Fe-phosphates in Jezero Crater as evidence for an ancient habitable environment on Mars

Domain: earth_science
Published: 2025-07-14
Editorial outcome: minor_revision

Abstract
Phosphorus is an essential component for life, and in-situ identification of phosphate minerals that formed in aqueous conditions directly contributes toward one of the main goals of the Mars 2020 Perseverance rover: to seek signs of ancient habitable environments. In Jezero crater, proximity science analyses within a conglomerate outcrop, “Onahu” demonstrate the presence of rare Fe3+-bearing phosphate minerals (likely metavivianite, ferrolaueite, (ferro)beraunite, and/or santabarbaraite) embedded in a carbonate-rich matrix. While Fe-phosphates have been inferred previously on Mars, this work presents the most definitive in-situ identification of martian Fe-phosphate minerals to date, using textural, chemical, spectral, and diffraction analyses of discrete green-blue grains. The Fe-phosphate minerals’ textural context along with comparisons to Earth analogs suggest they likely formed after oxidation of Fe2+-phosphate vivianite, the most common Fe-phosphate in sedimentary environments on Earth, often associated with microbial activity and organics. While there is no obvious evidence of biological inputs in Onahu, if the Fe-phosphates’ formation environment was similar to vivianite-rich sedimentary environments on Earth, these minerals likely originally precipitated in conditions favorable to potential martian life—in a low temperature, reducing aqueous medium with high concentrations of bio-limiting elements, and Fe-redox gradients that could provide an energy source. If the sample collected from Onahu (Otis_Peak) is returned to Earth, analysis of the Fe-phosphates may provide new insights into ancient habitable environments on Mars.

Introduction
On February 18, 2021 the Mars 2020 rover Perseverance landed on the floor of Jezero crater, a ~45 km diameter impact crater that once contained a lake1. The crater hosts an ancient, well-preserved depositional fan/delta complex composed of sediments transported and deposited by streams >3.2-3.8 billion years ago2–8. Detailed investigation of the sediments and rocks in the Jezero fan can thus provide insights into the physical and chemical conditions in ancient aqueous environments on Mars, the duration and nature of past habitable environments, and their potential to sustain biological or prebiotic processes.

At the time of this writing, Perseverance has collected 11 samples from the fan for potential return to Earth, all currently stowed within the rover’s Sample and Caching System (like the sample we discuss here) or cached at the Three Forks Sample Depot. Here, we report the detailed mineralogical and chemical analysis of a conglomerate outcrop, informally named Onahu, sampled by Perseverance on Mars solar day (Sol) 882 (sample name Otis_Peak). Onahu is located on the top of Jezero’s western fan and provides the most definitive in-situ identification of Fe-phosphate minerals on Mars, observed as large (~100s μm wide) green-blue grains (Fig. 1A).

Fe-phosphate minerals are rare on Earth and Mars, where Ca-phosphates are the dominant phosphate minerals (i.e., fluorapatite on Earth, and merrillite and chlorapatite on Mars9). Fe-phosphates have been inferred on Mars before in Gale and Gusev crater9–11, but, until now, have never been directly observed on the surface as we show here, using a combination of high-resolution imaging, chemical, and micro-structural analysis. Phosphate minerals are high value targets for Mars sample return because their chemistry and textural occurrence can be used to constrain the bioavailability of P for prebiotic chemical reactions and/or potential life9, past aqueous conditions on Mars9,12,13; and when igneous in origin, insights into magmatic volatile abundances14–16. Fe-phosphate minerals that precipitate in aqueous environments are particularly important astrobiological targets as they have been observed on Earth as products of dissimilatory Fe-reducing bacteria17,18, encrusting bacterial cells19, and growing on the hydroxyapatite components of bones, shells, and fossils that have reacted with Fe-rich fluids17,20. While biotic influences are not essential for Fe-phosphate formation on Earth or Mars, terrestrial Fe-phosphates are commonly associated with organic matter, either growing on or replacing plant detritus, or precipitating in environments where degradation of organic debris results in more reducing and P-rich conditions17.

This work summarizes the detailed textural, chemical, reflectance, and micro-structural diffraction analysis of the Onahu Fe-phosphates using the Planetary Instrument for X-ray Lithochemistry (PIXL) – a mapping X-ray fluorescence spectrometer21 – and images from the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument22, both mounted on the arm of the rover. The high-resolution PIXL and SHERLOC results are used to identify the Fe-phosphate mineral that is most likely present in Onahu, constrain the paleoenvironmental conditions that led to their formation, and assess if these conditions were habitable.

Results
Imaging and compositional analyses of Onahu
Onahu is in the Otis_Peak formation (fm), recognized by its fluvial deposits of planar laminated and cross-stratified sandstone to granule conglomerates. Large, scattered boulders (called the ‘rubbly unit’) overlie the Otis_Peak fm, which could have been sourced from the olivine-rich units common outside Jezero crater23–25. The laminated, poorly sorted medium-grained sandstones of the Skrinkle_Haven member (Tenby fm) lie beneath the Otis_Peak fm23.

The Fe-phosphates described here were identified in a 50mm circular abrasion patch informally named Ouzel_Falls abraded into the pebbly, cross-stratified conglomerate Onahu that outcrops on the top of the western Jezero fan (Fig. 1B, C). The outcrop’s surface was abraded to remove surface dust and coatings and to provide a relatively smooth surface for analyses by proximity science instruments (PIXL and SHERLOC).

This work focuses on X-ray fluorescence (XRF) maps (3 maps collected between Sols 789–811; Fig. 1D), X-ray diffraction analysis of individual Fe-phosphate grains, and reflectance spectral images collected by PIXL, as SHERLOC Raman spectroscopy maps did not include the Fe-phosphate-rich areas. However, microscopic images collected by SHERLOC cameras (color Wide Angle Topographic Sensor for Operations and eNgineering [WATSON] images with ~15.9 μm/pixel resolution) provide complementary high-resolution context images of the Ouzel_Falls abrasion patch.

Chemistry
PIXL maps show the Ouzel_Falls abrasion patch is predominantly composed of gravel- to pebble-sized clasts of olivine (Fo61), altered mafic clasts with elevated Fe+Mg/Si (potentially basaltic polymineralic clasts that have been altered to clay minerals), and relatively Mg-rich Mg-Fe-carbonates (Mg#~55) embedded in a fine-grained matrix. The matrix includes domains rich in Si and Mg-Fe-sulfate, and domains rich in relatively Fe-rich Fe-Mg-carbonate (Mg#~31), intermixed with fine-grained fragments of the major clast types (Fig. 2A-C, Supplementary Figs. S4-S7, and Tables S3, S4).

Fe-phosphates were identified in the carbonate-bearing matrix domain by their high P and Fe concentrations (up to 24.7 wt% P2O5 and 48.8 wt% FeO). These Fe-P “hot-spots” are closely correlated with the ~50–500 μm green-blue grains visible in WATSON images (Figs. 1A, 2B). Because the green-blue grains are similar to or smaller than the PIXL beam size, all the Fe-P-rich PMCs (PIXL Motion Counters = integers corresponding to each analysis point location in a PIXL map) contain elements that are present in surrounding minerals (e.g., Si and S) and are interpreted as mixtures of Fe-phosphate and surrounding matrix material. This mixing relationship is clear in Fig. 2D, which shows that the Fe-P hot-spots are consistent with mixtures of Fe-phosphate minerals and the major phases in Ouzel_Falls. Simulated models of these mixtures also produce trends similar to the empirical PIXL data presented here (see Supplementary Information and Figures S2, S3).

At PIXL’s resolution, the Fe-phosphates appear to be predominantly composed of only Fe and P based on the strong compositional and spatial correlation between the two elements (Fig. 2B, E). There are no other obvious correlations between high-P concentrations and other cations that are commonly found in phosphate minerals such as Ca, Al, Mg, Mn, and Na (Supplementary Fig. S1). No trace elements except Ni were detected in the Fe-phosphate-bearing PMCs (Supplementary Table S2).

Based on the lack of correlation with other major and trace elements, the composition of the Fe-phosphates is estimated here by determining the P/Fe ratio of the PMCs that contain the most Fe-phosphate (PMCs with P2O5 > 10 wt%, SiO2 < 30 wt%, SO3 < 5 wt%). The slope of a weighted orthogonal regression through these points (a weighted linear regression that accounts for errors in both x and y variables carried out using the scipy.odr package in Python26,27) indicates the Fe-phosphates have P/Fe molar ratios of approximately 0.67±0.11 (~2/3; Fig. 2E). The non-zero intercept from this regression (0.14±0.05) is consistent with mixing with surrounding Fe-bearing phases. According to our literature review and the RRUFF database of International Mineralogical Association approved minerals28, 19 known terrestrial Fe-phosphate minerals have P/Fe ratios in this range (see Supplementary Table S5). It should be noted however that the P/Fe ratio determined here is a first order estimate based on discretionary SiO2, P2O5, and SO3 thresholds (Figure S1), and there is likely more unquantified uncertainty as the Fe-phosphates are mixed with multiple end-members which is difficult to model with a linear correlation trend.

Minor phases identified by PIXL in the Ouzel_Falls abrasion include sulfates and spinels. The bulk composition of Ouzel_Falls contains significant sulfur (~6.6 wt% SO3; Table S2), most of which can be ascribed to Mg-Fe-sulfate minerals. Areas enriched in Mg and S were found across the PIXL scans, and typically associated with the Si-rich matrix (Supplementary Fig. S6). Ca-sulfate is uncommon in Ouzel_Falls and occurs in discrete monomineralic areas. Small grains of Cr-Ti spinel (likely Ti-bearing chromite) are rare and randomly distributed (Supplementary Fig. S8).

Reflectance
To identify which Fe-phosphate mineral with P/Fe ~2/3 is most likely present in Ouzel_Falls we examined their reflectance spectra in more detail using PIXL’s Micro Context Camera (MCC). The Fe-phosphates can be readily delineated using Green/Near Infrared (G/NIR) band ratio images, standing out as regions with the highest G/NIR in the abrasion patch (Fig. 3A-C).

Four of the grains with the highest P concentrations were selected for further analysis and comparison with terrestrial Fe-phosphate reflectance spectra mainly compiled by the RELAB database29. We focus on reflectance ratios baselined with the NIR band, as this band is dominant in discerning the grains from the matrix. The available bands of PIXL’s MCC are; ultraviolet [UV, 385 nm], green [G; 530 nm], blue [B, 450 nm], and near-infrared [NIR, 735 nm]. As such, detailed comparisons between PIXL spectral profiles which contain only 4 bands, and high resolution RELAB measurements are limited. Moreover, there is additional uncertainty due to particle size effects on the spectral profiles, as RELAB measurements are based on powdered material.

The Fe-phosphate grains in Ouzel_Falls have distinct UV/NIR and G/NIR (Fig. 3D). The average reflectance ratios of Grains 1-3 are relatively similar, varying between 0.91-0.94 G/NIR and 0.60-0.71 UV/NIR, while Grain 4’s averages are much lower with 0.79 G/NIR, and 0.46 UV/NIR, revealing an overall increase in NIR relative to other wavelengths in this grain and possibly a different Fe-phosphate phase. Furthermore, many of the individual pixels within Grains 1–3 have similar reflectance ratios to the more ferrous terrestrial Fe-phosphates (vivianite and metavivianite), while the individual pixels from the core of Grain 4 are more similar to terrestrial ferric phosphates (strunzite and strengite).

Only the cores of each grain were used for comparison to terrestrial Fe-phosphates in Fig. 3D in order to minimize any mixing with surrounding Ouzel_Falls matrix. In order to assess if any zoning is present, additional analysis of the cores and rims of each grain were carried out (Supplementary Materials and Figures S9-S16, Table S6). The G/NIR ratio of Grains 1–3 were noted to decrease from grain core to rim, either indicative of mixing between the green-blue phosphates with surrounding reddish matrix, and/or possible zoning between green cores and redder rims within the phosphates themselves. Grain 4 is again unique, with very similar G/NIR ratios from core to rim, and a very slight increase in G/NIR from core to rim – opposite to the trend observed in Grains 1–3. The relatively low G/NIR signature from the core of Grain 4 could be due to the presence of a redder Fe-phosphate phase in the core vs rim, or insufficient abrasion as this grain is slightly recessed relative to the other grains within the abrasion patch.

In summary, based on the reflectance ratios of individual pixels within each grain (Fig. 3D), the Fe-phosphates reflectance spectra in Ouzel_Falls likely represent mixtures of relatively ferrous phosphates and more oxidized phosphates with higher NIR reflectance, and/or mixtures with the high NIR matrix that surrounds each grain at Ouzel_Falls. However, the significantly higher NIR reflectance in Grain 4 in comparison to Grains 1–3 could indicate it is a separate, more oxidized phase (Fig. 3D). The decrease in G/NIR and UV/NIR as oxidation increases in Fe-phosphates has also been observed in experimentally oxidized synthetic vivianite30.

Bright regions in the G/NIR images are also associated with vitreous olivine grains, and as such, estimating the volume of Fe-phosphates across the abrasion patch using MCC imaging is ambiguous. If the PIXL scan areas are assumed to be representative of the entire rock, Onahu contains ~1 vol% Fe-phosphates (78 PMCs > 10 wt% P2O5 / 6495 PMCs total).

Crystal Structure
As terrestrial reflectance data from Fe-phosphates are limited, further analysis is needed to narrow down the most likely Fe-phosphate phase(s) at Ouzel_Falls. Here we use X-ray diffraction peaks detected by PIXL to assess the Fe-phosphates’ crystallinity and compare their respective diffraction patterns to terrestrial Fe-phosphates. X-ray diffraction peaks in the Ouzel_Falls Fe-P hotspots indicate that the Fe-phosphates are predominantly crystalline, i.e., with grain sizes larger than >30–50 μm in the region with diffraction peaks (Fig. 4, and Supplementary Information). Diffraction peaks were also detected from the olivine and carbonate. The clays and high-Si matrix regions either do not diffract or diffract poorly, indicating they are amorphous or micro-crystalline with grain sizes <30–50 μm31.

