Why Mars is Red - Ferrihydrite Detection in Martian Dust and  Implications for Early Mars

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. Quantitative spectroscopic analyses indicate that a hyperfine mixture of ferrihydrite and basalt 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.IntroductionIdentifying 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 dry, 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 ALH 84001 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) nor 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 MIMOSII 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 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 unconstrained 36–38. In addition, the Alpha Particle X-ray Spectrometer (APXS) instrument analyzed air fall dust on the science observation tray. These measurements39 indicated that the dust is compositionally similar to the bulk Mars is in and which is in with MER and ChemCam measurements that the amorphous iron oxide component observed in the “Rocknest” soil be to The at Mars which a and a detected when the was to This finding that H2O is bound to the amorphous component of the as the instrument hydrated or hydroxylated minerals in this In measurements that iron oxides wt. % of the X-ray amorphous component in Martian the specific mineralogical of these phases has Here, we present that ferrihydrite (Fe5O8H · nH2O) – a poorly hydrated iron oxide mineral – is the dominant phase in Martian dust. Through visible near-infrared spectral and observations with laboratory studies of we demonstrate that ferrihydrite the best spectral to the color of Further, to the particle size of Martian dust, we analyzed observations from the and Imaging combined with laboratory measurements of suggests that dust particles are in In addition, we show that ferrihydrite its mineralogical structure and into other iron oxide-hydroxide phases when to present-day Martian conditions its and for the past climate and on of Martian dust and The of Martian dust by key spectral features that with laboratory mixture of hyperfine in a measured under conditions spectra the a absorption at visible a in reflectance a hydration bands from and a deep absorption centered 3 µm the spectral properties the µm of hyperfine mixture provide for the presence of ferrihydrite in Martian dust. mixture in the visible – with Martian dust spectra by and Previous that the reflectance spectra of in the NIR are with the spectral properties of Martian dust. However, provide a for the portion of the Martian NIR in regions OH and H2O bands are typically observed in spectra. current a of Martian dust spectra from that are with mixture The Martian spectra analyzed for this include data from and OMEGA as well as measurements from Mars MER and is the of which is an observation of the and magnetic airfall dust component by the The magnet was designed to which is as ferrihydrite is under the Martian diurnal observed at the MER – The in with the suggests a weak absorption feature This spectral feature is likely due to from ferrihydrite and minerals present in the Martian dust, as by the mixture also Finally, we that the dust spectra at five different on the are similar in the visible the that Martian dust is well on a the visible spectral – iron oxides and distinct and spectral the deep absorption from to µm in of by the transition the broad band at µm in and to a in spectra of hematite and and the µm of the band in The specific band and is a of the structure the The and of these bands in and mixture spectra are with the Martian dust that a of of the Martian dust In the of mixture the and of the bands observed in the Martian spectra similar to previous studies of (e.g. ref. This suggests that ferrihydrite is a other iron oxides and to the hue of Mars of reflectance spectra of demonstrate that large amounts of ferrihydrite are to the red – of Mars of ferrihydrite in a mixture with basalt the visible nor the visible absorption at – ferrihydrite is to the and analyses indicate that as as and is to provide a with Martian dust spectra. Based on analyses of Fe and abundances in ferrihydrite and a of we that the poorly crystalline iron oxide in and approximately This with measurements of in the amorphous component of the which from in Martian However, the for Martian from and Pathfinder and the in Martian dust observed by and In the of ferrihydrite in best spectral the that of the significant X-ray amorphous iron component detected in Martian soils by be The portion of the Martian from to 2.5 µm is with the properties of the basalt used in the in and in this indicate in the or of the While laboratory are observations of the Martian surface indicate that dust in be In addition, the at 1.4 and 1.9 µm are observed in the mixture These features are because basalt is less ferrihydrite at these The reflectance properties of including the of and its are by the this and less has a on these Quantitative spectral analysis indicates ferrihydrite quantitative based on the suggests that the mixture a to the observations of Martian dust compared to the and This is by the of approximately for the in to for the other a that the spectra in the reflectance with indicating a spectral the laboratory and Martian small feature in the Martian dust spectra visible in the ChemCam be to the transition which is centered also This transition is by the strength and of the which is by the of the iron mixture less spectral in this The of this band is known to in and with this feature with these ferrihydrite samples collected from a in and in spectral in the This be due to the of ferrihydrite formation in to of oxidation and resulting in a of from ferrihydrite to the crystalline ferrihydrite and X-ray diffraction of indicate the presence of the However, in at in the structure of the ferrihydrite to the observed feature is that Martian dust contain small amounts of other as which contain a spectral of and goethite that wt. % goethite the observed experiments ferrihydrite of the observed 3-µm band in the Martian dust be to chemically bound water in This feature arises from the bound water within the structure (Fe5O8H · However, ferrihydrite of the 3-µm band observed in Martian dust suggesting the presence