary information\nLine 67: Add a comma after “mixtures”\nLine 71: Change “comparison” to “compared”\nLine 79: Fix the spelling of goethite\nLine 88: Explain why magnetite is not shows in Supplementary Fig. 5.\nSupplementary Fig. 6: Explain the source of the K in the synthetic ferrihydrite and whether it affects the ferrihydrite spectral\nproperties.\nLine 101: Add a comma after “hematite”\nSupplementary Fig. 8: Show the martian spectrum for comparison to laboratory spectra.\nLine 110: Add a comma after “basalt”\nLine 112: Change “a goethite” to “as goethite”\nLine 119: Describe and justify in the methods the type of Mg sulfate that was used in the mixtures (e.g., epsomite)\nSupplementary Fig. 10: Change c and d to a and b", "reviewer_author_response": "General reply to the reviewers\nThank you for your careful and thorough reviews that have significantly improved our manuscript. We\nwould like to highlight several key points addressed in our revision.\nFirst, regarding the crucial question of ferrihydrite stability on present day Mars raised by two reviewers.\nWe have substantially expanded our discussion of this topic, integrating both our experimental\nobservations (14-year stability under Earth conditions and 40-day Mars-simulation experiments) with\nrecent theoretical calculations by Sassi & Rosso (2022). Using their model we demonstrate that at Mars'\naverage temperature (210 K), the kinetic barriers to iron self-diffusion effectively prevent solid-state\ntransformation, with estimated timescales of 10^16 years far exceeding the age of the solar system.\nIn addition, several moderate changes have been implemented in response to your comments:\n1. We conducted an electron microprobe analysis of our synthetic ferrihydrite at MIT's Electron\nMicroprobe Facility, replacing our previous XRF data, which was erroneous. This analysis\nrevealed that in our preferred 1:2 and 1:3 ferrihydrite-to-basalt mixtures there is about 20-30\nwt. % poorly crystalline FeO, thereby increasing our previous estimation.\n2. Following Reviewer #3's suggestion, we performed complete XRD characterization of all iron\noxide-hydroxide phases. This analysis revealed that our primary \"schwertmannite\"\n(oxyhydroxysulfate) sample was actually akaganeite (hydroxide), likely due to a labeling error\nduring sample preparation 14 years ago. We have corrected this throughout the manuscript and\nfigures, particularly in Figure 2b which now shows the \"Akaganeite-basalt mix\" spectrum. We\nalso included a second, verified schwertmannite sample (containing minor natrojarosite) to\nmaintain comprehensive coverage of iron oxide phases.\n3. We expanded the spectral analysis to include additional mixing ratios, particularly addressing\nReviewer #2's concerns about hematite content. These new results are presented in\nsupplementary figures demonstrating that even small amounts of crystalline hematite would be\nspectrally detectable.\n4. We clarified the manuscript's structure and improved our discussion of formation timing,\nfocusing on the Hesperian period (~3 Ga). To illustrate the key concepts, we also included a new\nschematic Fig. 9. We also acknowledge that precise age determinations require additional\nconstraints beyond our current scope.\nThe manuscript has been revised throughout to incorporate these changes along with numerous other\nsuggested improvements. We thank the reviewers again for their valuable input. Please find our detailed\nresponses to individual comments below.\nSincerely,\nA. Valantinas and the team\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nReview report of the manuscript “Why Mars is Red - Ferrihydrite Detection in Martian Dust and\nImplications for Early Mars” submitted to Nature Communications, NCOMMS-24-49227.\nDear Authors,\nThank you for the opportunity to review your manuscript. I enjoyed reading the manuscript and seeing\nthe number of evidence that sub-micron ferrihydrite explains the observations. Overall, the manuscript\nproposes an interesting hypothesis supported by reasonable data and thoroughly describes the\nimplications of the finding. Thus, I consider that there is no need for major corrections. Several minor\ncomments are listed below, line by line.\nGood luck and all the best\nMinor comments\nLine 247: Figure 7 appears before Figure 6. Please reconsider the order.\nFixed.\nLine 318: Could you add an explanation or reference why the basalt may inhibit the transformation of\nferrihydrite?\nThis is a fair concern, thank you. We have decided to remove this statement from the manuscript.\nLine 357: Could you discuss the interpretation of the sub-micron size nature of ferrihydrite for the origin\nof the ferrihydrite? I suppose it is also an important constraint suggested by this study.\nThank you for this suggestion. We have included a short text on the interpretation of the sub-micron\nsize nature of ferrihydrite in the discussion section.\nLine 371: A space after (atmospheric photolysis) is lacking.\nThis gets fixed after implementing Nature style citations.\nLine 377: Could you add a brief explanation of why such a reaction occurs?\nThe oxidation reaction occurs because even though the thermodynamic equilibrium does not favor\nFe3+ formation, the continuous escape of H2 from the experimental system drives the reaction\nforward according to Le Chatelier's principle. As H2 is removed, the system continues to produce more\nH2 and Fe3+ to maintain equilibrium. The experiments demonstrated that iron oxidation could\nproceed without UV radiation or free oxygen, likely driven by this H2 loss process. This information is\nadded in the following sentence where we cite Chevrier et al. (2006).\nLine 494: Please reconsider the notation and explanation of Ferrihydrite-2L (2-line) in Fig. 8, as it is not\nreader-friendly.\nFixed.\nLine 518: Information on the origin of goethite seems to be lacking.\nFixed. This paragraph was reworded to improve clarity. Goethite sample is from the mine of En\nBournegade, Tarn, France.