# Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation

J. Bishop, J. Meusburger, C. Weitz, M. Parente, C. Gross, D. Talla, A.

Saranathan, Y. Itoh, M. Gruendler, A. Howells, et al.

► To cite this version:

J. Bishop, J. Meusburger, C. Weitz, M. Parente, C. Gross, et al.. Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation. Nature Communications, 2025, 16 (1), pp.7020. 10.1038/s41467-025-61801-2 . hal-05405965

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# Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation

Received: 22 January 2025

Accepted: 2 July 2025

Published online: 05 August 2025

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J. L. Bishop<sup>1,2✉</sup>, J. M. Meusburger<sup>2</sup>, C. M. Weitz<sup>3</sup>, M. Parente<sup>4</sup>, C. Gross<sup>5</sup>, D. Talla<sup>6</sup>, A. M. Saranathan<sup>4,7,8</sup>, Y. Itoh<sup>4,9</sup>, M. R. D. Gruendler<sup>1,10</sup>, A. E. G. Howells<sup>2,11</sup>, M. Yeşilbaş<sup>12</sup>, T. Hiroi<sup>13</sup>, B. Schmitt<sup>14</sup>, A. Maturilli<sup>15</sup>, M. Al-Samir<sup>5</sup>, T. F. Bristow<sup>1,2</sup>, B. Lafuente<sup>1,2</sup> & M. Wildner<sup>6</sup>

Sulfate minerals are significant components of the martian surface and provide clues about the martian geochemical environment. One unusual Fe-sulfate phase has been intriguing Mars scientists for over a decade due to its unique spectral bands that are distinct from any known minerals and its occurrence in layered sedimentary rocks. We describe here detection of ferric hydroxysulfate (Fe³⁺SO₄OH) and its implications for the geochemical history of Mars. Crystalline ferric hydroxysulfate is formed by heating hydrous Fe²⁺ sulfates to 100 °C or above and has a strong spectral band at 2.236 µm, similar to the spectral feature observed on Mars at Aram Chaos and on the plateau above Juventae Chasma. Hydrated sulfates at these locations likely formed through evaporative processes or low-temperature alteration. In contrast, Fe³⁺SO₄OH is more consistent with heating and oxidation of hydrated ferrous sulfates, potentially through deposition of lava, ash, or through hydrothermal processes.

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument in orbit at Mars has provided hyperspectral data for mapping numerous minerals that guide our understanding of the ancient geochemical history of Mars. A variety of sulfate minerals have been identified on Mars from orbital (e.g. refs. 1,2) and landed (e.g. refs. 3–5) missions through comparison with terrestrial minerals using spectral parameters, X-ray diffraction, and elemental abundances. An unusual spectral band at 2.236 µm was discovered in CRISM spectra of Mars at the plateau bordering Juventae Chasma⁶ and in Aram Chaos⁷ and has provided a challenge for mineral identification for over 15 years because this spectral band is not consistent with any known

minerals. Preliminary lab investigations proposed dehydrated iron sulfates as the source of this unexplained material<fnref n="1" />⁻⁸. Recent advances in lab experiments on iron sulfates and improvements in the calibration of CRISM images are enabling identification and characterization of these intriguing martian outcrops, which we discuss in this study.

## Results

### Investigation of martian outcrops with unique ~2.23 µm spectral band

Characterizing these outcrops with unusual spectral signatures in CRISM images has benefitted from improved processing techniques⁹