From the 19 Fe-phosphate minerals with P/Fe ratios ~2/3 (Supplementary Tables S5, S7), 10 were selected for diffraction pattern comparison with the Ouzel_Falls Fe-phosphates in order to identify the most likely phase(s). The 10 phosphates were selected either based on their similarity to the Ouzel_Falls phosphates in color and reflectance, their prevalence on Earth and Mars, and/or as compositional end-members for completeness, and include:

• Vivianite [Fe2+3(PO4)2·8H2O] — often blue/green in color with similar reflectance band ratios to some of the Ouzel_Falls Fe-phosphates, the most common Fe-phosphate in sedimentary environments on Earth17.
• Metavivianite [Fe2+Fe3+2(PO4)2(OH)2·6H2O] — often dark blue/green to black in color with similar reflectance band ratios to some of the Ouzel_Falls Fe-phosphates, and an oxidation product of vivianite30.
• Beraunite [Fe3+6(PO4)4O(OH)4·6H2O] — often red to red-brown, a ferric end-member for comparison.
• Ferroberaunite [Fe2+Fe3+5(PO4)4(OH)5·6H2O] — often green to brown or gray, and a more reduced form of beraunite.
• Strengite [Fe3+PO4·2H2O] — occurs in a variety of colors, compositionally unlikely (i.e., molar P/Fe ratio of 1:1), but reflectance is relatively similar to some of the Ouzel_Falls Fe-phosphates, and previously inferred to occur on Mars in the Paso Robles soils, Gusev Crater11.
• Rockbridgeite [(Fe2+0.5Fe3+0.5)Fe3+4(PO4)3(OH)5] — often blue/green in color, and an Fe-rich end-member.
• Sarcopside [Fe2+3(PO4)2] — occurs in a variety of colors, and an anhydrous end-member for comparison.
• Ferrolaueite [Fe2+Fe3+2(PO4)2(OH)2·8H2O] — typically brown to orange-brown, but its Mn-rich end-member laueite [Mn2+Fe3+2(PO4)2(OH)2·8H2O] was recently inferred to be present on Mars as an oxidation product of Mn-rich vivianite at the Groken Site, Gale Crater10. Laueite was also included in the diffraction analysis for completeness.
• Ferrostrunzite and Ferristrunzite [Fe3+Fe2+(PO4)2(OH)2·6H2O and Fe3+Fe2+(PO4)2(OH)3·5H2O] — typically yellow to light-brown, but their Mn-rich end-member strunzite [Mn2+Fe3+2(PO4)2(OH)2·6H2O] was also recently inferred to be on Mars with laueite as an oxidation product of Mn-rich vivianite at the Groken Site, Gale Crater10. Ferristrunzite has also been inferred in P-rich Paso Robles soil, Gusev Crater11. Strunzite was also included in the diffraction analysis for completeness and due to its similar reflectance band ratios to some of the Ouzel_Falls Fe-phosphates.

Out of the 12 Fe-phosphates listed above (also including laueite and strunzite), the intensity and position of the detected X-ray diffraction peaks are most consistent with metavivianite, beraunite, ferroberaunite, and/or ferrolaueite (Fig. 4). The results from diffraction analysis are consistent with reflectance spectra for each grain as those with the highest relative NIR reflectance (i.e., Grain 4 Fig. 4D) are identified as Fe3+-rich beraunite, while grains with relatively low NIR reflectance (and higher G/NIR ratios) are identified as mixed valence metavivianite and ferrolaueite (Grains 1–3, Fig. 4E, F).

Fe-Phosphate Identification
Chemical, reflectance, and diffraction analysis of the Fe-phosphates in Ouzel_Falls by PIXL indicate that the secondary phosphate minerals metavivianite, (ferro)beraunite, and/or ferrolaueite are likely present. This is the first time these rare Fe-phosphate minerals have been identified on Mars.

These minerals (as well as others of the 19 Fe-phosphate minerals with P/Fe molar ratios ~2/3) can also be differentiated based on their Fe-oxidation state and H2O contents. While PIXL is unable to directly measure these components, other instruments on the Perseverance Rover are sensitive to hydration and oxidation (SuperCam and SHERLOC). Unfortunately, these instruments did not target the Fe-phosphates in Ouzel_Falls.

Metavivianite, beraunite, and ferrolaueite can all form on Earth in association with the Fe-phosphate mineral vivianite30,32,33. Vivianite occurs as green-blue crystals and is the most common Fe-phosphate formed in sedimentary environments on Earth17. It is also found as a secondary phase in a variety of hydrothermal settings17. Metavivianite (triclinic) forms from vivianite (monoclinic) as Fe3+ increases, resulting in a degradation of crystal symmetry and darkening in color34. If Fe-oxidation continues, eventually the orange-brown amorphous pseudo-mineral santabarbaraite (Fe3+3(PO4)2(OH)3·5H2O) forms after metavivianite20,30. Reflectance spectra for santabarbaraite were not available for comparison at this time, however, we hypothesize that mixtures of dark green-blue to black metavivianite and orange-brown santabarbaraite and/or other Fe3+-phosphates could also produce the high NIR reflectance observed in the Ouzel_Falls phosphates (Fig. 3D).

Beraunite and ferrolaueite are much less common on Earth than vivianite, but both can occur associated with vivianite as Fe-oxidation products like metavivianite; or conversely be reduced to form vivianite32,33,35,36. Ferroberaunite is a relatively new mineral37, and slightly reduced form of beraunite that has been often misidentified as green beraunite in similar formation environments.

As all of these phases can occur together, it is possible that the Fe-phosphate assemblage at Ouzel_Falls comprises a mixture of (ferro)beraunite, metavivianite, ferrolaueite, santabarbaraite, vivianite, and/or other Fe-phosphate minerals that we are not able to differentiate at PIXL’s resolution, or where terrestrial analog data needed for comparison are limited. For example, no reflectance spectra for beraunite, ferroberaunite, or ferrolaueite were available for comparison at the time of this writing.

Fe-Phosphates Origin and Paleoenvironmental Conditions
In order to constrain the paleoenvironmental conditions that led to the formation of the Fe-phosphates in Ouzel_Falls, we first need to consider if they were incorporated into the Onahu conglomerate as detrital phases, or if they formed in place.

In either case, the Fe-phosphates described here are the first to be identified and sampled by Perseverance, indicating a unique formation environment (Ca-phosphates are common in Jezero crater9,38,39). Additionally, as metavivianite, ferrolaueite, (ferro)beraunite, and/or santabarbaraite are all known to form after oxidation of vivianite on Earth, it is assumed that they first formed as vivianite in a more reducing environment before being exposed to oxidizing conditions either within the Onahu conglomerate itself, or within the Jezero paleolake watershed.

The presence of metavivianite, ferrolaueite, (ferro)beraunite, or santabarbaraite in Ouzel_Falls suggests that the original vivianite crystals must have also undergone oxidation in-place if authigenic. The formation of separate phosphate-carbonate-rich, and Si-sulfate-rich domains in the conglomerate matrix (Fig. 2C) indicates that diagenesis in Onahu occurred under evolving paleoenvironmental conditions and potentially different redox states. It is hypothesized here that the vivianite originally precipitated in a reducing Fe2+-rich fluid associated with the formation of the carbonate-rich matrix (based on their co-occurrence), and was then eventually oxidized either from alteration by infiltrating oxidizing Si-S-rich fluids that formed the Si- and sulfate-rich (oxidized S) matrix in the remaining pore space and/or from exposure to the oxidizing martian atmosphere (Fig. 5). Based on the persistence of carbonate, it is assumed that if a Si- and sulfate-rich fluid infiltrated into the Fe-phosphates-carbonate matrix, the fluid was not strongly acidic, and/or the water-rock interaction was short lived. Regardless, vivianite is also relatively insoluble in low pH environments (in comparison to Ca-phosphates)9.

The Ouzel_Falls abrasion patch is also the most P-enriched PIXL target analyzed within the western fan (Supplementary Fig. S17; Table S2), particularly in comparison to other carbonate-rich siliciclastic rocks, indicating that, if authigenic, the Fe-phosphates likely formed from a localized P-rich fluid.

While the textural occurrence of Fe-phosphates within the Fe-rich carbonate-bearing matrix, the presence of two discrete matrix domains, and the overall P-enrichment in Ouzel_Falls support a model where vivianite precipitated and oxidized in-place, we should also consider if the Fe-phosphates may have precipitated from a similar process in sediments upstream within the Jezero paleolake watershed and were concentrated in the carbonate-rich matrix in Onahu through sorting.

Detrital origin
Terrestrial Fe-phosphates are not common detrital phases, and to our knowledge, have never been identified. Vivianite and metavivianite are soft (Mohs hardness of 1.5-2, and 2.5, respectively17,49) and unlikely to persist during extended fluvial or eolian transport. Beraunite, ferroberaunite, and ferrolaueite are slightly harder, with Mohs hardness of 3–432,37,50.

A source of detrital Fe-phosphates in Onahu could be upstream sediments where vivianite formed in a similar process as described above. However, vivianite and its oxidation products are also known to occur on Earth as secondary minerals in altered P-rich pegmatites36,51,52 or in metal-rich orebodies (with either igneous or sedimentary protoliths) and skarn deposits50,53. At this time, there is no evidence of pegmatites, metal-rich orebodies, and/or skarn deposits in the Jezero watershed (or elsewhere on Mars), but the discovery of highly evolved Fe-rich basalts on the Jezero crater floor38,54 indicates that more extreme differentiated lithologies could exist in the watershed that we are not able to identify with remote analyses alone.

The Jezero watershed is also known to contain significantly altered rock based on the presence of smectite, carbonate, and incision of the altered basement and mottled terrain units by fluvial valleys5. Aqueous alteration models demonstrate it is possible for Fe-phosphates to form during weathering of Ca-phosphate-bearing ultramafic rocks55; as such, they could have originally formed in the Jezero watershed from this process.

Regardless of the inferred origin of the Fe-phosphates, based on our current knowledge of the Jezero watershed, and comparisons to terrestrial environments, the Onahu Fe-phosphates most likely are not igneous or hydrothermal in origin and formed in a P- and Fe-rich aqueous environment at relatively low temperatures either as precipitates in sediments or as alteration products of P-rich rocks. If detrital, oxidation of vivianite could have occurred before or after transport and incorporation into Onahu.

Implications for habitability and Mars sample return
The identification of secondary Fe-phosphate minerals (metavivianite, ferrolaueite, (ferro)beraunite, and/or santabarbaraite) after vivianite in Ouzel_Falls provides evidence that many of the materials and energy source(s) needed for habitable conditions may have existed in the Jezero fan top, and/or within the Jezero paleolake watershed. Vivianite and its oxidation products have been inferred on Mars before9–11, but this is the first time the Fe-phosphates noted here have been identified using correlative high-resolution chemical, structural, reflectance, and imaging analyses.

Based on comparison to natural systems on Earth, the vivianite in Ouzel_Falls most likely precipitated in conditions favorable to potential martian life within a low temperature, anoxic, reducing aqueous medium at approximately neutral pH17, with high concentrations of bio-limiting elements including carbon and phosphorus, and Fe-redox gradients that could provide an energy source. For example, dissimilatory reduction of ferric ions by bacteria when phosphorus is available, to the ferrous form in vivianite, can enable access to metabolic energy56,57. These conditions are most likely to have occurred when Mars was warmer and wetter, around the time of the Jezero fan formation (>3.2 Ga2–8).

Although vivianite is thought to have been important in the biogeochemical P cycle on early Earth43,58,59, it is largely absent from the ancient geologic record, possibly due to oxidation/dissolution driven by diagenesis and metamorphism58. In contrast, the results presented here, combined with the well-preserved nature of the western fan8 suggest the Onahu outcrop experienced minimal metamorphic alteration following the formation of the fan ca. 3.2 Ga, thus extending the preservational window of vivianite. However, while the available evidence supports a potentially ancient origin for these well-preserved Fe-phosphates, we cannot rule out their formation in another more recent event.

Many of the conditions needed for vivianite formation are also suitable for the preservation of chemical biosignatures, if present60. Terrestrial Fe-phosphates have been observed encrusting bacteria, and growing on/replacing organic detritus and the hydroxyapatite components of bones and fossils17. Furthermore, authigenic carbonate minerals like those that occur in the Ouzel_Falls matrix are particularly conducive to the preservation of chemical and morphological biosignatures, from organic molecules to microbial fossils61,62.

Perseverance can provide immense amounts of information on martian paleoenvironments through the identification of rare minerals like the Fe-phosphates described here when high-resolution proximity science instruments are deployed. Still, textural, compositional, and isotopic analyses of the returned sample (Otis_Peak) will be essential to constrain the ancient environmental conditions, chemical reactions, or even microbial activity that produced these unusual minerals.

Methods
PIXL Analyses
The PIXL instrument produces maps of elemental abundances on martian surface materials using X-ray Fluorescence (XRF). PIXL’s focused X-ray beam interacts with the surface in a ~120 μm diameter spot size for fluoresced X-rays of 8 keV, and larger diameters for lower energies21,31,63. PIXL rasters its focused X-ray beam across a predefined area of interest, and so produces analyses of multiple spots that are correlated with beam positions; from which the PIXLISE software package64 produces maps of elemental abundances (see21,31,38,39 for more details on PIXL’s operation). PIXL maps can include thousands of individual X-ray fluorescence spectra, each with unique positions recorded as PIXL Motion Counters (PMCs - integers tracking each time PIXL moves onto a new software activity that correspond to locations of each PIXL analysis in a scanned area).