\nLine 519: Although the reference is cited (Schwertmann and Cornell, 2000), I suggest briefly explaining\nthe synthesis procedure, especially for ferrihydrite, as it is the key material of this study and would be of\ninterest to readers.\nThank you for this suggestion. We have added a detailed description of the ferrihydrite synthesis\nprocedure to the Methods section, including the key steps of Fe(NO3)3·9H2O dissolution, pH\nadjustment with KOH, and centrifugation protocols.\nLine 524: I suggest explicitly explaining that, from here, the description of the sample preparation begins,\nrather than just “First...” and “Second...”.\nFixed.\nLine 529: maghemeite => maghemite\nFixed.\nLine 530: Please add spaces between < and numbers.\nFixed.\nLine 530: Please reconsider the flow of paragraphs. For example, please state that this paragraph\ndescribes the preparation procedure for “7 samples” out of the 9 mentioned above paragraph and that\nthe preparation procedure for others, including ferrihydrite, is written in the next paragraph so that\nreaders can understand easily.\nFixed.\nLine 531: Please add a description of the origin and nature of the basalt used in this study.\nThank you for this suggestion. We have analyzed the mineralogical composition of the basalt sample\nusing XRD and added these results to the Supplementary file. The analysis reveals several silicate\nmineral phases including augite, forsterite, magnetite, anorthite, nepheline and sanidine. Based on\nquantitative XRD analysis showing high abundances of sanidine (12.5 wt%) and nepheline (6.0 wt%),\nthis sample shows characteristics of an alkaline basalt. While the exact sampling locality is unknown,\nwe know the sample originates from Virginia, USA. This information has been added to the Methods\nsection.\nReviewer #2 (Remarks to the Author):\nMars is a red planet. Previously, the red color on the surface of Mars was thought to be due to the\nubiquitous presence of the anhydrous iron oxide hematite, which is associated with dusts on the planet.\nHowever, this study shows that the ubiquitous iron oxide on Mars is not hematite, but ferrihydrite. I\nthink the finding is quite novel and has major implications for the past aqueous environment on Mars,\nwhich is worth publishing in the journal Nature Communication.\nI agree with the authors that significant ferrihydrite with basalt can explain the red color of the dust on\nMars. However, I am still concerned that (1) the presence of hematite rather than ferrihydrite is not\ncompletely ruled out (2) the ferrihydrite used in the study is the reasonable standard material for\ncomparison with the spectra of Martian dust. In order to reach the authors' conclusion, these concerns\nneed to be addressed.\nRegarding the first point, the author made the systematic study using ferrihydrite and basalt to\nreproduce the spectra of the Mars spectra. However, they only showed the comparison with the 1:2\nhematite-basalt mixture in Figure 2. Is it really impossible to reproduce the Mars spectra with hematite\nnot ferrihydrite? As the authors also mentioned, the MSL measured the chemical compositions and\nmineralogy of the Rocknest site (dust sample) in Gale. The quantitative analysis of the XRD pattern\nsuggests the presence of 1% hematite in the sample. Does the 1% hematite never explain the color of\nthe Martian dust?\nWe appreciate this important question. Our team thoroughly investigated different hematite-to-basalt\nratios and found that none could adequately reproduce the Martian dust spectrum. We have included\na supplementary figure demonstrating this finding (see Fig. 1 here). As shown, even a 1:99 weight ratio\nproduces a spectrum distinctly different from that of Martian dust, while a 1:9 ratio exhibits clear\nhematite spectral signatures (broad blue absorption at ~0.5 um, shoulder at 0.6 um and deep\nabsorption at 0.85 um) in the VNIR range that are not observed in Martian spectra. These results\nstrongly suggest that hematite cannot be the dominant iron oxide phase in Martian dust. This point\nwas also added to the manuscript main text.\nFigure 1. Hematite-basalt mix spectra of different weight ratios in comparison to Martian dust OMEGA\nspectrum. The 1:99 weight ratio produces a spectrum distinctly different from that of Martian dust,\nwhile a 1:9 ratio exhibits clear hematite spectral signatures (broad blue absorption at ~0.5 µm, shoulder\nat 0.6 µm and deep absorption at 0.85 µm) in the VNIR range that are not observed in Martian spectra.\nThese results strongly suggest that hematite cannot be the dominant iron oxide phase in Martian dust.\nSpectra normalized at 1.0 µm.\nRegarding the second point, the color (or visible spectrum) of ferrihydrite must vary with conditions. The\nauthors also mentioned that the spectrum of natural ferrihydrite is different from the synthesized\nmaterial. Possible ranges of spectra of ferrihydrite are characteristic of other iron oxides, especially for\nhematite.\nThank you for this important observation. While ferrihydrite's spectral properties show some variation\ndepending on formation conditions, the fundamental spectral differences between iron oxide phases\nremain diagnostically distinct. Our comparative analysis of natural and synthetic ferrihydrites versus\ncrystalline hematite shows clear spectroscopic differences (see Fig. 2 here). These differences arise\nfrom crystal structure variations, which are confirmed by distinct X-ray diffraction patterns for each\nphase (Supplementary Fig. 15). The XRD patterns provide independent validation of the mineralogical\ndifferences observed in our VNIR spectral analysis.\nFigure 2. Natural and synthetic ferrihydrite comparison to hematite used in this study. Diagnostic\nfeatures marked with arrows. For clarity spectra normalized at 740 nm.\nMinor comments:\nLine 123-128: The motivation and purpose of this study should be written in the end of the introduction.\nThank you for this suggestion. We have added the following content to the beginning of the last\nparagraph: “While MSL's measurements established that iron oxides comprise ~20 wt. % of the X-ray\namorphous component in Martian soils, the specific mineralogical nature of these phases has\nremained elusive.”