of hydrated investigated these hydrated phases the Martian dust 3-μm band by with The of to mixture both the on the of the 3 band and the at observed in the OMEGA dust This suggests that both ferrihydrite and sulfates to the hydration signature in Martian dust, with recent ChemCam results that sulfates as of hydration in Martian Furthermore, under conditions, ferrihydrite also contains adsorbed of this adsorbed water is likely in the hyper-arid Martian environment. This is by a of ferrihydrite to present-day Martian conditions which showed a rapid of the H2O band and In the 1.4 µm band The observation that the and bands the band is under conditions of low water is with from previous Finally, X-ray diffraction analyses in before and the indicate that ferrihydrite a phase due to and laboratory observations indicate particle size phase spectral from observations and laboratory indicate that surface including ferrihydrite on Mars particle of observations an when reflectance color are phase a in studies of Martian soils using data from the and MER The of both and laboratory size at phase suggesting a particle size as the of ferrihydrite a at phase and particles an color ratio particles and a The particle size from these observations with laboratory mixture samples the particle from observations and laboratory are with from and observations of surface by the The of ferrihydrite as the phase in Martian dust several ferrihydrite stable under present Martian the presence of poorly crystalline ferrihydrite for the of water on ancient is the of surface oxidation on early is a mineral on that can to stable and crystalline phases with are two pathways for ferrihydrite to into other The known as by dry at which ferrihydrite to and into However, present-day Martian conditions are by an of approximately and low water resulting from a water in the laboratory under Martian conditions demonstrate that ferrihydrite adsorbed H2O its poorly crystalline as by X-ray diffraction analysis In addition, that at Martian conditions transformation of ferrihydrite to goethite is to be as the low kinetic to iron that years for the transformation both and suggests that the and dry conditions on present-day Mars the transformation of The other is which and in aqueous resulting in the of goethite and This is on pH and water low pH and low water to this Using the and the from et we that a of ferrihydrite approximately years to into hematite under conditions of pH and a water of However, and been shown to or at the transformation of ferrihydrite to other In addition, under a and wet Mars (e.g. water of in acidic and conditions, the transformation be acidic and conditions for been present on early as by observations of the minerals with rover experiments (e.g. and from the transformation of ferrihydrite been or in the aqueous environments of ancient ferrihydrite is with other minerals containing and/or (e.g. (also samples from laboratory which of silicate and ferrihydrite spectral with observed on the Martian surface This suggests that ferrihydrite on Mars is likely present in its form in association with volcanic The presence of a component in the dust two different it indicate that ferrihydrite formed in an aqueous associated with or that the basalt is from regions through weathering. The and on Mars and of basalt particles with The fine grain size of ferrihydrite constraints on its formation ferrihydrite from it forms small of likely due to rapid or forms The size of ferrihydrite in and its to Martian spectral data suggests of these under similar rapid conditions, followed by This is with of aqueous alteration which typically crystalline Furthermore, it is to that on ferrihydrite is in volcanic as and forms as one of the poorly crystalline phases from alteration of and (e.g. The abundance of these minerals on the chemistry of the volcanic In volcanic to and are less Furthermore, the early alteration of iron with amounts of and which typically form during under and presence of ferrihydrite on Mars provide crucial insights into the of aqueous and oxidative weathering during the planet's early at surface This formation with climate and models by conditions, the of surface and also environmental conditions that followed the significant period of formation of during the period, approximately 3 billion years In the was a phase of volcanic (e.g. that with water or and conditions for formation of However, the of amorphous material including and ferrihydrite suggests of aqueous as ferrihydrite formation rapid This that ferrihydrite formation on Mars was likely a with continuous as or The presence of sulfates and ferrihydrite in the dust, indicated by 3-µm band analysis with the transition acidic and conditions in the Finally, the ubiquitous hue observed on the Martian likely due to the presence of suggests an period of and through and dust This the of ferrihydrite on the Martian the of surface oxidation on Mars is in of abundance of ferrihydrite in the surface and dust. In we show the redox of that conditions are for its In aqueous ferrihydrite forms through two pathways either the of or the oxidation of In laboratory the of as ferric to the formation of the of the crystallinity of the resulting ferrihydrite which is by a in the of X-ray diffraction from to However, in the Martian oxidation of Fe2+ is likely due to the large abundance of silicate minerals as or (e.g. The key is the Fe2+ was by components in the or by in surface on and shown that the oxidation of and of crystalline in through two either using by the breakdown of or through chemical by with water The hydroxyl be due to lower formation and as surface However, chemistry experiments also that in the of or iron oxidation can in and H2O These experiments showed the formation of ferric as goethite and as well as minerals including no was early weathering which to phases as hydrogen to suggesting a oxidation that without In the oxidation of surface on early Mars by has been by recent studies of Gale which identified oxide in However, of the been proposed as and also has been identified as a Fe2+ it to the formation of crystalline iron phases in experiments of On Earth, the of is from it crucial to this on Mars to the for past and the