PIXL obtained three XRF raster scans on the Ouzel_Falls abrasion patch (Sols 789, 790, and 811) at step sizes of 0.125 mm, for a total scan area of 97 mm2 (Fig. 1D). Energy-dispersive XRF spectra were acquired for each point for 10 s each (6495 PMCs total), with an additional 30 s of dwell time when PIXL’s adaptive sampling algorithm was triggered on rare phases of interest (such as phosphate minerals) in order to improve signal-to-noise ratios at these locations (123 PMCS, e.g.,65). PIXL’s micro context camera (MCC) also acquired images of the abrasion patch illuminated by ultraviolet (UV, 385 nm), blue (B, 450 nm), green (G, 530 nm) and near-infrared (735 nm) light emitting diodes (LEDs) with a spatial resolution of ~50 μm/pixel. The preflight calibration of the LED-MCC system allows multispectral analysis of these four wavelengths66,67. The XRF and MCC data were then analyzed using PIXLISE, allowing for detailed examination of elemental compositions and X-ray spectral characteristics correlated to context images. PIXLISE fully incorporates instrument calibration of optics, geometry, XRF quantification as well as the in-situ conditions of topographic effects and positional drift caused by diurnal thermal environment, enabling accurate correlation between XRF spectra and context images to within 50 microns68.

Mineral mixing modeling
PIXL’s analytical spot size varies between elements because lighter elements fluoresce at lower energies63. The beam diameter varies inversely with energy, meaning higher energy X-rays are focused to a smaller diameter. High-energy X-rays excite more high-energy fluorescence, so the relationship between PIXL’s analytical spot size and X-ray energy results in a larger analytical volume for lighter atoms and a correspondingly inflated measured distribution. While these effects are well-characterized and accounted for in a homogenous sample, they become more complex at grain boundaries. Mixing relationships between neighboring mineral grains appear nonlinear, complicating efforts to determine the “unmixed” end-member compositions of mineral phases that are smaller than, or close to, PIXL’s spot size (including the Fe-phosphates of interest here). Thus, to constrain the composition of Fe-phosphates, we modeled compositional mixing between the main minerals in Ouzel_Falls and various Fe-phosphate endmembers, taking the energy-dependent variation in spot size for different elements into account. See Supplementary Fig. S3 and Supplementary Information.

Crystallographic mapping and diffraction phase matching
PIXL has two detectors, and so can separate anisotropic X-ray diffraction peaks occurring from crystalline materials from isotropic X-ray fluorescence peaks when energies of PIXL’s incident radiation happen to be in a diffracting condition with d-spacings and orientation of crystal lattices in the target materials69. Diffraction, as well as surface roughness, can result in misestimation of elemental concentrations, as such, all XRF compositions reported here have been corrected for these effects using the method outlined in ref. 38.

When detected, X-ray diffraction can also be used to assess crystallinity, grain size, and grain texture of a target31,38,39 and to distinguish among minerals with similar elemental compositions70. Here, diffraction patterns from the Fe-P-rich areas of the Ouzel_Falls abrasion patch were compared to modeled diffraction patterns for several candidate Fe-phosphate minerals to identify the most likely phases (see Supplementary Information).

Sampling
The Perseverance rover attempted twice to collect a sample from the Onahu outcrop near and on the Ouzel_Falls abrasion patch, but the rock was too friable to acquire an intact core. A final sampling attempt was carried out ~30 meters NNW of the Onahu outcrop in a workspace informally named Emerald_Lake, in the same stratigraphic layer as Onahu. The Otis_Peak sample core was successfully acquired from this location.

As PIXL was not used to analyze the Emerald_Lake workspace, data from the SuperCam, WATSON, and CacheCam instruments on Perseverance are used here to confirm its similarity to Onahu (see Supplementary Information, Table S8 and Figures S18, S19).