\nLine 126: What does it mean “stable”? What is the definition of the term of “stable” in this study?\nWe added a clarification saying “We show that ferrihydrite maintains its mineralogical structure and\ndoes not transform into other iron oxide phases when exposed to simulated present-day Martian\nconditions” and removed the word “stable”.\nLine 173: The dust sample from the Rocknest site contains hematite. Ferrihydrite is a metastable phase\nand changes to hematite and/or goethite. I think the mixture of ferrihydrite and hematite is a reasonable\nscenario for Martian dust, although the contribution of ferrihydrite may be much higher than hematite.\nThe authors examined the inclusion of goethite with ferrihydrite (Supplementary Figure 8). The inclusion\nof hematite with ferrihydrite should be investigated, which is related to the main comment (1).\nThank you for this insightful suggestion. We have investigated ferrihydrite-hematite-basalt mixtures\nand added two new figures to address this point:\n1. Supplementary Fig. 9b demonstrates that while 10 wt% hematite produces identifiable\nspectral signatures, very small amounts of hematite (<1 wt%) in ferrihydrite-basalt mixtures\nare spectrally undetectable.\n2. Supplementary Fig. 9c provides a direct comparison between 1 wt% hematite and 1 wt%\ngoethite in these mixtures, showing similar spectra with goethite exhibiting a slightly more\npronounced shoulder feature.\nThese results indicate that while we cannot exclude the presence of minor amounts of crystalline\nhematite through spectroscopy alone, any significant hematite content would be detectable in our\nspectral analysis.\nLine 237-: However, the periods that ferrihydrite survives must be 8-9 orders of magnitude longer than\nthe experimental duration. These experiments may not provide evidence that ferrihydrite is stable on\nMars. This comment is related to the second comment in the Minor Comments.\nThank you for raising this important question about ferrihydrite's long-term stability on Mars. While\nour experimental timescales are indeed much shorter than geological periods, both theoretical\ncalculations and empirical observations strongly support ferrihydrite's stability under Martian\nconditions. The key lies in understanding the temperature dependence of transformation kinetics.\nRecent calculations by Sassi & Rosso (2022) reveal that ferrihydrite transformation requires iron cation\nself-diffusion within the crystal lattice. This process has high activation energy barriers (0.57-2.47 eV\nper unit cell), making it extremely slow even at Earth temperatures (300 K). Under these conditions,\ncalculated transformation times range from decades to millions of years - far longer than the months\nto years observed in natural settings where water-mediated processes dominate. When we extend\nthese calculations to Mars' average temperature (210 K), the transformation becomes effectively\nimpossible - timescales reach 10^16 years for the fastest pathway and 10^25 years for the slowest, far\nexceeding the age of the solar system itself (see Fig. 3 here).\nOur experimental observations provide additional support for these predictions. We found that our 2-\nline ferrihydrite remained stable for 14 years under ambient lab conditions, and even aggressive\ndehydration and exposure to present-day Martian conditions (UV, low pressure, CO2 atmosphere) for\n40 days had no effect on its crystallinity (e.g. main text Fig. 7).\nThis highlights an important distinction between Earth and Mars environments. On Earth, ferrihydrite\ntypically transforms through either solid-state transformation at elevated temperatures (>400 K) or\ndissolution-reprecipitation reactions mediated by liquid water. On Mars, the combination of low\ntemperatures and absence of liquid water effectively \"freezes\" both transformation pathways. While\nferrihydrite remains thermodynamically metastable compared to goethite, the extreme suppression of\niron self-diffusion at 210 K prevents the structural reorganization needed for crystallization. Therefore,\nwhat appears unstable under Earth conditions can persist indefinitely in the Martian environment.\nFigure 3. Solid-state transformation timescales of ferrihydrite to goethite.\nLine 247: Figure 7 should be Figure 6.\nFixed.\nReviewer #3 (Remarks to the Author):\nThe goal of this manuscript is to constrain the mineral assemblage of the martian dust primarily using\nvisible/near-infrared spectroscopy. This manuscript presents visible/near-infrared reflectance data of\nMars-relevant iron-bearing phases measured under modern martian atmospheric conditions (e.g.,\ncomposition, radiation, pressure) and compares the laboratory spectra to martian orbital and in-situ\nspectra. The manuscript finds that a mixture of ferrihydrite and basalt, along with magnesium sulfate\nand goethite, best match the visible/near-infrared spectra from Mars. The manuscript concludes that the\npresence of ferrihydrite in martian dust can help constrain the early aqueous history of Mars and that\nthe ferrihydrite formed just before the martian surface became hyperarid.\nThe topic of the manuscript will be of broad interest to the Mars science community because it\naddresses a long-standing question: What is the martian dust made of? The manuscript will also be of\nbroad interest to the planetary spectroscopy community because it presents visible/near-infrared\nreflectance data of a variety of iron-bearing phases measured under Mars-like conditions. The methods,\nresults, and conclusions are generally sound, but I think the manuscript should be rearranged a bit to\nhighlight the important results and conclusions more clearly. For example, some data in the\nsupplementary information should be moved to the main text, and a brief description of the\nmethodology should be moved to the main text. I have a few substantial comments below, followed by\nminor line-by-line comments (most of which are suggested grammatical changes). If the authors have\nany questions about my review, they should feel free to email me, Liz Rampe, at\nelizabeth.b.rampe@nasa.gov.