<footnote n="1">¹SETI Institute, Mountain View, CA, USA. ²NASA Ames Research Center, Space Science and Astrobiology Division, Moffett Field, CA, USA. ³Planetary Science Institute, Tucson, AZ, USA. ⁴Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA, USA. ⁵Freie Universität Berlin, Institute of Geological Sciences, Berlin, Germany. ⁶Department of Mineralogy and Crystallography, University of Vienna, Vienna, Austria. ⁷Science Systems and Applications, Inc, Lanham, MD, USA. ⁸NASA Goddard Space Flight Center, Greenbelt, MD, USA. ⁹Johns Hopkins University Applied Physics Lab, Laurel, MD, USA. ¹⁰University of California San Diego, La Jolla, CA, USA. ¹¹Blue Marble Space Institute of Science, Seattle, WA, USA. ¹²Umeå University, Umeå, Sweden. ¹³Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA. ¹⁴University of Grenoble Alpes, CNRS, IPAG, Grenoble, France. ¹⁵German Aerospace Center (DLR), Institute of Space Research, Berlin, Germany. ✉e-mail: jbishop@seti.org</footnote>
and mapping algorithms¹⁰. These remote sensing advances have
uncovered previously unknown outcrops on Mars through analyses
of smaller spot sizes with clearer spectra e.g. ref. 11. Analyses of
CRISM images at the Juventae Plateau and Aram Chaos using these
new techniques revealed additional outcrops of units containing this
unique ~2.23 µm band along with other spectral features
(Figs. 1 and 2). At the Juventae Plateau region, the outcrop containing
the ~2.23 µm band is observed mixed with hydration bands similar to
polyhydrated sulfates, and High Resolution Imaging Science Experi-
ment (HiRISE) images show that thin units containing the 2.23 µm
band occur primarily stratigraphically above the polyhydrated sul-
fate unit and in some cases also below the polyhydrated sulfate unit
(Fig. 1, Supplementary Fig. S1, Supplementary data 1). Because these
units are only 2-3 CRISM pixels wide in many cases (~40–50 m),
spectra of the 2.23 µm band unit likely include a mixture of that phase
and the polyhydrated sulfate unit below it. We analyzed the CRISM
spectra of this 2.23 µm band material in comparison with lab spectra
of hydrated sulfates and several species having bands somewhat
near 2.23 µm. These include montmorillonite (Na₀.₃Al₂(Si₄O₁₀)
(OH)₂·nH₂O) with a band at 2.21 µm, nontronite (Na₀.₃ Fe₂(Si₄O₁₀)
(OH)₂·nH₂O) with a band at 2.29 µm, gypsum (CaSO₄·2H₂O) with a
doublet at 2.22 and 2.26 µm, and jarosite (KFe₃(SO₄)₂(OH)₆) with a
band at 2.26 µm and shoulders at 2.22 and ~2.3 µm. None of these
minerals are consistent with the CRISM spectra of this outcrop hav-
ing a single band near 2.23 µm. In addition to this enigmatic 2.23 µm
band, this outcrop contains features near 1.43, 1.94, and 2.4 µm that
are consistent with polyhydrated sulfates including rozenite
(FeSO₄·4H₂O) and epsomite (MgSO₄·7H₂O) (Fig. 1d).

Outcrops containing a band near 2.23 µm in CRISM spectra are
also observed at several locations at Aram Chaos. The units containing

this unique spectral feature are mapped in red (Fig. 2, Supplementary
Figs. S2, S3, and Supplementary data 1) and occur as small patches, but
span several CRISM pixels, enabling improved isolation of the spectral
properties of this material compared to the Juventae Plateau region.
The 2.236 µm band is narrower in these spectra and is accompanied by
additional sharp features at 1.48, 1.82, 2.19, and 2.37 µm that are
attributed to ferric hydroxysulfate (Fig. 2). In Aram Chaos this 2.23 µm
band material is most commonly observed adjacent to and beneath
monohydrated sulfate outcrops mapped in green. Spectra of the
monohydrated sulfate minerals szomolnokite (FeSO₄·H₂O) and kie-
serite (MgSO₄·H₂O) both have a band at 2.40 µm and another band
near 2.1 µm that varies with composition. This band occurs at 2.09 µm
in spectra of szomolnokite and at 2.13 µm in spectra of kieserite. Some
of the monohydrated sulfate outcrops also include weak bands near
2.23 µm, indicating partial alteration to form a mixed phase containing
some monohydrated sulfate and some Fe³⁺SO₄OH (Fig. 3). The poly-
hydrated sulfate spectra contain bands near 1.44 and 1.93-1.95 µm and a
drop in reflectance near 2.42 µm (Fig. 3), similar to spectra of rozenite
and starkeyite (MgSO₄·4H₂O).