Data availability
As per guidelines of the NASA Mars 2020 mission, all data underlying the study are available in the public domain, and specifically reside in the Planetary Data System (PDS) Geosciences Node. Compositional and spectral data from PIXL used in this study are available in the PDS Geosciences Node at https://doi.org/10.17189/152264571. The processed chemical PIXL data are available in the Supplementary Information. SuperCam chemical data is available in the PDS Geosciences Node at https://doi.org/10.17189/152264672. ACI and WATSON images can be found in the PDS Geosciences Node at https://planetarydata.jpl.nasa.gov/img/data/mars2020/mars2020_imgops/data_aci_imgops/73 and https://planetarydata.jpl.nasa.gov/img/data/mars2020/mars2020_imgops/data_watson_imgops/74, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General review:\nThe identification of Fe-phosphates in Jezero crater is a novel and very interesting discovery. The paper drills into this\nfinding to be specific about the mineralogy of the Fe-phosphates. There are minimal XRD data or reflectance data to make a\nstrong case of the specific minerals identified. So this review offers some suggestions for presenting a clearer and perhaps\nstronger case for ferrolaueite and/or santabarbaraite (in addition to the beraunite, metavivianite identifications). I find the\nXRD data and comparison to beraunite and metavivianite peaks the most compelling and recommend showing a similar\ngraph for the ferrolaueite spot with assigned ferrolaueite peaks. Perhaps this Figure S9 (with an added ferrolaueite\nexample) should find its way into the main paper (rather than supplemental info).\nGenerally, the findings in this paper are significant and novel, but the manuscript needs another going-over for\nimprovements and clarity prior to publication.\nSpecific comments:\nAbstract: The abstract weighs heavily on the likelihood of possible habitable environments on Mars and potential links to\nmicrobial activity, subjects that aren’t the main point of the paper, which is identifying Fe-phosphates in Jezero (and those\nbeing the first strong evidence of specific Fe-phosphates). Perhaps the authors might want to emphasize the main findings\npresented in the paper (Fe-phosphates and their formation in neutral pH, low temperature fluids with subsequent\noxidation…) and de-emphasize the links to possible habitable environments on Mars (we don’t know much more about this\nuntil the samples are analyzed back on Earth). To shift the focus a bit, consider changing Line 55 to remove the bit about\nmicrobial activity and organics and replace it with the inference of neutral pH and low temperature fluids. Just a suggestion.\nLine 51: change “have been previously inferred” to “have been inferred previously”\nLine 54: Perhaps remove “the” to read “…oxidation of Fe2+-phosphate vivianite”\nLines 111, 115: remove the periods after “fm.” except when at the end of a sentence\nLine 114: remove the hyphen in “poorly-sorted”\nLine 130: remove the s in “clays”\nLine 142: Add a period to the end of the sentence\nFigure 2G: Although the various Fe-phosphates were identified using diffraction phase mapping (at least beraunite and\nmetavivianite shown in Figure S9), it seems more likely that due to the similarity of color and proximity of grains there would\nlikely only be one Fe-phosphate mineral present, as opposed to the 3 different ones listed in Figure 2G. In addition, the\ngreen-blue color to my eye looks similar for the different minerals indicated, yet beraunite and ferrolaueite are not green-blue\nminerals on Earth (as also mentioned in Table S4, Table S5, and later in the paper in Lines 237-261). Please address this\nconcern of specificity. Can the data really discriminate 3 different green-blue Fe-phosphate minerals? What are the other\nfeatures in Figure S9A and S9C and can a single Fe-phosphate be isolated and identified in each panel? That being said,\nthe top left oval in Figure 2G identifies beraunite and metavivianite, yet Figure S9E and S9F show the same oval but only list\nberaunite (not metavivianite). Be consistent and list the identifiable Fe-phosphates. I understand that the star in that oval is\nberaunite (and I only know the full oval contains another b/c of Figure 2G) but the Figure S9F oval labeled “beraunite” is then\nmisleading.\nPaper has used P:Fe (Lines 169, 171, 275, 316, etc.) and P/Fe (Lines 160, 166, 180, 246, Supp., etc.). Choose one\napproach and use it consistently (including in the Supplemental files) or clarify why both P:Fe and P/Fe are used.\nLine 171: Perhaps add “terrestrial” to read “19 known terrestrial Fe-phosphate minerals….”\nLines 173-175: I’m curious about possible presence of Fe-sulfates. There was little discussion about this possibility. The\npaper stated that most sulfur was “ascribed to Mg-sulfate minerals.” Paper also said “excess SO3 is likely from alteration that\nhas not been properly accounted for (Table S2)”. Can any more be said to address the presence/absence of Fe-sulfates (as\nthey would possibly be indicative of more acidic water than the paper ascribes for Fe-phosphates)?\nLine 184: The word “and” needs to come out of “green and wavelengths”.\nLine 190 and Figure 3 caption: Change “Re-Lab” to “RELAB”. Also the RELAB files listed and shown in Figure 3D are\nsamples originally supplied by Dr. Janice Bishop of SETI to get measured at RELAB. You may want to reach out to her to\nfind out if the sample names were correct b/c she and I and Dr. Darby Dyar were working on a suite of phosphates some of\nwhich got measured at RELAB and subsequently were XRD’ed to assure composition. Some of our samples were originally\nmisidentified and the old names might be associated with the RELAB database. Janice used a different numbering scheme\nfor some of my or Darby’s samples, so I’m not sure which ones these are but we could double-check their identification.\nFigure 3: Why is only the “Grain 1” spectrum shown in Figure 3C? If these Fe-phosphates are able to be discriminated via\nXRD, I would presume the VNIR spectra would be slightly different from each other. I’d like to see all of the Fe-phosphate\ngrain spectra shown in Fig. 3C (not just Grain 1) – if not all the Fe-phosphate ovals, then at least add Grain 2 and Grain 3\nspectra. How different are they? Do any of them actually look like the terrestrial phosphate reflectance spectra? This\ndisconnect is a little problematic for me.\nLines 191-192 and Figure 3: I don’t really see the similarity of the reflectance spectrum in Figure 3C with any of the spectra in\nFigure 3D. And the “Grain 1” spot on Figure 3A looks to be identified as ferrolaueite on Figure 2G, but a lab spectrum of\nferrolaueite isn’t shown in Figure 3D for comparison. I think that needs to be discussed or clarified more. It would be good to\nhave tick marks on the axes as well (for all the graphs).\nLines 198-199: “Most similar to mixtures of relatively ferrous phosphates…and … (like beraunite)….” I don’t see that really,\nthe metavivianite and vivianite reflectance spectra show a positive feature around 450 nm but the Grain 1 spectrum shows a\ndip. Please elaborate – maybe state up front that this is related to the G/NIR v UV/NIR plots not the specific reflectance\nspectra themselves, or something like that.\nLine 223: The authors mention that the reflectance data for Fe-phosphates is limited (which is still true); however, some have\nbeen published (albeit at limited spectral ranges) and could be referenced. I’m first author on 2 of them, so a reference is\nabsolutely not required (and they are abstracts), but I want to make the authors aware of these. For example:\nChapter 4, Figure 4.5d in Bishop (2019) Visible and Near-infrared reflectance spectroscopy: Laboratory spectra of geologic\nmaterials, in Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry\nof Planetary Surfaces (J. L. Bishop, J. F. Bell III, and J. E. Moersch, editors). Cambridge University Press, Cambridge, UK,\nDOI: 10.1017/9781316888872\nLane M.D., Bishop J.L., Dyar M.D., Parente M., King P.L. & Hyde B.C. (2007) The ferric sulfate and ferric phosphate minerals\nin the light-toned Paso Robles rover track soils: A multi-instrument analysis. 7th Int'l Mars Conf., Abstract #3331.\nLane M.D., Mertzman S.A., Dyar M.D. & Bishop J.L. (2011) Phosphate minerals measured in the visible-near infrared and\nthermal infrared: Spectra and XRD analyses. 42nd Lunar Planetary Science Conf., Abstract #1013.\nLines 228 and 265: Why don’t you show the XRD spectra of the ferrolaueite sample in Figure S9 and label it on Figure S9F\nlike it is on Figure 2G? That seems important so reader can verify the findings and b/c it is stated as one of the 3 Fe-P\nminerals identified at Ouzel_Falls and is “the first time these rare Fe-phosphate minerals have been identified on Mars.” But I\nhaven’t really seen the XRD or spectral evidence for ferrolaueite presented in the manuscript.\nLine 285: I also don’t know of a santabarbaraite VNIR spectrum in the literature. What about a ferrolaueite spectrum? State\nthat it, too, is unavailable in the literature or show it on Figure 3D.\nLine 292: perhaps change “co-occur” to “occur together”\nLine 295: change “is limited” to “are limited”\nLine 358: Perhaps add a reference for the hardness of vivianite and metavivianite\nLine 467: Change [data] “is used” to [data] “are used”", "reviewer_2": "The paper brings the result of the investigation of a phosphate rich area in Jezero Crater, on Mars, by the Mars 2020\nPerseverance rover. The rover collected mineralogical and chemical data of a conglomerate outcrop, informally named\nOnahu. Although Ca-phosphates are the main phosphates recognized on Mars, the minerals present in the outcrop were\nshown to be the first directly observed Fe-phosphates on the planet. The authors were able to study and determinate, with\nhigh confidence, that the grains include minerals link beraunite, ferrolaueite and metavivianite, while other mineral phases\nwere proposed but not confirmed. The origin scenarios of such minerals were proposed based on the knowledge of analog\nenvironments in which they form on Earth, listing the most and least likely hypotheses. The authigenic hypothesis talks\nabout the formation of the Fe-phosphates in aqueous environments (on Earth, a process that may involve life) and later\noxidized in-place. Meanwhile, the detrital hypothesis would involve the movement of grains from upstream outcrops,\nalthough there is no evidence of such rocks that could provide the grains of the Fe-phosphates. To conclude, the authors\ndiscuss the implications of this discovery for the habitability and possibility of life as we know, being the minerals a source of\nphosphorus and energy, and capable of preserving biogenic signatures.\nPhosphorus is an important element for life as we know it, and understanding of its bioavailability is crucial for a better\ninterpretation of the ancient habitability of Mars. During the exploration of the planet, it had being demonstrated the presence\nof P-rich areas and, while it was proposed the existence of Fe-phosphates on those areas, no direct study had identified\nwhich mineral phases were present. Although there are limitations in the techniques that can be coupled on rovers to\nexplore Mars, the study was able to obtain highly reliable results that confirmed suspicions of the presence of Fe-\nphosphates and evaluate the mineral phases using classical techniques like X-ray diffraction and fluorescence. It was a pity that\ninstruments and analyses, such as RAMAN spectroscopy, could not target the grains. However, hopefully, even more\nstudies may be performed in a sample collected in the vicinities of the outcrop. All described processes associated with Fe-\nphosphate (and other mineral phases from the outcrop) are known from abiotic environments. The bioavailability of\nphosphate is important for life to thrive, whoever, the presence of Fe-phosphates also has a great repercussion for the\npossibility of chemolithoautotrophic life. Processes of biological energetic gain from Fe-redox reactions associated to Fe-\nphosphates are known, like the mentioned biologically induced reduction of ferric ions and formation of vivianite. However,\nthere are also other processes like the biologically induced oxidation of Fe-phosphates that, together with the environmental\nabiotically induced oxidation, could generate minerals (biosignatures) with Fe in a more oxidized state and with\nastrobiological interest.\nDue to the quality of the text, the novelty of the data and the important consequences of the results, the\nrecommendation of this paper is for publication with minor reviews.\nFind, below, specific comments and some points that can be improved.\n1. Line 99, 105, 107 (figure 1) and 151 (figure 2): It is important to acknowledge the source and exact reference of the images\nused in the figure captions. However, as these texts are already long, perhaps the original name of the image (e.g.\n(SI1_0789_0736995354_613FDR_N0390926SRLC000 36_000095J01.IMG) could be referenced elsewhere in the paper or\nin supplementary materials.\n2. Line 175: This paragraph discusses the presence of sulfate-rich phases, focusing on the identification of Ca-sulfate and\nMg-sulfate (more detailed data are presented in Supplementary Figure S6). In many cases on Mars, the formation of ferric\nsulfate minerals (like jarosite) is associated with low pH aqueous environments. Was the unequivocal presence of iron-rich\nsulfates associated with this outcrop observed?\n3. Line 209 (Figure 3C): The relative reflectance spectra for the Grain 1 is shown in the figure, but is not specified if it is a\nsingle pixel or the average for the grain. As a suggestion, data from Grains 2 and 3 could be also included in the graph for\ncomparison.\n4. Line 222: Due to the importance of the X-ray diffraction for the identification of the Fe-phosphates, the authors may\nconsider that the diffractograms could be included in the main paper, like the Figure S9 from thee Supplementary Figures.\n5. Line 226-228: The Fe-phosphates are described as “predominantly crystalline, i.e., with grain sizes larger than >30-50 μm\nin the region with diffraction peaks”. Although not stated the presence of amorphous or micro-crystalline phases (beyond\nclay), this may corroborate the hypothesis of presence of the amorphous santabarbaraite (well discussed later between lines\n284-288). While the presence of beraunite (fully oxidized ferric phosphate) was confirmed, more evidence of the presence of\nsantabarbaraite (more closed related to vivianite/metavivianite) would be very interesting. In lines 285-286, the authors says\nthat the “reflectance spectra for santabarbaraite were not available for comparison at this time”. Santabarbarite is not a\ncommon mineral, but do not seem hard to be acquired or borrowed to obtain its reflectance spectrum. In the absence of the\nspectrum of santabarbaraite in the database used, did the authors consider obtaining the spectrum of the mineral\nthemselves? Could the reflectance data of santabarbaraite help to further corroborate the hypothesis of the presence of this\nphase in the outcrop?\n6. Line 341-344: The possibility of biogenic action in the ferrous phosphate formation is well stated (lines 320-322). However\nthe possibility of biological oxidation of iron/reduced Fe-phosphates, like vivianite/metavivianite, in presence of sulfate was\nnot explored. This could expand the possibilities for discussing the habitability of this Martian environment.\n7. Lines 342-343: The authors stated that “then eventually oxidized either from alteration by infiltrating oxidizing Si-S-rich\nfluids that formed the Si- and sulfate-rich (oxidized SO42-) matrix”. The terminology oxidized SO42- is confusing since there\nis no oxidized and reduced sulfate, but oxidized and reduced form of sulfur. I recommend to change to (oxidized S).\n8. Line 383, Figure 4: In the second image (“Fluid Infiltration”), please correct the phosphate formula (PO4 to PO43-). As a\nsuggestion to this picture, the yellow used to Sulfate (last picture) is very difficult to see. Please consider changing the color\nif it is possible to.\n9. Line 396 (and in figure 4): For chemical accuracy, the terminology “~ neutral pH” is not the most adequate, since pH is just\na number; the medium can be acid, alkaline or neutral. So, the correct forms are “neutral medium” or “pH around 7”. I suggest\nthe second form, avoiding the use of “~” that is more informal.\n10. Line 396: The terminology “reducing water” is also non adequate. Please prefer “reducing aqueous medium with pH\naround 7 and high concentrations of bio-limiting elements…”\n11. Line 387 Topic “Implications for Habitability and Mars Sample Return”. It is not clear for me the reasons for the authors to\nmention “Mars sample return” in this topic since they vaguely mention the topic in lines 409-410. The authors should\nconsider changing the topic name to “Implications for Martian habitability and biosignature detection”.\n12. Supplementary material table S5. Ferroberaunite. Just correct the molecular formula of water, in which 2 is not\nsubscripted. Also, Ferristrunzite. Add “2+” in Fe in the molecular structure.\n13. Supplementary material table S6, 5th line. The reference to “FeOT” as a major oxide is correct or is it a typing mistake?", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_4": "SUMMARY:\nKizovski et al 2024 present evidence gathered by the Perseverance rover for Fe-phosphate minerals on the surface of Mars.