\nBroad comments/suggestions\n(1) Abstract: I suggest mentioning the results of the irradiation experiments in the abstract. The spectral\nmeasurements of different Fe phases under martian conditions (e.g., UV radiation, CO2-rich\natmosphere) will be important data for the planetary community to interpret spectra collected from a\nvariety of planetary bodies.\nThank you for the suggestion. Abstract updated.\n(2) Introduction:\na. The introduction to Fe-oxides/oxyhydroxides in the martian dust is thorough, but I suggest adding\nmore about how ferrihydrite and other Fe phases are used to interpret the past martian climate (e.g.,\nunder what conditions does ferrihydrite form and transform into other phases?).\nThank you for this suggestion. We added two sentences to address this in the first introduction\nparagraph (and additional references).\nb. CaSSIS data are not described in the introduction. I suggest introducing these data, what they’re used\nfor, and how they’ve been previously interpreted to justify the analyses of CaSSIS data in the manuscript.\nThank you for this suggestion. We have added a brief introduction to CaSSIS in the last paragraph:\n\"...using multi-angular observations from the Colour and Stereo Surface Imaging System (CaSSIS)\nonboard ESA's Trace Gas Orbiter, which, combined with laboratory spectrogoniometric measurements\nof ferrihydrite, suggests that dust particles are predominantly subµm (<1 µm) in size.\" Due to Nature\nCommunications' 5000-word limit for the main text, we have placed the detailed historical usage and\nexamples of CaSSIS observations in the Methods section, which is more appropriate for the journal's\nstyle.\nc. Add a paragraph between the Introduction and Results sections that briefly describes the mixtures\nthat were created so that the reader knows what’s coming in the results.\nThank you for this suggestion. While Nature Communications' format requires the Methods section at\nthe end of the article, we have addressed this concern by mentioning our key experimental approach\nin the introduction's final paragraph: \"Through systematic visible near-infrared (VNIR) spectral\nanalysis, combining orbital and in-situ observations with laboratory studies of novel submicron\nferrihydrite-basalt mixtures...\" This previews the experimental work while maintaining the journal's\nrequired structure.\n(3) Methods:\na. Describe the composition of the basalt in the mineral mixtures (i.e., mineral assemblage, geochemical\ncomposition) to help the reader better interpret the spectra.\nThank you for this suggestion. We have analyzed and added the basalt's mineralogical composition in\nSupplementary Fig. 13. XRD analysis reveals a mineral assemblage of augite, forsterite, magnetite,\nanorthite, nepheline (6.0 wt%) and sanidine (12.5 wt%). The high sanidine and nepheline content,\ncombined with low SiO2 (40.83%), indicates this is an alkaline basalt. This mineralogical assemblage\nsuggests an evolved alkaline magmatic system rather than a typical tholeiitic basalt composition.\nb. Further explain why Fe-oxides aside from hematite were not finely ground for spectral measurements.\nWe included this information in the methods section: “Given the extensive time required for the\nspecialized grinding procedure and the primary focus on comparing ferrihydrite and hematite as\npotential major components of Martian dust, we prioritized these two phases for submicron size\npreparation. The other iron (oxy)hydroxides with particle sizes ranging from <40 µm to <10 µm were\nincluded to demonstrate their distinct spectral properties and rule out their dominance as the primary\niron-bearing phase in Martian dust, which could be accomplished effectively with their standard\npowder preparations.”\nc. Add information about the natural samples that were measured (e.g., sites, collection procedures,\nprevious publications) in the supplementary information or methods.\nThank you for this suggestion. We have expanded our sample descriptions and collection procedures\nin the Methods section. Note that none of these samples have been published before. Most of the\niron oxide-hydroxide samples come from the University of Arkansas collection. The natural\nferrihydrites were recently collected by the lead author.\n(4) Results: Spectra of basalt-ferrihydrite-Mg sulfate mixtures show a good match to the 3-micron feature\non Mars, but these data are in the supplementary information and sulfates are not mentioned in the\nintroduction. I suggest briefly discussing the importance of sulfates on Mars in the introduction and\nmoving the basalt-ferrihydrite-Mg sulfate mixture methods and results to the main body of the\nmanuscript. Explain how Mg sulfate fits into the timing of the formation of ferrihydrite if a ferrihydrite-\nbasalt-Mg sulfate-goethite mixture best explains the vis/NIR data.\nThank you for this suggestion. We agree that the 3-micron band spectral match is an important novel\nfinding that deserves distinction in the main text. We have moved the ferrihydrite-basalt-Mg sulfate\nmixture spectral data from the supplementary information to the main text. In the introduction, we've\nadded a brief mention of sulfates, noting their identification in rocks and dust in Gale Crater by MSL\ninstruments, with appropriate citations. We've also included a concise discussion of the spectral\nresults and implications in the results section. However, given Nature Communications' 5000-word\nlimit and our current manuscript length, we had to be selective in expanding the introduction, results\nand discussion. We focus on the clear ferrihydrite-basalt-sulfate spectral relationship at 3 microns,\nwhile noting that the potential minor goethite component (suggested by the 600 nm feature) requires\nfurther study to confirm. This allows us to highlight the key novel findings while maintaining\nmanuscript concision. The sulfate discussion is kept brief but impactful, emphasizing the spectral\nevidence while acknowledging that detailed analysis of formation timing relationships would require\nmore extensive treatment better suited for future work.