planet's habitability. Finally, previous studies provided insights into Mars oxidation the presence of ferrihydrite in the Martian surface remains a of the for to the redox conditions to formation on In indicate that a hydrated iron oxide-bearing is the iron oxide component of Martian dust, to previous of anhydrous minerals the This hydrated phase likely formed during water in the of early under oxidative The presence of ferrihydrite suggests that Mars to a that has to the present preserving this mineral phase the planet's Mars regolith samples to that contain analyses of these including and stable provide crucial redox conditions, and water on Mars different iron oxide phases were used in this magnetite, and and were at the of the of and and were under conditions for years before investigated in this The samples were in and can be in ferrihydrite the is as of was in we of to the pH the The of were by The samples were several with water and at of was in water and in a at for approximately of product that was and was by and 3 of in of water to followed by of at the to temperature, the was and The presence of in the spectra was noted as a during this possibly due to the of with of using in which was to The pH was from 3 to using of under resulting in of of a reddish The pH was to and the was and was by a mixture of with a ratio of which was with an of approximately in a at The is from the of and goethite is a from the of is from µm particle size as shown in was from The basalt used in this is from the of and from an in However, we include mineralogical analysis of this as shown in The ferrihydrite samples shown in are from a in and were collected under by The ferrihydrite from the was collected in ferrihydrite samples were collected as aqueous and under The ferrihydrite poorly no phase transformation in pH measurements of the water circumneutral for both and The of the iron oxide phases and of a large and samples using a dry we used a at the of properties samples it was to to the particle the size to be and determined for each used of and Particle goethite maghemite and two ferrihydrite and to Martian dust particle This was at the of using a similar to the by The a oxide with of and The initial of 50 µm were for with the was and for in with the were from the by using The of in was and the in under a to the dry The resulting particle size was by the for the and the on ferrihydrite and hematite as components of Martian dust, we these two phases for size The other iron with particle from µm to µm were to demonstrate distinct spectral properties and as the phase in Martian dust, which be with of basalt with iron oxides including and at a ferrihydrite and hematite we from to with The specific used in this are in the 3-µm band and presence of sulfates in Martian dust, we of and While anhydrous from was spectral analysis revealed hydration features of These hydration features bound water within the adsorbed indicating hydration has were through and using a and and spectra analyzed in this were under conditions for The of each was with which is a imaging that from to The (NIR) up to µm was at with a equipped with a reflectance a of each iron oxide we the and at resulting in the spectra shown in this of in the and NIR were at using analysis also other (oxy)hydroxide as and goethite was at In addition, 3 µm observations of the were also at were in dry air to The used were the Spectrometer and the The spectra were in a similar to provide spectra up to is and for the and and in the the of of mixture spectra and the spectra by and we a each observed the were and at The observed was from the data at The observed was by the observed by its was by the observed to the laboratory in the This the the two these and the of the is the of data in the spectra. in the and OMEGA were and analyzed approximately observations from the and Previous studies the capability to phase to surface and particle size as well as under low ratio the and for spectral analysis of Martian the dust spectra from each we used the for to regions of over and stable was the of in size by or other were past we also observations phase because of dominant for each observation we using the of dust from the Mars This as a for the conditions during the of and when surface reflectance of ferrihydrite were using which is a at the of In to was to in the and in the phase of to phase was also equipped with the which spectral the and reflectance data was using a diffuse and in of each the samples were using a each surface was with the material using fine of The was to the surface of dust resulting from airfall in the of was used to Mars surface conditions (e.g. ref. dry was by a through the noted The were in that are under a that both and reflectance samples were to a at and reflectance spectra were measured without the samples were with a with spectral to be to that of the measured by which has The used were used to the of the calibration targets when to the of the at 50 from the through a The spectra were to the measured at and for measurements using a reflectance showed no significant absorption nm) air and likely due to the absorption was The samples were to both and for with a at under conditions spectral measurements were at spectra over the to were measured with an with a spectral of and and spectral of 1.4 for the to and for the to The data are by the instrument to data at with a were measured at a of and with provided by an spectra were measured to that is in an well the which was to a diffuse reflectance and for its absorption properties to the the spectral data spectra of the dark and were and to provide spectra over the were collected at Mars before and the was with a were with the used for at a lower to of the The spectra were collected to an the and were to the data collected with the at of the was by a at and and the was to collected spectra. spectra and spectra were with the of and and spectra were for each diffraction were with a continuous from to on a with a two was equipped with a a a a a and a at an of and of The X-ray was at and using a of that been to and were into an X-ray amorphous that been to conditions were a due to was using the to the for

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