\nThis evidence includes textural, chemical, spectral, and diffraction analyses of grains discovered in a conglomerate outcrop\nnamed “Onahu”. The evidence for the presence of Fe-phosphates in general is compelling, representing the most definitive\nevidence for their presence on Mars presented thus far. The study also interrogates the specific minerals that occur in this\nlocation and begins to build a paragenetic model of their occurrence. Data is assessed to be most consistent with the\nminerals beraunite, metavivianite, and ferrolaueite. The fact that these minerals occur within a conglomerate made up\nlargely of ~basalt is unusual, but the key to their occurrence appears to be that this is a highly alternated conglomerate that\ncontains carbonate-rich matrix material. Indeed, the Fe-phosphates are apparently restricted to the matrix. Both authigenic\nand a detrital models of origin for the minerals are discussed, followed by a brief discussion of the implications of the\nfindings for habitability. Overall, the paper makes a compelling contribution to the literature. I have some questions and\ncomments about the proposed paragenetic models and the discussion about habitability. On this basis, I recommend minor\nrevisions for the paper.\nCOMMENTS:\nIs the origin of the minerals authigenic or detrital?\nI did not fully understand the arguments presented regarding an authigenic or detrital origin of the Fe-phosphate minerals.\nThe authors seem to provide really quite compelling evidence against a detrital origin. In particular, the minerals are:\n1) Only found in the carbonate-rich alteration matrix.\n2) Soft minerals that have never been observed to survive the conditions of transport required to form a conglomerate.\n3) Form on Earth either in surficial weathering zones or in oxidised hydrothermal systems in granitic pegmatites.\nAs far as I can tell, these points would seem to render a detrital origin extremely unlikely.\n1) The alteration matrix formed in place after the conglomerate had already lithified, as indicated in the authors’ Fig. 4. A\ndetrital origin would presumably require a different texture i.e., where the minerals are no longer present in what looks pretty\nclearly to be a post-deposition texture.\n2) Minerals that are both very soft and low abundance are hard to preserve as detrital phases. Physical\nfeasibility of detrital secondary late paragenetic Fe-phosphates seems tricky.\n3) Perseverance is sampling only surficial weathering zones. This lines up nicely with these minerals being weathering\nproducts rather than detrital having formed in some earlier alteration phase of the material that was then transported to form\nthe conglomerate.\nAll of this is simply to say that, whilst the paper does not give a definitive answer either way and presents a detrital origin as\nbasically equally plausible to an authigenic one, I do not think the evidence seems to support that level of ambiguity. It would\nprobably be better if the discussion came down more clearly on the side of authigenesis, as per Fig. 4, having listed the\nissues with a competing detrital hypothesis.\nWhen did the minerals form?\nSomething else that was unclear to me is when in the life of Mars the authors imagine the minerals may have formed? I\nknow we cannot give any exact dates but could the authors contextualise the sequence of events in the history of Mars?\nMars is strongly oxidised today and ceased to have widespread aqueous activity a while ago. Is Onahu telling us something\nabout the changing surficial conditions, i.e., from reducing to oxidising to dry?\nI think this would be useful to comment on. The authors hint at it with “evolving paleoenvironmental conditions” but do not go\non to say if this fits with emerging evidence for the environmental evolutionary timeline of Mars and the crater in general.\nI am also wondering about the timing because the Onahu conglomerate is apparently >3.2 billion years old. I am a tad\nsceptical that these secondary late paragenesis Fe-phosphates could be 3.2 billion years old, given how recent and\nephemeral the same minerals are when found in weathering zones on Earth. Then again, Mars is far less active than Earth\nso perhaps the same rules do not apply. Could the authors add a rough timeline of events to their figure 4 along with a\ncorresponding broader environmental history of the crater and of Mars as a whole?\n- Prebiotic chemistry and habitability\nThe implications of identifying Fe-phosphates for habitability and prebiotic chemistry on Mars are definitely substantial.\nHowever, I think the paper could go a little further in specifying what these implications are. Mention is mainly made of the\nfact that the conditions were probably habitable. However, this seems quite unsurprising. What is more exciting from my\nperspective is that vivianite is an important mineral in the early P cycling literature. Therefore, we should be able to put some\nnumbers on P availability to early life and to prebiotic chemistry. Could the authors double check the literature and report\nlikely phosphate concentrations in the altering fluids that are consistent with vivianite precipitation? And again, echoing the\ncomments above, the timing of all this is important. Can we learn from these minerals about P availability 3+ billion years\nago, or only somewhat more recently?"}, "author_response": "Response to Reviews for Nature Communications manuscript NCOMMS-24-65274\nFe-phosphates in Jezero Crater: Chemical, Structural, and Spectral Evidence for an\nAncient Habitable Environment on Mars\nT.V. Kizovski et al.\nWe thank all reviewers for their thorough and extremely helpful comments that have greatly\nstrengthened the paper and helped us present our Fe-phosphate identifications more clearly.\nResponses to each comment are provided in blue below.\nReviewer #1 (Remarks to the Author):\nGeneral review:\nThe identification of Fe-phosphates in Jezero crater is a novel and very interesting discovery. The paper\ndrills into this finding to be specific about the mineralogy of the Fe-phosphates. There are minimal XRD\ndata or reflectance data to make a strong case of the specific minerals identified. So this review offers\nsome suggestions for presenting a clearer and perhaps stronger case for ferrolaueite and/or santabarbaraite\n(in addition to the beraunite, metavivianite identifications). I find the XRD data and comparison to\nberaunite and metavivianite peaks the most compelling and recommend showing a similar graph for the\nferrolaueite spot with assigned ferrolaueite peaks. Perhaps this Figure S9 (with an added ferrolaueite\nexample) should find its way into the main paper (rather than supplemental info).\nGenerally, the findings in this paper are significant and novel, but the manuscript needs another going-\nover for improvements and clarity prior to publication.\nSpecific comments:\nAbstract: The abstract weighs heavily on the likelihood of possible habitable environments on Mars and\npotential links to microbial activity, subjects that aren’t the main point of the paper, which is identifying\nFe-phosphates in Jezero (and those being the first strong evidence of specific Fe-phosphates). Perhaps the\nauthors might want to emphasize the main findings presented in the paper (Fe-phosphates and their\nformation in neutral pH, low temperature fluids with subsequent oxidation…) and de-emphasize the links\nto possible habitable environments on Mars (we don’t know much more about this until the samples are\nanalyzed back on Earth). To shift the focus a bit, consider changing Line 55 to remove the bit about\nmicrobial activity and organics and replace it with the inference of neutral pH and low temperature fluids.\nJust a suggestion.\nThank you for the suggestion, and we agree the first identification of Fe-phosphates in Jezero and the\nancient environmental conditions they formed in are the main findings of this paper. However, we also\nfeel that it's important to state potential ties between Fe-phosphates and potential microbial activity in the\nabstract to emphasize their importance upfront to readers of all disciplines. We have however updated the\nabstract so that we now also focus on the formation conditions of the Fe-phosphates as suggested (see\nbelow).\n“Phosphorus is an essential component for life, and in-situ identification of phosphate minerals that\nformed in aqueous conditions directly contributes toward one of the main goals of the Mars 2020\nPerseverance rover: to seek signs of ancient habitable environments. In Jezero crater, proximity\nscience analyses within a conglomerate outcrop, “Onahu” demonstrate the presence of rare Fe3+-\nbearing phosphate minerals (likely metavivianite, ferrolaueite, (ferro)beraunite, and/or\nsantabarbaraite) embedded in a carbonate-rich matrix. While Fe-phosphates have been inferred\npreviously on Mars, this work presents the most definitive in-situ identification of martian Fe-\nphosphate minerals to date, using textural, chemical, spectral, and diffraction analyses of discrete\ngreen-blue grains. The Fe-phosphate minerals’ textural context along with comparisons to Earth\nanalogs suggest they likely formed after oxidation of Fe2+-phosphate vivianite, the most common\nFe-phosphate in sedimentary environments on Earth, often associated with microbial activity and\norganics. While there is no obvious evidence of biological inputs in Onahu, if the Fe-phosphates’\nformation environment was similar to vivianite-rich sedimentary environments on Earth, these\nminerals likely originally precipitated in conditions favorable to potential martian life — in a low\ntemperature, reducing aqueous medium with high concentrations of bio-limiting elements, and Fe-\nredox gradients that could provide an energy source. If the sample collected from Onahu\n(Otis_Peak) is returned to Earth, analysis of the Fe-phosphates will provide new insights into\nancient habitable environments on Mars.”\nLine 51: change “have been previously inferred” to “have been inferred previously”\nAgreed and changed.\nLine 54: Perhaps remove “the” to read “…oxidation of Fe2+-phosphate vivianite”\nAgreed and changed.\nLines 111, 115: remove the periods after “fm.” except when at the end of a sentence\nAgreed and changed.\nLine 114: remove the hyphen in “poorly-sorted”\nAgreed and changed.\nLine 130: remove the s in “clays”\nAgreed and changed.\nLine 142: Add a period to the end of the sentence\nThank you for catching this, this has been updated.\nFigure 2G: Although the various Fe-phosphates were identified using diffraction phase mapping (at least\nberaunite and metavivianite shown in Figure S9), it seems more likely that due to the similarity of color\nand proximity of grains there would likely only be one Fe-phosphate mineral present, as opposed to the 3\ndifferent ones listed in Figure 2G. In addition, the green-blue color to my eye looks similar for the\ndifferent minerals indicated, yet beraunite and ferrolaueite are not green-blue minerals on Earth (as also\nmentioned in Table S4, Table S5, and later in the paper in Lines 237-261). Please address this concern of\nspecificity. Can the data really discriminate 3 different green-blue Fe-phosphate minerals? What are the\nother features in Figure S9A and S9C and can a single Fe-phosphate be isolated and identified in each\npanel? That being said, the top left oval in Figure 2G identifies beraunite and metavivianite, yet Figure\nS9E and S9F show the same oval but only list beraunite (not metavivianite). Be consistent and list the\nidentifiable Fe-phosphates. I understand that the star in that oval is beraunite (and I only know the full\noval contains another b/c of Figure 2G) but the Figure S9F oval labeled “beraunite” is then misleading.\nCan the data really discriminate 3 different green-blue Fe-phosphate minerals? The analysis we present\nindicates which mineral (if any) is most likely, but we cannot make definitive claims without additional\nanalysis not possible remotely. However, we have only identified regions (and individual PMCs are now\nalso included in the figure) with a p-value of 0.1 or less when compared to the other candidates (i.e., we\nare at least 90% confident in the selection based on the available data). Other PMCs where the confidence\nis lower have been left unidentified. This could be due to many reasons such as overprinting, beam\nmixing, grain fracturing, deformations, etc. At the request of Reviewer #2, Figure S9 has been moved to\nthe main text. This new figure includes both region identification (as previously) and identification of\nindividual PMCs, including all the PMCs that can be confidently identified and the locations of those that\ncannot be identified. Region identification is then based on the analysis of individual PMCs instead of just\nfocused on the region as previously. Due to this additional layer of analysis, one of the regions is now\nambiguous and could be either beraunite or ferroberaunite, or a mixture of both. We believe this analysis\nand the new figure addresses these concerns.\nWhat are the other features in Figure S9A and S9C? The spectra in the original Fig S9A and S9C contain\nmany peaks. Most of the peaks that appear equally in both detectors are characteristic X-ray fluorescence\npeaks. Also visible are the scattered Rh L-lines from the source at ca. 2.7keV. Peaks that appear in one\ndetector and not the other detector over a short energy range (not an offset over the entire energy range)\nare most likely due to crystalline diffraction due to its anisotropic nature. The updated figure identifies\nthese peaks with arrows to make this clearer to the reader.\nPaper has used P:Fe (Lines 169, 171, 275, 316, etc.) and P/Fe (Lines 160, 166, 180, 246, Supp., etc.).\nChoose one approach and use it consistently (including in the Supplemental files) or clarify why both\nP:Fe and P/Fe are used.\nThank you for catching this, we have now changed it so all P,Fe ratios are P/Fe.\nLine 171: Perhaps add “terrestrial” to read “19 known terrestrial Fe-phosphate minerals….”\nAgreed and changed.\nLines 173-175: I’m curious about possible presence of Fe-sulfates. There was little discussion about this\npossibility. The paper stated that most sulfur was “ascribed to Mg-sulfate minerals.” Paper also said\n“excess SO3 is likely from alteration that has not been properly accounted for (Table S2)”. Can any more\nbe said to address the presence/absence of Fe-sulfates (as they would possibly be indicative of more\nacidic water than the paper ascribes for Fe-phosphates)?\nWe thank the reviewer for pointing this out. The sulfates do indeed contain Fe in addition to Mg and we\nhave corrected this to state there are Mg,Fe-sulfates throughout the manuscript and added an additional\nsupplementary figure (Figure S6) showing that these sulfates in Ouzel_Falls include both Mg and Fe (but\ncorrelation with Mg is most evident). We believe the sulfates likely formed after the Fe-phosphates in a\nsecondary fluid infiltration event as per Figure 5 as they are more associated with the Si-rich matrix vs.\nthe carbonate-bearing matrix the Fe-phosphates are embedded in. Vivianite is actually relatively insoluble\nin low pH environments (especially when compared to more common martian phosphates such as apatite\nand merrillite - Hausrath et al., 2024). While S-rich fluids may have interacted with the Fe-phosphates in\nthe secondary fluid infiltration event, the persistence of carbonate indicates the acidity of the secondary\nfluid that created the Si and S-rich matrix domain was relatively low, or the water-rock/sediment\ninteraction was short lived. We have also removed the Fe-phosphate end-member from Table S2 as on\nreflection we would prefer to carry out a more robust “unmixing” effort (subject of future work) than the\nsimple “stripping” we originally did here. We instead report the average of the Fe-phosphate compositions\nwith less sulfate mixing (now with a threshold of <5 wt% SO ). We have added some text to the\n3\nauthigenic origin section of the paper to reinforce this that is highlighted below:\n“The presence of metavivianite, ferrolaueite, beraunite, and/or santabarbaraite in Ouzel_Falls\nsuggests that the original vivianite crystals must have also undergone oxidation in-place if authigenic.\nThe formation of separate phosphate-carbonate-rich, and Si-sulfate-rich domains in the\nconglomerate matrix (Figure 2C) indicates that diagenesis in Onahu occurred under evolving\npaleoenvironmental conditions and potentially different redox states. It is hypothesized here that the\nvivianite originally precipitated in a reducing Fe2+-rich fluid associated with the formation of the\ncarbonate-rich matrix (based on their co-occurrence), and was then eventually oxidized either from\nalteration by infiltrating oxidizing Si-S-rich fluids that formed the Si- and sulfate-rich (oxidized S)\nmatrix in the remaining pore space and/or exposure to the oxidizing martian atmosphere (Figure 5).\nBased on the persistence of carbonate, it is assumed that if a Si- and sulfate-rich fluid did alter the\nFe-phosphates, the fluid was not strongly acidic, and/or the water-rock/sediment interaction was\nshort lived. Regardless, vivianite is also relatively insoluble in low pH environments (in comparison\nto Ca-phosphates; Hausrath et al., 2024).”\nLine 184: The word “and” needs to come out of “green and wavelengths”.\nThank you for catching this, this has been updated.\nLine 190 and Figure 3 caption: Change “Re-Lab” to “RELAB”. Also the RELAB files listed and shown\nin Figure 3D are samples originally supplied by Dr. Janice Bishop of SETI to get measured at RELAB.