\n(5) Discussion: The discussion of how ferrihydrite formed on Mars could be better developed. There are\nsome seemingly contradictory statements in the discussion about the timing of the formation of\nferrihydrite. Did ferrihydrite form early or late in Mars’ history? Did it form within a rock and then\nbecome physically eroded to get incorporated into the dust? Or did it form from water-rock interaction\nof unconsolidated eolian materials? If it originally formed in a rock then was physically eroded, why\naren’t there other secondary phases common on Mars in the martian dust (e.g., smectite)?\nThank you for this important observation. We have revised the discussion to remove potentially\ncontradictory statements about timing and clarified our key findings and scope.\nWe note that the goal of this paper is to determine the iron oxide phase in the dust and not to identify\nthe source of the dust. While our data suggest ferrihydrite formed when liquid water was present on\nMars (perhaps 3 billion years ago during the Late Hesperian period), we leave precise age\ndeterminations and source mechanisms for future work.\nRegarding the apparent absence of smectites in Martian dust - our observations align with terrestrial\nanalogs where ferrihydrite commonly forms in volcanic settings through early alteration of basaltic\nmaterials. Under cold and wet conditions, the early alteration of basaltic glass primarily produces iron\nhydroxides with minor amounts of imogolite and kaolinite, rather than smectites which typically\nrequire prolonged weathering under warmer, wetter conditions. This is analogous to young Andosols\non Earth where ferrihydrite and allophane dominate over smectites.\nLine-by-line comments\nMain text\nAbstract: define all acronyms in the abstract.\nAs it currently stands the abstract is 230+ words long and Nature allows only 200. Therefore, we have\ndecided to delete the acronyms.\nThe manuscript uses “um” and “micron” interchangeably. I suggest using one term and being consistent\nthroughout the manuscript. I also suggest changing ampersands to “and”.\nFixed.\nLine 28: add an alpha symbol before “Fe2O3”\nFixed.\nLine 31: clarify that the mineralogy of the surface dust is unresolved from MSL data (e.g., “…, but the\nminerals present in the dust remains unresolved).\nFixed.\nLine 45: This introductory sentence indicates that the Fe-oxide assemblage only tells us about ancient\nclimates. From your measurements of Fe-oxides/oxyhydroxides under modern martian conditions, the\nsecondary Fe assemblage can also tell us about modern climate and how long those conditions may have\npersisted.\nIncluded the words “past and present”.\nLine 46: add a comma after “Earth”\nFixed.\nLine 48: add “typically” after “The” because the martian surface isn’t red everywhere.\nIncluded a specifier “dust-covered”.\nLine 55: Hyphenate “near-infrared”\nFixed.\nLine 56: Define the acronym “ESA”\nFixed.\nLine 58: Add a space between “)(“\nThis goes away with Nature-style referencing.\nLine 69: Hyphenate “surface-to-volume”\nFixed.\nLine 76: Explain what bond vibrations cause the 1.4 and 1.9 um bands\nFixed.\nLine 77: Add a comma after “e.g.”\nFixed.\nLine 79: Specify what “coarse-grained” means (e.g., what is the crystallite size?)\nSpecified that mm-sized hematite spherules (aka blueberries) were identified.\nLine 81: Add “e.g.,” before “Morris et al.”\nFixed.\nLine 82: add a comma after “(Fe3+/FeT)” and delete “more”\nFixed.\nLine 84: Define “microcrystal” size range\nFixed. Added “nanocrystal (<10 nm)”.\nLine 88: Change “while” to “whereas”\nFixed.\nLine 92: Change “spectral” to “spectra”\nFixed.\nLine 107: hyphenate “dust-covered” and change “dune” to “shadow”\nFixed.\nLine 110: The abundance of Fe oxides in the amorphous component in Rocknest is unconstrained. We\nhave calculated the abundance of Fe in the amorphous component using CheMin mineral abundances\nand APXS elemental abundances, but we don’t know the speciation of the Fe (e.g., oxides,\noxyhydroxides, sulfates).\nUpdated the sentence to include this information.\nLine 116: Change “at” to “in the” and change “soils” to “soil”\nFixed.\nLine 121: Change “crystalline phyllosilicate minerals” to “hydrated or hydroxylated minerals”\nFixed.\nLine 128: After reading the introduction, I suggest adding a paragraph on the methods before moving\ninto the results to help prepare the reader for what data they will see in the results. For example, I was\nsurprised to learn that you made measurements of mineral mixtures and not just individual minerals.\nThank you for this suggestion. Due to Nature Communications' strict word limit and format\nrequirements, we cannot add a separate methods paragraph before the Results. However, we have\naddressed this by expanding the final introduction paragraph to preview our experimental approach:\n\"Through systematic visible near-infrared (VNIR) spectral analysis, combining orbital and in-situ\nobservations with laboratory studies of novel submicron ferrihydrite-basalt mixtures...\" This provides\nreaders with key methodological context while maintaining journal formatting requirements.\nLine 133: clarify whether the FH-basalt powder was measured under martian conditions\nMentioned that this spectrum was acquired under ambient conditions. The reason being is to highlight\nthat even under ambient conditions hydration features except the notable 3-micron band are not\nobserved.\nLine 148: Delete the comma between CRISM and OMEGA\nFixed.\nLine 168: Remove the accent in “akaganeite”\nFixed.\nLine 181: Delete “VIS”\nFixed.\nLine 182: Add a comma after “slope”\nFixed.\nLine 186: Clarify what the amount of oxidized iron is smaller than\nThank you for noting this unclear statement. We have removed this sentence about relative amounts\nof oxidized iron since it was ambiguous and potentially confusing. Instead, we focus on the\nsubsequent, more concrete discussion of total FeO content, which provides clearer context for our\nfindings.