\nYou may want to reach out to her to find out if the sample names were correct b/c she and I and Dr. Darby\nDyar were working on a suite of phosphates some of which got measured at RELAB and subsequently\nwere XRD’ed to assure composition. Some of our samples were originally misidentified and the old\nnames might be associated with the RELAB database. Janice used a different numbering scheme for some\nof my or Darby’s samples, so I’m not sure which ones these are but we could double-check their\nidentification.\nWe thank the reviewer for this point and for facilitating discussions with Dr. Bishop to ensure the\nRELAB samples were correctly identified. As per Dr. Bishop, we have removed Beraunite from Figure 3\nas its identity was incorrect in RELAB, all other phosphates were confirmed to be correct by XRD and\nother analyses according to Dr. Bishop (and Dr. Lane).\nFigure 3: Why is only the “Grain 1” spectrum shown in Figure 3C? If these Fe-phosphates are able to be\ndiscriminated via XRD, I would presume the VNIR spectra would be slightly different from each other.\nI’d like to see all of the Fe-phosphate grain spectra shown in Fig. 3C (not just Grain 1) – if not all the Fe-\nphosphate ovals, then at least add Grain 2 and Grain 3 spectra. How different are they? Do any of them\nactually look like the terrestrial phosphate reflectance spectra? This disconnect is a little problematic for\nme.\nThe spectra for each of the highlighted grains have now been added to the supplement, with Figure 3\nnoting the differences in G/NIR and UV/NIR between them. It is difficult to directly compare the shape of\nPIXL reflectance spectra, which includes 4 wavelengths only, to the resolution of RELAB data which is\ncomprised of thousands of wavelengths. As such, we have updated this figure to rely more on ratios to\nshow the differences between the grains, while still showing the “raw” spectra in the supplement with\nterrestrial data. As is illustrated in new Figure 3A and old Figure 3C, the reflectance Grains 1-4 have\nsimilar reflectance properties, but slight differences are discernable, especially when looking at individual\npixels. For example, higher G/NIR ratios for Grains 1-3 indicate more Fe2+-enrichment in these\nphosphates (complimenting XRD data), but Grain 4 shows the lowest G/NIR signature indicating higher\nFe3+ contents, again complimenting XRD data.\nLines 191-192 and Figure 3: I don’t really see the similarity of the reflectance spectrum in Figure 3C with\nany of the spectra in Figure 3D. And the “Grain 1” spot on Figure 3A looks to be identified as ferrolaueite\non Figure 2G, but a lab spectrum of ferrolaueite isn’t shown in Figure 3D for comparison. I think that\nneeds to be discussed or clarified more. It would be good to have tick marks on the axes as well (for all\nthe graphs).\nAs discussed above, we believe the resolution difference between RELAB and PIXL spectra make direct\ncomparisons of reflectance spectra “shape” difficult and have focussed only on reflectance ratios in the\nmain text to make comparisons more clear. Tick marks have been added to the graphs. There is no\navailable ferrolaueite reflectance spectrum for comparison - this has now been made clear in the text in\nthe Fe-phosphate Identification section of the discussion.\n“As all of these phases can occur together, it is possible that the Fe-phosphate assemblage at\nOuzel_Falls comprises a mixture of (ferro)beraunite, metavivianite, ferrolaueite, santabarbaraite,\nvivianite, and/or other Fe-phosphate minerals that we are not able to differentiate at PIXL’s\nresolution, or where terrestrial analog data needed for comparison are limited. For example, no\nreflectance spectra for beraunite, ferroberaunite, or ferrolaueite were available for comparison at\nthe time of this writing.”\nLines 198-199: “Most similar to mixtures of relatively ferrous phosphates…and … (like beraunite)….” I\ndon’t see that really, the metavivianite and vivianite reflectance spectra show a positive feature around\n450 nm, but the Grain 1 spectrum shows a dip. Please elaborate – maybe state up front that this is related\nto the G/NIR v UV/NIR plots not the specific reflectance spectra themselves, or something like that.\nThank you for the suggestion, we have now made it clear in the text and Figure 3 that the similarities are\nbased on ratios only due to resolution differences between RELAB and PIXL spectra.\nLine 223: The authors mention that the reflectance data for Fe-phosphates is limited (which is still true);\nhowever, some have been published (albeit at limited spectral ranges) and could be referenced. I’m first\nauthor on 2 of them, so a reference is absolutely not required (and they are abstracts), but I want to make\nthe authors aware of these. For example:\nChapter 4, Figure 4.5d in Bishop (2019) Visible and Near-infrared reflectance spectroscopy: Laboratory\nspectra of geologic materials, in Remote Compositional Analysis: Techniques for Understanding\nSpectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces (J. L. Bishop, J. F. Bell III, and J. E.\nMoersch, editors). Cambridge University Press, Cambridge, UK, DOI: 10.1017/9781316888872\nLane M.D., Bishop J.L., Dyar M.D., Parente M., King P.L. & Hyde B.C. (2007) The ferric sulfate and\nferric phosphate minerals in the light-toned Paso Robles rover track soils: A multi-instrument analysis. 7th\nInt'l Mars Conf., Abstract #3331.\nLane M.D., Mertzman S.A., Dyar M.D. & Bishop J.L. (2011) Phosphate minerals measured in the visible-\nnear infrared and thermal infrared: Spectra and XRD analyses. 42nd Lunar Planetary Science Conf.,\nAbstract #1013.\nThank you for these helpful references! Unfortunately, all of the phosphate mineral spectra presented in\nthese references contain significant amounts of other cations such as Al,Mg, and Ca (in addition to Fe),\nand as such, are not good matches for the Ouzel-Falls Fe-phosphates and were not included here.\nLines 228 and 265: Why don’t you show the XRD spectra of the ferrolaueite sample in Figure S9 and\nlabel it on Figure S9F like it is on Figure 2G? That seems important so reader can verify the findings and\nb/c it is stated as one of the 3 Fe-P minerals identified at Ouzel_Falls and is “the first time these rare Fe-\nphosphate minerals have been identified on Mars.” But I haven’t really seen the XRD or spectral evidence\nfor ferrolaueite presented in the manuscript.\nOur apologies for not including it, the spectra included were simply two representative spectra and this\nwas a clear oversight on our part. We have now included a ferrolaueite XRD spectra in the updated figure\nwhich has now been moved to the main text to emphasize its importance to our interpretation.\nLine 285: I also don’t know of a santabarbaraite VNIR spectrum in the literature. What about a\nferrolaueite spectrum? State that it, too, is unavailable in the literature or show it on Figure 3D.\nAgreed and changed.\nLine 292: perhaps change “co-occur” to “occur together”\nAgreed and changed.\nLine 295: change “is limited” to “are limited”\nAgreed and changed.\nLine 358: Perhaps add a reference for the hardness of vivianite and metavivianite\nThese have now been added.\nLine 467: Change [data] “is used” to [data] “are used”\nAgreed and changed.\nReviewer #2 (Remarks to the Author):\nThe paper brings the result of the investigation of a phosphate rich area in Jezero Crater, on Mars, by the\nMars 2020 Perseverance rover. The rover collected mineralogical and chemical data of a conglomerate\noutcrop, informally named Onahu. Although Ca-phosphates are the main phosphates recognized on Mars,\nthe minerals present in the outcrop were shown to be the first directly observed Fe-phosphates on the\nplanet. The authors were able to study and determinate, with high confidence, that the grains include\nminerals link beraunite, ferrolaueite and metavivianite, while other mineral phases were proposed but not\nconfirmed. The origin scenarios of such minerals were proposed based on the knowledge of analog\nenvironments in which they form on Earth, listing the most and least likely hypotheses. The authigenic\nhypothesis talks about the formation of the Fe-phosphates in aqueous environments (on Earth, a process\nthat may involve life) and later oxidized in-place. Meanwhile, the detrital hypothesis would involve the\nmovement of grains from upstream outcrops, although there is no evidence of such rocks that could\nprovide the grains of the Fe-phosphates. To conclude, the authors discuss the implications of this\ndiscovery for the habitability and possibility of life as we know, being the minerals a source of\nphosphorus and energy, and capable of preserving biogenic signatures.\nPhosphorus is an important element for life as we know it, and understanding of its bioavailability is\ncrucial for a better interpretation of the ancient habitability of Mars. During the exploration of the planet,\nit had been demonstrated the presence of P-rich areas and, while it was proposed the existence of Fe-\nphosphates on those areas, no direct study had identified which mineral phases were present. Although\nthere are limitations in the techniques that can be coupled on rovers to explore Mars, the study was able to\nobtain highly reliable results that confirmed suspicions of the presence of Fe-phosphates and evaluate the\nmineral phases using classical techniques like X-ray diffraction and fluorescence. It was a pity that\ninstruments and analyses, such as RAMAN spectroscopy, could not target the grains. However, hopefully,\neven more studies may be performed in a sample collected in the vicinities of the outcrop. All described\nprocesses associated with Fe-phosphate (and other mineral phases from the outcrop) are known from\nabiotic environments. The bioavailability of phosphate is important for life to thrive, whoever, the\npresence of Fe-phosphates also has a great repercussion for the possibility of chemolithoautotrophic life.\nProcesses of biological energetic gain from Fe-redox reactions associated to Fe-phosphates are known,\nlike the mentioned biologically induced reduction of ferric ions and formation of vivianite. However,\nthere are also other processes like the biologically induced oxidation of Fe-phosphates that, together with\nthe environmental abiotically induced oxidation, could generate minerals (biosignatures) with Fe in a\nmore oxidized state and with astrobiological interest.\nDue to the quality of the text, the novelty of the data and the important consequences of the results, the\nrecommendation of this paper is for publication with minor reviews.\nFind, below, specific comments and some points that can be improved.\n1. Line 99, 105, 107 (figure 1) and 151 (figure 2): It is important to acknowledge the source and exact\nreference of the images used in the figure captions. However, as these texts are already long, perhaps the\noriginal name of the image (e.g. (SI1_0789_0736995354_613FDR_N0390926SRLC000\n36_000095J01.IMG) could be referenced elsewhere in the paper or in supplementary materials.\nAgreed, we have now moved the long image names to a table in the supplement.\n2. Line 175: This paragraph discusses the presence of sulfate-rich phases, focusing on the identification of\nCa-sulfate and Mg-sulfate (more detailed data are presented in Supplementary Figure S6). In many cases\non Mars, the formation of ferric sulfate minerals (like jarosite) is associated with low pH aqueous\nenvironments. Was the unequivocal presence of iron-rich sulfates associated with this outcrop observed?\nWe thank the reviewer for pointing this out. The sulfates do indeed likely contain Fe in addition to Mg\nand we have corrected this to state there are Mg,Fe-sulfates throughout the manuscript and added an\nadditional supplementary figure (Figure SX) showing that the sulfates in Ouzel_Falls include both Mg\nand Fe (although there is more obvious correlation with Mg). We believe the sulfates likely formed after\nthe Fe-phosphates in a secondary fluid infiltration event as per Figure 5 as they are more associated with\nthe Si-rich matrix vs. the carbonate-bearing matrix the Fe-phosphates are embedded in. Vivianite is\nactually relatively insoluble in low pH environments (especially when compared to more common\nmartian phosphates such as apatite and merrillite - Hausrath et al., 2024). While S-rich fluids may have\ninteracted with the Fe-phosphates in the secondary fluid infiltration event, the persistence of carbonate\nindicates the acidity of the secondary fluid that created the Si and S-rich matrix domain was relatively\nlow, or the water-rock/sediment interaction was short lived. We have added some text to the\nauthigenic origin section of the paper to reinforce this that is highlighted below:\n“The presence of metavivianite, ferrolaueite, beraunite, and/or santabarbaraite in Ouzel_Falls\nsuggests that the original vivianite crystals must have also undergone oxidation in-place if authigenic.\nThe formation of separate phosphate-carbonate-rich, and Si-sulfate-rich domains in the\nconglomerate matrix (Figure 2C) indicates that diagenesis in Onahu occurred under evolving\npaleoenvironmental conditions and potentially different redox states. It is hypothesized here that the\nvivianite originally precipitated in a reducing Fe2+-rich fluid associated with the formation of the\ncarbonate-rich matrix (based on their co-occurrence), and was then eventually oxidized either from\nalteration by infiltrating oxidizing Si-S-rich fluids that formed the Si- and sulfate-rich (oxidized S)\nmatrix in the remaining pore space and/or exposure to the oxidizing martian atmosphere (Figure 5).\nBased on the persistence of carbonate, it is assumed that if a Si- and sulfate-rich fluid did alter the\nFe-phosphates, the fluid was not strongly acidic, and/or the water-rock/sediment interaction was\nshort lived. Regardless, vivianite is also relatively insoluble in low pH environments (in comparison\nto Ca-phosphates; Hausrath et al., 2024).”\n3. Line 209 (Figure 3C): The relative reflectance spectra for the Grain 1 is shown in the figure, but is not\nspecified if it is a single pixel or the average for the grain. As a suggestion, data from Grains 2 and 3\ncould be also included in the graph for comparison.\nI can see how this was not clear, and we have updated Figure 3 to only focus on the reflectance ratios for\nthe various individual grains. The ratios averages and individual points are plotted and indicated in the\nlegend for clarity.\nThe individual spectra for each of the highlighted grains have now been added to the supplement to\nincrease the readability of Figure 3. Additionally, it is difficult to directly compare the shape of PIXL\nreflectance spectra, which includes 4 wavelengths only, to the resolution of terrestrial RELAB data which\nis comprised of thousands of wavelengths. Figure 3 now focuses on comparison to reflectance ratios only\nso that comparison to terrestrial RELAB data is easier to portray.\n4. Line 222: Due to the importance of the X-ray diffraction for the identification of the Fe-phosphates, the\nauthors may consider that the diffractograms could be included in the main paper, like the Figure S9 from\nthe Supplementary Figures.\nWe agree, and this figure has now been updated and added to the main text (Figure 4).\n5. Line 226-228: The Fe-phosphates are described as “predominantly crystalline, i.e., with grain sizes\nlarger than >30-50 μm in the region with diffraction peaks”. Although not stated the presence of\namorphous or micro-crystalline phases (beyond clay), this may corroborate the hypothesis of presence of\nthe amorphous santabarbaraite (well discussed later between lines 284-288). While the presence of\nberaunite (fully oxidized ferric phosphate) was confirmed, more evidence of the presence of\nsantabarbaraite (more closed related to vivianite/metavivianite) would be very interesting. In lines 285-\n286, the authors says that the “reflectance spectra for santabarbaraite were not available for comparison at\nthis time”. Santabarbarite is not a common mineral, but do not seem hard to be acquired or borrowed to\nobtain its reflectance spectrum. In the absence of the spectrum of santabarbaraite in the database used, did\nthe authors consider obtaining the spectrum of the mineral themselves? Could the reflectance data of\nsantabarbaraite help to further corroborate the hypothesis of the presence of this phase in the outcrop?\nWe thank the reviewer for the suggestion and did consider obtaining the spectrum of the mineral\nourselves but we're not able to source it in time for submission or for this revision but are hoping this will\nbe an exciting topic for future work. As santabarbaraite is often associated with the oxidation of vivianite\nwe can infer it is a likely phase contributing to the high near infrared signal in the reflectance.\n6. Line 341-344: The possibility of biogenic action in the ferrous phosphate formation is well stated (lines\n320-322). However, the possibility of biological oxidation of iron/reduced Fe-phosphates, like\nvivianite/metavivianite, in presence of sulfate was not explored. This could expand the possibilities for\ndiscussing the habitability of this Martian environment.\nWe thank the reviewer for the suggestion, but would like to point out that Fe-Mg sulfates are texturally\nseparate from Fe-phosphates/carbonate indicating there were multiple water-rock/sediment interaction\nevents/periods, with different fluid chemistries, pH, and S activity. Our data does not support that sulfates,\nor S-rich fluids, were present at the time of vivianite crystallization and as such, we do not believe that\nfurther discussion is necessary. The evidence for 2 separate fluid infiltration events is detailed in the\nAuthigenic Origin section, and Figure 5. If the reviewer is referring to how vivianite and sulfate can\npersist in the same rock, we refer back to our response to comment 2 above and reiterate that vivianite is\nactually relatively insoluble in low pH environments (especially when compared to more common\nmartian phosphates such as apatite and merrillite - Hausrath et al., 2024).