\nLine 218: Is it possible that the shoulder feature at ~600 nm could be from chemical substitutions or\nimpurities in ferrihydrite (e.g., Si)?\nThank you for this interesting suggestion. While ferrihydrite commonly contains impurities on Earth,\nwe find it unlikely that the 600 nm feature is related to chemical substitutions for several reasons:\n1. Previous studies of Si-rich ferrihydrites (Bishop & Murad, 2002) do not show this spectral\nshoulder.\n2. This spectral region is highly characteristic of iron oxide crystal field transitions, with several\nphases (goethite, hematite, akaganeite) showing absorption features around 650 nm that\nresult in similar spectral shoulders (Scheinost et al. 1998; see Supplementary Fig. 4b).\nLine 231: Add a comma after “e.g.”.\nFixed.\nLine 231: I suggest changing the angle to Angstroms.\nFixed.\nLine 234: At the end of the sentence, consider adding a clarifying clause like: “…may contain small\namounts of other (oxy)hydroxides such as goethite, which contain a ~600 nm spectral feature.”\nFixed.\nLine 234: Add a comma after “basalt”\nFixed.\nLine 241: This is the first mention of mixtures with sulfate. I suggest adding some text about sulfates in\nthe introduction to motivate the measurement of mixtures with sulfate.\nWe added a brief mention of sulfates in the introduction section. “ChemCam also detected sulfates in\nthe rocks(Rapin et al., 2019) and dust(David et al., 2022) in Gale Crater indicating an ancient evaporitic\nand acidic environment”\nLines 249 and 250: hyphenate “3.0-um” and “1.9-um”\nFixed.\nLine 280: hyphenate “iron-oxide”\nFixed.\nLine 284: Although there is very little H2O in the martian atmosphere, the relative humidity can be very\nhigh sometimes, right? Consider changing “extremely low humidity” to something like “extremely low\nwater vapor concentration”.\nCorrect. We updated this sentence to include “extremely low water vapor content”.\nLine 289: The ferrihydrite structure did not change over the course of the 40-day experiments presented\nhere. The ferrihydrite in the martian dust has been exposed to these conditions for many\nthousands/millions of years. Would you expect the ferrihydrite structure to change over those much\nlonger timescales?\n(copy pasted from the response to rev #2)\nThank you for raising this important question about ferrihydrite's long-term stability on Mars. While\nour experimental timescales are indeed much shorter than geological periods, both theoretical\ncalculations and empirical observations strongly support ferrihydrite's stability under Martian\nconditions. The key lies in understanding the temperature dependence of transformation kinetics.\nRecent calculations by Sassi & Rosso (2022) reveal that ferrihydrite transformation requires iron cation\nself-diffusion within the crystal lattice. This process has high activation energy barriers (0.57-2.47 eV\nper unit cell), making it extremely slow even at Earth temperatures (300K). Under these conditions,\ncalculated transformation times range from decades to millions of years - far longer than the months\nto years observed in natural settings where water-mediated processes dominate. When we extend\nthese calculations to Mars' average temperature (210K), the transformation becomes effectively\nimpossible - timescales reach 10^16 years for the fastest pathway and 10^25 years for the slowest, far\nexceeding the age of the solar system itself (see Fig. 3 here).\nOur experimental observations provide additional support for these predictions. We found that our 2-\nline ferrihydrite remained stable for 14 years under ambient lab conditions, and even aggressive\ndehydration and exposure to present-day Martian conditions (UV, low pressure, CO2 atmosphere) for\n40 days had no effect on its crystallinity (e.g. main text Fig. 7).\nThis highlights an important distinction between Earth and Mars environments. On Earth, ferrihydrite\ntypically transforms through either solid-state transformation at elevated temperatures (>400K) or\ndissolution-reprecipitation reactions mediated by liquid water. On Mars, the combination of low\ntemperatures and absence of liquid water effectively \"freezes\" both transformation pathways. While\nferrihydrite remains thermodynamically metastable compared to goethite, the extreme suppression of\niron self-diffusion at 210K prevents the structural reorganization needed for crystallization. Therefore,\nwhat appears unstable under Earth conditions can persist indefinitely in the Martian environment.\nLine 294: Add the calculations using the Arrhenius equation to the supplementary information\nDone.\nLine 301: Add “e.g.,” before Fairen et al\nFixed.\nLine 305: Add “e.g.,” before Klingelhofer and consider adding references for the jarosite detection by\nCheMin (e.g., Rampe et al., 2017; Morrison et al., 2024)\nRampe, E. B., Ming, D. W., Blake, D. F., Bristow, T. F., Chipera, S. J., Grotzinger, J. P., et al. (2017).\nMineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater, Mars.\nEarth and Planetary Science Letters, 471, 172-185.\nMorrison, S. M., Blake, D. F., Bristow, T. F., Castle, N., Chipera, S. J., Craig, P. I., et al. (2024). Expanded\ninsights into Martian mineralogy: Updated analysis of Gale crater’s mineral composition via CheMin\ncrystal chemical investigations. Minerals, 14(8), 773.\nFixed and added.\nLine 309: Add “e.g.,” before Bishop and Murad\nFixed.\nLine 318: Please explain how basalt may inhibit the transformation of ferrihydrite to a more crystalline\nphase.\nWe have removed this sentence from the manuscript.\nLine 323: Add “e.g.,” before Childs and Parfitt\nFixed.\nLine 326: Add references that show ferrihydrite is more prevalent than allophane and imogolite in Fe-\nrich volcanic materials\nThanks for this suggestion. This statement is related to the previous reference so we have moved the\nParfitt et al (1988) reference here.\nLine 328: In the discussion, further explain why the presence of ferrihydrite provides insight into early\nMars history and not more modern environments. Consider defining “early history” (e.g., number of\nbillions of years ago).