\n7. Lines 342-343: The authors stated that “then eventually oxidized either from alteration by infiltrating\noxidizing Si-S-rich fluids that formed the Si- and sulfate-rich (oxidized SO42-) matrix”. The terminology\noxidized SO42- is confusing since there is no oxidized and reduced sulfate, but oxidized and reduced\nform of sulfur. I recommend changing to (oxidized S).\nThis was an oversight, thank you very much for the correction, which has now been changed to oxidized\nS.\n8. Line 383, Figure 4: In the second image (“Fluid Infiltration”), please correct the phosphate formula\n(PO4 to PO43-). As a suggestion to this picture, the yellow used to Sulfate (last picture) is very difficult to\nsee. Please consider changing the color if it is possible to.\nThank you for the suggestion, we have made the color darker and easier to see.\n9. Line 396 (and in figure 4): For chemical accuracy, the terminology “~ neutral pH” is not the most\nadequate, since pH is just a number; the medium can be acid, alkaline or neutral. So, the correct forms are\n“neutral medium” or “pH around 7”. I suggest the second form, avoiding the use of “~” that is more\ninformal.\nThank you for the suggestion, this has now been changed to “reducing aqueous medium with\napproximately neutral pH”, we have updated figure 4 (now figure 5) to refer to “~neutral fluids” due\nto space constraints.\nPlease also note that in the authigenic origin section we have updated the text to state that: “…vivianite\noften forms in natural systems at circumneutral pH (e.g. pH 6-917).” To give a more specific range\nrelated to natural terrestrial environments. Reference 17 = Rothe et al., 2016.\n10. Line 396: The terminology “reducing water” is also non adequate. Please prefer “reducing aqueous\nmedium with pH around 7 and high concentrations of bio-limiting elements…”\nThis has now been changed to “reducing aqueous medium with approximately neutral pH and high\nconcentrations of bio-limiting elements including carbon and phosphorus…”.\n11. Line 387 Topic “Implications for Habitability and Mars Sample Return”. It is not clear for me the\nreasons for the authors to mention “Mars sample return” in this topic since they vaguely mention the topic\nin lines 409-410. The authors should consider changing the topic name to “Implications for Martian\nhabitability and biosignature detection”.\nI understand this comment, and appreciate the suggestion, but I do think that Mars Sample Return needs\nto be referenced here as we are intending to stress both the implications for habitability and biosignature\ndetection on Mars, and the importance of Mars Sample Return to confirm this possibility as summed up\nin the last paragraph.\n12. Supplementary material table S5. Ferroberaunite. Just correct the molecular formula of water, in\nwhich 2 is not subscripted. Also, Ferristrunzite. Add “2+” in Fe in the molecular structure.\nThank you for catching this, it has been updated!\n13. Supplementary material table S6, 5th line. The reference to “FeOT” as a major oxide is correct or is it\na typing mistake?\nThis is referencing FeO Total. We have updated that row so it is spelling out as FeO Total instead of\nFeOT.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of\nthe Nature Communications initiative to facilitate training in peer review and to provide appropriate\nrecognition for Early Career Researchers who co-review manuscripts.\nThank you for your efforts!\nReviewer #4 (Remarks to the Author):\nSUMMARY:\nKizovski et al 2024 present evidence gathered by the Perseverance rover for Fe-phosphate minerals on the\nsurface of Mars. This evidence includes textural, chemical, spectral, and diffraction analyses of grains\ndiscovered in a conglomerate outcrop named “Onahu”. The evidence for the presence of Fe-phosphates in\ngeneral is compelling, representing the most definitive evidence for their presence on Mars presented thus\nfar. The study also interrogates the specific minerals that occur in this location and begins to build a\nparagenetic model of their occurrence. Data is assessed to be most consistent with the minerals beraunite,\nmetavivianite, and ferrolaueite. The fact that these minerals occur within a conglomerate made up largely\nof ~basalt is unusual, but the key to their occurrence appears to be that this is a highly alternated\nconglomerate that contains carbonate-rich matrix material. Indeed, the Fe-phosphates are apparently\nrestricted to the matrix. Both authigenic and a detrital models of origin for the minerals are discussed,\nfollowed by a brief discussion of the implications of the findings for habitability. Overall, the paper makes\na compelling contribution to the literature. I have some questions and comments about the proposed\nparagenetic models and the discussion about habitability. On this basis, I recommend minor revisions for\nthe paper.\nCOMMENTS:\nIs the origin of the minerals authigenic or detrital?\nI did not fully understand the arguments presented regarding an authigenic or detrital origin of the Fe-\nphosphate minerals. The authors seem to provide really quite compelling evidence against a detrital\norigin. In particular, the minerals are:\n1) Only found in the carbonate-rich alteration matrix.\n2) Soft minerals that have never been observed to survive the conditions of transport required to form a\nconglomerate.\n3) Form on Earth either in surficial weathering zones or in oxidised hydrothermal systems in granitic\npegmatites.\nAs far as I can tell, these points would seem to render a detrital origin extremely unlikely.\n1) The alteration matrix formed in place after the conglomerate had already lithified, as indicated in the\nauthors’ Fig. 4. A detrital origin would presumably require a different texture i.e., where the minerals are\nno longer present in what looks pretty clearly to be a post-deposition texture.\n2) Minerals that are both very soft and low abundance are hard to preserve as detrital phases. Physical\nfeasibility of detrital secondary late paragenetic Fe-phosphates seems tricky.\n3) Perseverance is sampling only surficial weathering zones. This lines up nicely with these minerals\nbeing weathering products rather than detrital having formed in some earlier alteration phase of the\nmaterial that was then transported to form the conglomerate.\nAll of this is simply to say that, whilst the paper does not give a definitive answer either way and presents\na detrital origin as basically equally plausible to an authigenic one, I do not think the evidence seems to\nsupport that level of ambiguity. It would probably be better if the discussion came down more clearly on\nthe side of authigenesis, as per Fig. 4, having listed the issues with a competing detrital hypothesis.\nWe thank the reviewer for the support and helpful comment. While we agree with the points laid out here,\nwe would prefer to still consider a detrital origin. Pegmatitic environments are very unlikely on Mars as\nyou have stated above, but similar environments to what we have observed here at Onahu may not be.\nAfter the analysis presented in this work, the rover has continued further into the delta channel giving\nmore opportunities to find similar environments and minerals. In order to leave room for these potentially\nnearby occurrences we would prefer to leave the detrital origin as a possibility. We also note that Fe-\nphosphates like vivianite could potentially form from alteration of Ca-phosphate-bearing ultramafic rocks\nupstream of the delta (stated in the Detrital Origin section).\nWhen did the minerals form?\nSomething else that was unclear to me is when in the life of Mars the authors imagine the minerals may\nhave formed? I know we cannot give any exact dates but could the authors contextualise the sequence of\nevents in the history of Mars? Mars is strongly oxidised today and ceased to have widespread aqueous\nactivity a while ago. Is Onahu telling us something about the changing surficial conditions, i.e., from\nreducing to oxidising to dry?\nI think this would be useful to comment on. The authors hint at it with “evolving paleoenvironmental\nconditions” but do not go on to say if this fits with emerging evidence for the environmental evolutionary\ntimeline of Mars and the crater in general.\nI am also wondering about the timing because the Onahu conglomerate is apparently >3.2 billion years\nold. I am a tad sceptical that these secondary late paragenesis Fe-phosphates could be 3.2 billion years\nold, given how recent and ephemeral the same minerals are when found in weathering zones on Earth.\nThen again, Mars is far less active than Earth so perhaps the same rules do not apply. Could the authors\nadd a rough timeline of events to their figure 4 along with a corresponding broader environmental history\nof the crater and of Mars as a whole?\nWe thank the reviewer for the suggestions, but hesitate to speculate more on timing of the Fe-phosphates’\nformation. Figure 4 (now Figure 5) does show a rough sequence of events, and as fluids are involved, this\nis ofcourse more likely to have occurred when Mars was warmer and wetter - around the age of the delta -\nbut we cannot rule out that the vivianite could have formed in another discrete alteration event closer to\npresent. We have however, added some text to the Implications for Habitability and Mars Sample Return\nsection to make this ambiguity more clear and some back up information on why we think ancient Fe-\nphosphates could indeed persist (based on the presence of Ca-phosphates throughout the rover’s traverse\nwhich are much less inert than the Fe-phosphates). New text is highlighted below.\n“Based on comparisons to Earth analogs, the vivianite in Ouzel_Falls most likely precipitated in\nconditions favorable to potential martian life within a low temperature, reducing aqueous medium\nwith pH ~7 and high concentrations of bio-limiting elements including carbon and phosphorus, and\nFe-redox gradients that could provide an energy source. For example, dissimilatory reduction of\nferric ions by bacteria when phosphorus is available, to the ferrous form in vivianite, can enable\naccess to metabolic energy 54,55. While these conditions are most likely to have occurred when Mars\nwas warmer and wetter, around the time of the Jezero delta’s formation (>3.2 Ga2–8), we are not able\nto rule out if the Fe-phosphate minerals formed in another discrete alteration event closer to present.\nHowever, Fe-phosphates are also known to be more resistant to dissolution in low pH environments\nthan the primary igneous and detrital Ca-phosphate minerals that have been identified throughout\nJezero crater’s floor and delta9. The persistence of these comparatively soluble phosphate minerals\nthroughout the rover’s traverse indicates that it is possible the Fe-phosphate minerals in Onahu could\nhave originally formed billions of years ago.”\nPrebiotic chemistry and habitability\nThe implications of identifying Fe-phosphates for habitability and prebiotic chemistry on Mars are\ndefinitely substantial. However, I think the paper could go a little further in specifying what these\nimplications are. Mention is mainly made of the fact that the conditions were probably habitable.\nHowever, this seems quite unsurprising. What is more exciting from my perspective is that vivianite is an\nimportant mineral in the early P cycling literature. Therefore, we should be able to put some numbers on P\navailability to early life and to prebiotic chemistry. Could the authors double check the literature and\nreport likely phosphate concentrations in the altering fluids that are consistent with vivianite\nprecipitation? And again, echoing the comments above, the timing of all this is important. Can we learn\nfrom these minerals about P availability 3+ billion years ago, or only somewhat more recently?\nThank you for this suggestion! We have included the following statement in the Authigenic Origin\nstatement that I believe addresses the P concentrations needed for vivianite:\n“In anoxic lakes on Earth, the P/Fe2+ ratio needed for authigenic formation [of vivianite] is relatively\nhigh, typically ~1:6 or more, and >1:30 in laboratory experiments40.”\nAs noted in the response to the comment above, we hesitate to give an exact timing on when the Fe-\nphosphates formed, but have added some more text to address that ambiguity.\nREVIEWERS' COMMENTS\nNote to the editor:\nThank you for all of your help with this manuscript. Please note we have made some\nchanges to the wording in the final section related to the timing of the Fe-phosphates’\nformation on Mars beyond what was requested by reviewers. After additional review\nand discussions with co-authors we think we have found a much better way to discuss\nthe potential age of the Fe-phosphates. Please see these changes below.\nBased on comparisons to natural systems on Earth, the vivianite in Ouzel_Falls most likely\nprecipitated in conditions favorable to potential martian life within a low temperature, reducing\naqueous medium with approximately neutral pH17, with high concentrations of bio-limiting Deleted: and\nelements including carbon and phosphorus, and Fe-redox gradients that could provide an energy\nsource. For example, dissimilatory reduction of ferric ions by bacteria when phosphorus is\navailable, to the ferrous form in vivianite, can enable access to metabolic energy57,58. These Deleted: While t\nconditions are most likely to have occurred when Mars was warmer and wetter, around the time of\nthe Jezero fan formation (>3.2 Ga2–8). Deleted: delta’s\nAlthough vivianite is thought to have been important in the biogeochemical P cycle on early\nEarth44,59,60, it is largely absent from the ancient geologic record, possibly due to Formatted: Superscript\noxidation/dissolution driven by diagenesis and metamorphism59. In contrast, the results presented Deleted: , we are not able to rule out if the Fe-phosphate\nhere, combined with the well-preserved nature of the western fan8 suggest the Onahu outcrop minerals formed in another discrete alteration event closer to\npresent. However, Fe-phosphates are also known to be more\nexperienced minimal metamorphic alteration following the formation of the fan ca. 3.2 Ga, thus resistant to dissolution in low pH environments than the\nprimary igneous and detrital Ca-phosphate minerals that have\nextending the preservational window of vivianite. However, while the available evidence supports been identified throughout Jezero crater’s floor and delta9.\nThe persistence of these comparatively soluble phosphate\na potentially ancient origin for these well-preserved Fe-phosphates, we cannot rule out their minerals throughout the rover’s traverse indicates that it is\npossible the Fe-phosphate minerals in Onahu could have\nformation in another more recent event.\noriginally formed billions of years ago.\nWe believe that this more clearly states what we were trying to convey without getting\ninto “wordy” comparisons between Ca-phosphate and Fe-phosphate stability in\ndifferent pH conditions.\nPlease also note, we also had a minor typo in the supplementary that needed to be\nupdated under “Detailed Mineral Mixing Modelling and Results”. The FWHM were not\nupdated from a previous version and have now been corrected (there is no change to\nthe model, just the text below).\nWe use their effective beam sizes (FWHM) for various elements which varies between\napproximately 204 to 139 μm from Na to Fe, respectively, to model mixing between the main Deleted: 230\nphases in Ouzel_Falls and theoretical stoichiometric phosphate mineral endmembers in order to Deleted: 140\nattempt to determine the composition of the Fe-phosphates.\nDeleted: Cr\nDeleted: \nThe line-by-line response to reviewer #1 is below.\nReviewer #1 (Remarks to the Author):\nRe-review of Kizovski et al. (Melissa Lane)\nIt’s been a pleasure to read and review this work by Kizovski et al. The results are very\ninteresting and novel. I appreciate that the authors did more digging to verify the phosphate\nmineral reflectance spectra and ease my concerns. I do have some minor edits listed\nbelow that should be considered, but I recommend the paper be accepted for publication.\nThank you very much for your thorough and helpful reviews that have significantly\nstrengthened the paper. Please find our response to the review in red below.\nLine 61: Change “will provide” to “may provide”\nDone\nFig. 1 caption: I like that the figure numbers have been moved to a table. I would remove\n“for clarity” that is unnecessary to say.\nDone\nLines 153-155: Choose either hyphens or commas and use throughout paper. Personally,\nI’d prefer “relatively Mg-rich Mg-Fe-carbonates” and “Mg-Fe-sulfate” and “relatively Fe-rich\nFe-Mg-carbonate”.\nWe agree with your preference and have updated to hyphens throughout the manuscript.\nLine 171: Define PMC\nDone\nLine 265: Change which to that\nDone\nLine 267: Change which to that\nDone\nLine 273: Change “which is” to “that are”\nHere I think “which is” is more appropriate as this sentence is referring to the Fe-phosphate\nmodelling problem.\nLine 282: I think you should use ~2:3 to be similar to line 269. (It is okay to have the slash in\nP/Fe and use 2:3.)\nAll P/Fe ratios changed to 2/3 to be consistent.\nLine 289: Define MCC\nDone\nLine 291: Change which to that; also change “is limited” to “are limited”\nDone\nLine 292: Change “grain size effects” to “particle size effects”\nDone\nLine 298: Change “Further to this” to “Furthermore”\nDone\nLine 516: Change “phosphates reflectance spectra” to “phosphate reflectance spectra”\nAs there are multiple Fe-phosphates we are referring to here, we would prefer to keep the\nwording as is.