\nAs mentioned previously, we decided to focus the discussion only on the Hesperian period and\nprovide an approximate date of 3 billion years ago. We also note that determining the exact timing of\nferrihydrite formation requires additional constraints beyond the scope of this study and we leave this\nfor future investigations.\nLine 337: Add “e.g.,” before Carr and Head\nFixed.\nLine 350: The statement that ferrihydrite may have formed from the last widespread aqueous episode\nseems to contradict the statement that ferrihydrite formed during early Mars history.\nThank you for this comment. We have decided to remove this contradictory sentence from the\nmanuscript.\nLine 360: Add a comma after “solutions”\nFixed.\nLine 366: Add “e.g.,” before Burns\nFixed.\nLine 385: Change “+2” to “2+”\nFixed.\nLine 398: Capitalize “Mars Sample Return”\nFixed.\nLine 398: Change “bring back regolith samples” to “deliver regolith samples to Earth”\nFixed.\nLine 400: Consider elaborating on what we can learn about ferrihydrite and how it formed in the\nreturned samples. What types of measurements shows be performed on the regolith? Could the short-\nrange order of ferrihydrite tell us if ferrihydrite is pure or beginning to transform- to another, more\ncrystalline Fe phase? Could the physical relationship between ferrihydrite and other phases tell us what\nhas been altered to form ferrihydrite?\nThese are interesting questions. While many analyses can be performed as the reviewer suggests but\nwe believe isotope studies of ferrihydrite may provide crucial clues. Fe, H and O isotopic systems in\nferrihydrite can help reconstruct the aqueous geochemical conditions, water availability, and potential\nfor habitability during its formation on Mars. For example, Fe isotopes (δ⁵⁶Fe) record redox conditions\nduring formation by their sensitivity to Fe²⁺/Fe³⁺ ratios. These measurements could also indicate\npotential microbial activity, as biological Fe oxidation produces distinctive isotopic signatures.\nFurthermore, O isotopes (δ¹⁸O) preserve information about water sources and temperature during\nferrihydrite formation. This data helps determine formation conditions and provides clues about\nevaporation/precipitation cycles. Finally, H isotopes (δD) record information about water sources and\nindicate the degree of water-rock interaction, and the D/H ratios also help constrain atmospheric\nwater loss through time. Discussion on these points is beyond the scope of this paper and therefore\nwe only include a brief mention of the isotopes in the last sentence of the paper.\nLine 403: Hyphenate “light-toned”\nFixed.\nFigure 2b: Include the spectrum of basalt for comparison to the mixtures. Show spectra of mixtures with\nother Fe phases (e.g., akaganeite) either here or in the supplementary information.\nThank you for this suggestion. Fig. 2b now includes also a mixture of akaganeite.\nFigure 5c: Label ferrihydrite peak positions in angstroms\nFixed.\n. Show the search-match results in the supplementary information.\nGiven the distinctive and well-documented X-ray diffraction pattern of 2-line ferrihydrite, we believe\nthe raw XRD patterns shown in the paper, with peaks labeled in angstroms, are sufficient to\ndemonstrate phase identification. The characteristic broad peaks at ~2.6 Å and ~1.5 Å are diagnostic of\n2-line ferrihydrite and are unambiguous in our data. In addition, we also included a reference to a\nreview paper on ferrihydrite by Jambor & Dutrizac (1998) to support our identification.\nFigure 5d: Show the LIBS spectrum for comparison to the shoulder feature in Mars data.\nLIBS spectrum of dust added.\nFigure 5: Could impurities in ferrihydrite contribute to the shoulder at ~650 nm?\nWhile ferrihydrite is known to include impurities owing to its nanoscale particle size and adsorptive\nproperties we find it unlikely that the 650 nm feature is caused by an impurity. See our previous\ncomment.\nFigure 6: Explain the reason for the difference in the curves at high phase angles (e.g., there is a roll off in\nMars data but not in the lab data)\nWe clarified in the caption the differences in roll off point between the Martian data and laboratory\nexperiments.\nFigure 7a: Explain the reason for the shift in the 1.4-um band\nIt happens when adsorbed water is present and contributes to a 1.41 micron band (which also has\ncontributions from structural OH). The adsorbed water absorption is at longer wavelengths than the\nstructural water, so when the adsorbed water is lost, the band minimum can shift to lower\nwavelengths. Similar effects have been observed in Cloutis et al. (2008) and Poitras et al. (2018). We\ninclude this statement and references in this caption.\nLine 488: Hyphenate “background-removed”\nFixed.\nLine 491: Hyphenate 1.9-um\nFixed.\nFigure 8: Explain how the figure was made in the methods section. Also explain what “Ferrihydrite-2L”\nmeans.\nWe have included a short text on the pe vs ph diagram in the caption of this figure. The “Ferrihydrite-\n2L” was renamed to Ferrihydrite (2-line) in the diagram. In the caption we explain what is 2-line\nFerrihydrite.\nLine 499: Move the RMSE calculation section after the VNIR and FTIR spectroscopy section.\nFixed.\nLine 500: Clarify that the spectra are VNIR reflectance spectra (e.g., “To compare the goodness of fit of\nour lab mixture VNIR reflectance spectra and the VNIR reflectance spectra acquired by orbiters…\")\nFixed.\nLine 501: Add a comma after “rovers”\nDone.\nLine 518: Add a comma after “goethite”\nThis sentence was updated based on rev 1 suggestion.\nLine 520: Clarify whether iron oxide powders were analyzed via XRD before the experiments to confirm\npurity and that the minerals were not altered over the course of 14 years.