\nLine 583: I’d remove the split infinitive and use “Diffraction peaks also were detected….”\nI’d prefer to leave this as “Diffraction peaks were also detected..” for flow and clarity.\nLine 620: Change which to that\nDone\nLine 624: Use 2:3 (or whatever you choose for Lines 282 and 269)\nDone, all are 2/3\nLine 676: Add to say “This discovery {or choose what word you like} is the first time….”\nI’d prefer to leave this as “This is the first time these rare Fe-phosphate minerals have been\nidentified on Mars.” I don’t think the word “discovery” is necessary.\nLine 677: Change to 2:3\nDone, all P/Fe ratios were changed to 2/3.\nLine 677: Switch “can also be” to “also can be”\nThank you for the suggestion, but we prefer “can also be” for flow and clarity.\nLine 682: Switch “can all form” to “all can form”\nThank you for the suggestion but I think “Metavivianite, beraunite, and ferrolaueite can all\nform on Earth..” flows better.\nLine 683: Delete “known as” {it isn’t necessary}\nDone\nLine 684: Change forming to formed\nDone\nLine 689: You can delete the word “here”\nDone\nLine 727: As previously suggested, I would change “Fe,Mg-carbonate” to “Fe-Mg-\ncarbonate” (or at least be consistent throughout the paper)\nThank you, this has been updated so only hyphens are used as suggested.\nLine 729: Change “or filling” to “or filled”\nDone\nLine 734: Add a comma to read “e.g., pH...)\nDone\nLines 736-737: The hyphen is not needed in “biologically influenced systems”\nThe hyphen has been removed.\nLine 743: Something is amiss with how this sentence reads. Should it be “…through\nthe same process as when organic carbon oxidation …”?\nThank you for catching this, this has been updated as suggested.\nLine 748: Change “has been previously inferred” to “previously has been inferred”\nThank you for the suggestion, but we prefer “has been previously inferred” for flow and\nclarity.\nLine 750: Change “to have originally precipitated” to “to have precipitated originally”\nThank you for the suggestion, but we prefer “to have originally precipitated” for flow and\nclarity.\nLine 751: Change “Overtime” to “Over time” and “was likely oxidized” to “likely was\noxidized”\nWe have changed overtime to over time, but prefer “was likely oxidized”.\nLine 756: Change “must have also undergone” to “also must have undergone”\nThank you for the suggestion, but we prefer “must have also undergone” for flow and clarity.\nLine 761: Change “was then eventually oxidized” to “then eventually was oxidized”\nThank you for the suggestion, but we prefer “was then eventually oxidized” for flow and\nclarity.\nLine 762: For clarity, add to read “…pore space and/or from exposure….”\nDone\nLine 781: Add a hyphen for “Fe-phosphates”\nDone\nLine 789: Add to read “…identified as such.”\nThank you for the suggestion, but we prefer the original wording.\nLine 789: the comma should move to read “Mohs hardness of 1.5-2 and 2.5, respectively”\nDone\nLine 870: Don’t capitalize fluorescence\nFluorescence was capitalized before defining an acronym (X-ray Fluorescence = XRF)\nconsistent with what is done throughout the paper.\nLines 875-876: Move this definition of PMC up to its first usage (Line 171)\nThe full definition of PMC is already included on line 138 where it is first used “PMCs (PIXL\nMotion Counters = integers corresponding to each analysis point location in a PIXL map)”\nLine 882: Move the full definition of MCC up to its first usage (Line 289)\nMCC is already defined at its first use (Line 185) “PIXL’s Micro Context Camera (MCC)…”\nLine 905: For clarity, I suggest adding word to read “…X-ray diffraction peaks occurring from\ncrystalline ….”\nDone\nLine 911: Change “grain size” to “particle size”\nThank you for the suggestion, but we prefer “grain size” for clarity.\nLine 927: Remove the comma after “Lane” before “and”\nDone\nLine 928: Change Fe-phosphates to Fe-phosphate\nSince there are multiple Fe-phosphates we are referring to here so would prefer to keep this\nas is.\nTable S6 caption: Change “was” to “were”\nDone\nFigure S9 caption: Change “789.” To “789,”\nDone\nFigure S19: Make Sample lower case\nDone\nFigure S2 caption: Add comma after e.g.\nDone\nTable S2: It may only be on my end, but there is a gray square in the formula for\nFerrirockbridgeite\nThis issue does exist in the pdf but not the word file. We will make sure to carefully check\nthe proof so this does not occur in the final version.\nFigure S11, second line: Add to read “…higher in the rim. This relationship may be due to\na….”\nDone\nUnder Diffraction Phase Matching: Change “Fe-phosphates minerals” to “Fe-phosphate\nminerals”\nSince there are multiple Fe-phosphates we are referring to here so would prefer to keep this\nas is.\nWe sincerely thank all of the reviewers below for their time and effort!\nReviewer #2 (Remarks to the Author):\nI would like to express my sincere appreciation to the authors for their detailed response\nand effort in revising the manuscript. In my assessment, the authors have satisfactorily\naddressed all previously raised concerns, and the article is now suitable for publication. I\nam grateful for the opportunity to review this work and contribute to its refinement.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review and\nto provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReviewer #4 (Remarks to the Author):\nThe authors have done a solid job of revising the paper in response to my comments and I\nam now more than happy to recommend acceptance."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Re-review of Kizovski et al. (Melissa Lane)\nIt’s been a pleasure to read and review this work by Kizovski et al. The results are very interesting and novel. I appreciate that the authors did more digging to verify the phosphate mineral reflectance spectra and ease my concerns. I do have some minor edits listed below that should be considered, but I recommend the paper be accepted for publication.\nLine 61: Change “will provide” to “may provide”\nFig. 1 caption: I like that the figure numbers have been moved to a table. I would remove “for clarity” that is unnecessary to say.\nLines 153-155: Choose either hyphens or commas and use throughout paper. Personally, I’d prefer “relatively Mg-rich Mg-Fe-carbonates” and “Mg-Fe-sulfate” and “relatively Fe-rich Fe-Mg-carbonate”.\nLine 171: Define PMC\nLine 265: Change which to that\nLine 267: Change which to that\nLine 273: Change “which is” to “that are”\nLine 282: I think you should use ~2:3 to be similar to line 269. (It is okay to have the slash in P/Fe and use 2:3.)\nLine 289: Define MCC\nLine 291: Change which to that; also change “is limited” to “are limited”\nLine 292: Change “grain size effects” to “particle size effects”\nLine 298: Change “Further to this” to “Furthermore”\nLine 516: Change “phosphates reflectance spectra” to “phosphate reflectance spectra”\nLine 583: I’d remove the split infinitive and use “Diffraction peaks also were detected….”\nLine 620: Change which to that\nLine 624: Use 2:3 (or whatever you choose for Lines 282 and 269)\nLine 676: Add to say “This discovery {or choose what word you like} is the first time….”\nLine 677: Change to 2:3\nLine 677: Switch “can also be” to “also can be”\nLine 682: Switch “can all form” to “all can form”\nLine 683: Delete “known as” {it isn’t necessary}\nLine 684: Change forming to formed\nLine 689: You can delete the word “here”\nLine 727: As previously suggested, I would change “Fe,Mg-carbonate” to “Fe-Mg-carbonate” (or at least be consistent throughout the paper)\nLine 729: Change “or filling” to “or filled”\nLine 734: Add a comma to read “e.g., pH...)\nLines 736-737: The hyphen is not needed in “biologically influenced systems”\nLine 743: Something is amiss with how this sentence reads. Should it be “…through the same process as when organic carbon oxidation …”?\nLine 748: Change “has been previously inferred” to “previously has been inferred”\nLine 750: Change “to have originally precipitated” to “to have precipitated originally”\nLine 751: Change “Overtime” to “Over time” and “was likely oxidized” to “likely was oxidized”\nLine 756: Change “must have also undergone” to “also must have undergone”\nLine 761: Change “was then eventually oxidized” to “then eventually was oxidized”\nLine 762: For clarity, add to read “…pore space and/or from exposure….”\nLine 781: Add a hyphen for “Fe-phosphates”\nLine 789: Add to read “…identified as such.”\nLine 789: the comma should move to read “Mohs hardness of 1.5-2 and 2.5, respectively”\nLine 870: Don’t capitalize fluorescence\nLines 875-876: Move this definition of PMC up to its first usage (Line 171)\nLines 882: Move the full definition of MCC up to its first usage (Line 289)\nLine 905: For clarity, I suggest adding word to read “…X-ray diffraction peaks occurring from crystalline ….”\nLine 911: Change “grain size” to “particle size”\nLine 927: Remove the comma after “Lane” before “and”\nLine 928: Change Fe-phosphates to Fe-phosphate\nTable S6 caption: Change “was” to “were”\nFigure S9 caption: Change “789.” To “789,”\nFigure S19: Make Sample lower case\nFigure S2 caption: Add comma after e.g.\nTable S2: It may only be on my end, but there is a gray square in the formula for Ferrirockbridgeite\nFigure S11, second line: Add to read “…higher in the rim. This relationship may be due to a….”\nUnder Diffraction Phase Matching: Change “Fe-phosphates minerals” to “Fe-phosphate minerals”", "reviewer_2": "I would like to express my sincere appreciation to the authors for their detailed response and effort in revising the\nmanuscript. In my assessment, the authors have satisfactorily addressed all previously raised concerns, and the article is\nnow suitable for publication. I am grateful for the opportunity to review this work and contribute to its refinement.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_4": "The authors have done a solid job of revising the paper in response to my comments and I am now more than happy to\nrecommend acceptance."}, "author_response": "The line-by-line response to reviewer #1 is below.\nReviewer #1 (Remarks to the Author):\nRe-review of Kizovski et al. (Melissa Lane)\nIt’s been a pleasure to read and review this work by Kizovski et al. The results are very\ninteresting and novel. I appreciate that the authors did more digging to verify the phosphate\nmineral reflectance spectra and ease my concerns. I do have some minor edits listed\nbelow that should be considered, but I recommend the paper be accepted for publication.\nThank you very much for your thorough and helpful reviews that have significantly\nstrengthened the paper. Please find our response to the review in red below.\nLine 61: Change “will provide” to “may provide”\nDone\nFig. 1 caption: I like that the figure numbers have been moved to a table. I would remove\n“for clarity” that is unnecessary to say.\nDone\nLines 153-155: Choose either hyphens or commas and use throughout paper. Personally,\nI’d prefer “relatively Mg-rich Mg-Fe-carbonates” and “Mg-Fe-sulfate” and “relatively Fe-rich\nFe-Mg-carbonate”.\nWe agree with your preference and have updated to hyphens throughout the manuscript.\nLine 171: Define PMC\nDone\nLine 265: Change which to that\nDone\nLine 267: Change which to that\nDone\nLine 273: Change “which is” to “that are”\nHere I think “which is” is more appropriate as this sentence is referring to the Fe-phosphate\nmodelling problem.\nLine 282: I think you should use ~2:3 to be similar to line 269. (It is okay to have the slash in\nP/Fe and use 2:3.)\nAll P/Fe ratios changed to 2/3 to be consistent.\nLine 289: Define MCC\nDone\nLine 291: Change which to that; also change “is limited” to “are limited”\nDone\nLine 292: Change “grain size effects” to “particle size effects”\nDone\nLine 298: Change “Further to this” to “Furthermore”\nDone\nLine 516: Change “phosphates reflectance spectra” to “phosphate reflectance spectra”\nAs there are multiple Fe-phosphates we are referring to here, we would prefer to keep the\nwording as is.\nLine 583: I’d remove the split infinitive and use “Diffraction peaks also were detected….”\nI’d prefer to leave this as “Diffraction peaks were also detected..” for flow and clarity.\nLine 620: Change which to that\nDone\nLine 624: Use 2:3 (or whatever you choose for Lines 282 and 269)\nDone, all are 2/3\nLine 676: Add to say “This discovery {or choose what word you like} is the first time….”\nI’d prefer to leave this as “This is the first time these rare Fe-phosphate minerals have been\nidentified on Mars.” I don’t think the word “discovery” is necessary.\nLine 677: Change to 2:3\nDone, all P/Fe ratios were changed to 2/3.\nLine 677: Switch “can also be” to “also can be”\nThank you for the suggestion, but we prefer “can also be” for flow and clarity.\nLine 682: Switch “can all form” to “all can form”\nThank you for the suggestion but I think “Metavivianite, beraunite, and ferrolaueite can all\nform on Earth..” flows better.\nLine 683: Delete “known as” {it isn’t necessary}\nDone\nLine 684: Change forming to formed\nDone\nLine 689: You can delete the word “here”\nDone\nLine 727: As previously suggested, I would change “Fe,Mg-carbonate” to “Fe-Mg-\ncarbonate” (or at least be consistent throughout the paper)\nThank you, this has been updated so only hyphens are used as suggested.\nLine 729: Change “or filling” to “or filled”\nDone\nLine 734: Add a comma to read “e.g., pH...)\nDone\nLines 736-737: The hyphen is not needed in “biologically influenced systems”\nThe hyphen has been removed.\nLine 743: Something is amiss with how this sentence reads. Should it be “…through\nthe same process as when organic carbon oxidation …”?\nThank you for catching this, this has been updated as suggested.\nLine 748: Change “has been previously inferred” to “previously has been inferred”\nThank you for the suggestion, but we prefer “has been previously inferred” for flow and\nclarity.\nLine 750: Change “to have originally precipitated” to “to have precipitated originally”\nThank you for the suggestion, but we prefer “to have originally precipitated” for flow and\nclarity.\nLine 751: Change “Overtime” to “Over time” and “was likely oxidized” to “likely was\noxidized”\nWe have changed overtime to over time, but prefer “was likely oxidized”.\nLine 756: Change “must have also undergone” to “also must have undergone”\nThank you for the suggestion, but we prefer “must have also undergone” for flow and clarity.\nLine 761: Change “was then eventually oxidized” to “then eventually was oxidized”\nThank you for the suggestion, but we prefer “was then eventually oxidized” for flow and\nclarity.\nLine 762: For clarity, add to read “…pore space and/or from exposure….”\nDone\nLine 781: Add a hyphen for “Fe-phosphates”\nDone\nLine 789: Add to read “…identified as such.”\nThank you for the suggestion, but we prefer the original wording.\nLine 789: the comma should move to read “Mohs hardness of 1.5-2 and 2.5, respectively”\nDone\nLine 870: Don’t capitalize fluorescence\nFluorescence was capitalized before defining an acronym (X-ray Fluorescence = XRF)\nconsistent with what is done throughout the paper.\nLines 875-876: Move this definition of PMC up to its first usage (Line 171)\nThe full definition of PMC is already included on line 138 where it is first used “PMCs (PIXL\nMotion Counters = integers corresponding to each analysis point location in a PIXL map)”\nLine 882: Move the full definition of MCC up to its first usage (Line 289)\nMCC is already defined at its first use (Line 185) “PIXL’s Micro Context Camera (MCC)…”\nLine 905: For clarity, I suggest adding word to read “…X-ray diffraction peaks occurring from\ncrystalline ….”\nDone\nLine 911: Change “grain size” to “particle size”\nThank you for the suggestion, but we prefer “grain size” for clarity.\nLine 927: Remove the comma after “Lane” before “and”\nDone\nLine 928: Change Fe-phosphates to Fe-phosphate\nSince there are multiple Fe-phosphates we are referring to here so would prefer to keep this\nas is.\nTable S6 caption: Change “was” to “were”\nDone\nFigure S9 caption: Change “789.” To “789,”\nDone\nFigure S19: Make Sample lower case\nDone\nFigure S2 caption: Add comma after e.g.\nDone\nTable S2: It may only be on my end, but there is a gray square in the formula for\nFerrirockbridgeite\nThis issue does exist in the pdf but not the word file. We will make sure to carefully check\nthe proof so this does not occur in the final version.\nFigure S11, second line: Add to read “…higher in the rim. This relationship may be due to\na….”\nDone\nUnder Diffraction Phase Matching: Change “Fe-phosphates minerals” to “Fe-phosphate\nminerals”\nSince there are multiple Fe-phosphates we are referring to here so would prefer to keep this\nas is.\nWe sincerely thank all of the reviewers below for their time and effort!\nReviewer #2 (Remarks to the Author):\nI would like to express my sincere appreciation to the authors for their detailed response\nand effort in revising the manuscript. In my assessment, the authors have satisfactorily\naddressed all previously raised concerns, and the article is now suitable for publication. I\nam grateful for the opportunity to review this work and contribute to its refinement.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review and\nto provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReviewer #4 (Remarks to the Author):\nThe authors have done a solid job of revising the paper in response to my comments and I\nam now more than happy to recommend acceptance."}]

Decision letter