\nThank you for this important comment. We have conducted comprehensive XRD analyses of all iron\noxide-hydroxides used in this study to verify their mineralogical purity and stability. This analysis\nrevealed that our primary \"schwertmannite\" sample was misidentified and is actually akaganeite,\nlikely due to a labeling error when the samples were originally prepared 14 years ago. We have\nupdated Figure 2b accordingly to show the \"Akaganeite-basalt mix\" spectrum. Fortunately, we have\nanother synthetic schwertmannite sample that we decided to include in this study. This second\nschwertmannite sample contains natrojarosite as an impurity phase, and we have included the XRD\npattern and spectral data for confirmation in the supplementary file. Using this sample we also made a\nschwertmannite-basalt mixture in Figure 2b to address your earlier comment regarding the inclusion\nof additional iron hydroxide phases. The XRD patterns confirm at all other iron oxide samples\nmaintained their original mineralogical structure and crystallinity over the storage period. Complete\nXRD characterization data for all phases are now provided in Supplementary Figure 15.\nLine 524: Add a comma after “samples”\nFixed.\nLine 524: Specify whether sieving was done dry or wet\nSpecified.\nLine 526: Change “sample” to “samples”\nFixed.\nLine 528: Fix the spelling of goethite\nFixed.\nLine 531: Add a comma after “size”\nThis sentence was rewritten to address another reviewer’s point.\nLine 531: Explain the source of the basalt in the mixtures\nWe explain this in the second paragraph of the methods section. The basalt sample used in this study\nis from the University of Bern geological collection and was sampled from an unknown locality in\nVirginia, USA. However, we include here mineralogical analysis of this sample as shown in the\nSupplementary file.\nLine 540: State the temperature under which the ethanol was evaporated\nIt was evaporated under ambient temperature. This information is included in the text now.\nLine 545: Explicitly state the proportions of minerals and basalt in the mixtures in a table in the methods\nor supplementary information\nAll mixing ratios used in this work are now shown in Supplementary Table 1.\nLine 554: I suggest rearranging the sentence to “Spectra of ferrihydrite-basalt mixtures in the VNIR and\nNIR range were acquired…”\nFixed.\nLine 556: Specify which phases and mixtures were measured at RELAB\nFixed.\nLine 559: Delete “spectral”\nFixed.\nLine 560: Define the spectral resolution of the instruments.\nDefined.\nLine 576: Define “MCS”\nFixed.\nLine 576: Define what “This” refers to (e.g., “This model…”)\nFixed.\nLine 597: Hyphenate “mm-thick”\nFixed.\nLine 599: State for how long the samples were exposed to martian atmospheric and radiation conditions\nbefore spectral measurements. Also state the temperature of the measurements.\nThis is a fair concern. We updated this paragraph to include all the required information.\nLine 599: Add a comma after “atmosphere”\nFixed.\nLine 601: Delete “was used”\nFixed.\nLine 623: The sentence that starts on line 623 doesn’t read correctly to me. I think “data.” should be\nadded after “signal-to-noise”, and “reflectance” should be the start of a new sentence.\nFixed.\nLine 633: Add a comma after “0 deg”\nFixed.\nLine 640: Be more descriptive of “pre-“ and “post-exposed”. For example, “Samples that had not been\nexposed to Mars-like atmospheric pressure and radiation were front-loaded…”\nFixed.\nLine 641: Change “sample was” to “samples were”\nFixed.\nSupplementary information\nLine 67: Add a comma after “mixtures”\nFixed.\nLine 71: Change “comparison” to “compared”\nFixed.\nLine 79: Fix the spelling of goethite\nFixed.\nLine 88: Explain why magnetite is not shows in Supplementary Fig. 5.\nWe have included a picture of magnetite in this figure.\nSupplementary Fig. 6: Explain the source of the K in the synthetic ferrihydrite and whether it affects the\nferrihydrite spectral properties.\nThank you for the suggestion. Our electron microprobe analysis detected ~3 wt.% K in our synthetic\nferrihydrite (Supplementary Fig. 7), which originates from the KOH used during sample preparation as\ndescribed in our methods. Since K+ ions do not have electronic transitions or vibrational features in\nthe visible to near-infrared wavelength range (0.35-2.5 µm), this trace amount of potassium does not\ninfluence the spectral properties of our ferrihydrite samples. The observed spectral features are\ndominated by Fe3+ electronic transitions and OH/H2O vibrational bands characteristic of ferrihydrite.\nLine 101: Add a comma after “hematite”\nFixed.\nSupplementary Fig. 8: Show the martian spectrum for comparison to laboratory spectra.\nWe added a spectrum of Martian dust acquired by ChemCam in this figure.\nLine 110: Add a comma after “basalt”\nFixed.\nLine 112: Change “a goethite” to “as goethite”\nFixed.\nLine 119: Describe and justify in the methods the type of Mg sulfate that was used in the mixtures (e.g.,\nepsomite)\nFixed.\nSupplementary Fig. 10: Change c and d to a and b\nFixed.\nReferences\nChevrier, V., Mathé, P.-E., Rochette, P., Grauby, O., Bourrié, G., & Trolard, F. (2006). Iron weathering\nproducts in a CO2+(H2O or H2O2) atmosphere: Implications for weathering processes on the\nsurface of Mars. Geochimica et Cosmochimica Acta, 70(16), 4295–4317.\nhttps://doi.org/https://doi.org/10.1016/j.gca.2006.06.1368\nJambor, J. L., & Dutrizac, J. E. (1998). Occurrence and Constitution of Natural and Synthetic Ferrihydrite, a\nWidespread Iron Oxyhydroxide. Chemical Reviews, 98(7), 2549–2586.\nhttps://doi.org/10.1021/cr970105t\nSassi, M., & Rosso, K. M. (2022). Ab Initio Evaluation of Solid-State Transformation Pathways from\nFerrihydrite to Goethite. ACS Earth and Space Chemistry, 6(3), 800–809.\nhttps://doi.org/10.1021/acsearthspacechem.2c00026"}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors fully address my concerns. I am happy to recommend acceptance of the manuscript to the journal Nature Communication.", "reviewer_3": "Thank you to the authors for their detailed and thoughtful responses to my comments. I believe the authors have sufficiently\naddressed my comments, and I recommend the manuscript be published in Nature Communications without further edits."}, "author_response": ""}]

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