**Formation of Fe³⁺SO₄OH in the lab**

The potential mineral Fe³⁺SO₄OH is formed in the lab by heating dry powders of hydrated ferrous sulfates at temperatures from 100 to 300 °C¹²,¹³. Several Fe³⁺SO₄OH samples were produced in the lab (Table 1) and all include several narrow features due to vibrational bands, overtones, and combinations at 1.49, 1.83, 2.19, 2.236, 2.37, 2.61 and 2.89 µm. Additional Fe electronic absorptions produce a band near 0.94-0.96 µm that is broadened towards ~0.8 µm with a reflectance maximum near 0.7 µm, a drop in reflectance near 0.5 µm, and a narrow band at 0.43 µm, consistent with Fe³⁺ (Table 2, Fig. 3, Supplementary

<figure><img src="image_4.png" /><figcaption>**Fig. 1 |** Spectrally distinct units at the Juventae Plateau. **a** Mars Orbital LASER Altimeter (MOLA) map of equatorial Mars with black boxes indicating locations of the Juventae Plateau and Aram Chaos (red indicates higher elevations and blue lower elevations). **b** View of the plateau above Juventae Chasma with compositional units from CRISM image FRT00005814 showing pyroxene-bearing basalt in green, polyhydrated sulfates in blue, and the Fe³⁺SO₄OH-bearing phase in red over a High-</figcaption></figure>

Resolution Stereo Camera (HRSC) Digital Terrain Model (DTM). c Same CRISM compositional units over a HiRISE DTM (5x vertical exaggeration) with the basalt units split into basalt-1 in dark cyan and basalt-2 in medium green, scale bar for foreground. d CRISM spectra of unusual units with a band near 2.23 µm (top) and lab spectra of several minerals (bottom). Note that none of these minerals are a good spectral match to the CRISM spectra of the Fe³⁺SO₄OH-bearing outcrop.
<figure><img src="image_5.png" /><figcaption>**Fig. 2** | Spectrally distinct units at Aram Chaos. a HRSC view of Aram Chaos indicating locations of CRISM images. b Oblique view with CRISM mineral maps overlain on HRSC with red as Fe³⁺SO₄OH, green as monohydrated sulfate (MHS), and blue as polyhydrated sulfate (PHS) (2x vertical). c Mineral map for CRISM image FRT000098B2. d Oblique view of CRISM image FRT000098B2 overlain on HRSC (2x vertical). e Mineral map for CRISM image HRL0000646A. f Oblique view of CRISM image HRL0000646A overlain on HRSC (2x vertical). g CRISM FRT000098B2 spectra compared with lab spectra of minerals: spectrum 1 (red</figcaption></figure>

diamonds) contains a strong 2.236 µm band, similar to lab spectra of Fe³⁺SO₄OH (black, top); spectrum 2 (green circles) has bands at 2.13 and 2.40 µm, consistent with kieserite (MgSO₄·H₂O, dark gray, center); spectrum 3 (blue squares) is characteristic of PHS, similar to starkeyite (MgSO₄·4H₂O, black, bottom); spectrum 4 (orange circles) has bands at 2.11, 2.225, and 2.40 µm and is attributed to a mixture of Fe³⁺SO₄OH and MHS (note - mapped as green in image due to dominant MHS features); szomolnokite (Fe²⁺SO₄H₂O, light gray, center) spectrum shown for comparison.

data 2). These vibrational and electronic bands are distinct and readily distinguishable from the spectral properties of rozenite and szomolnokite e.g. refs. 6,14. Rozenite is characterized by Fe²⁺ bands near 0.98 and 1.18 µm with a reflectance maximum near 0.56 µm, while the Fe²⁺ band for szomolnokite occurs at ~0.94 µm with a broad reflectance maximum near 0.65–0.75 µm. The HOH stretching overtone occurs at 1.45 µm for rozenite and at 1.52 µm for szomolnokite, while the HOH stretch plus bend combination vibrations occur at 1.95 µm for rozenite

and at 2.09 µm for szomolnokite. The H₂O vibrations observed for rozenite are similar to those observed for many types of polyhydrated sulfates, in contrast to the constrained H₂O sites for the monohydrated sulfates szomolnokite and kieserite that are shifted to longer wavelengths e.g. refs. 15,16. This difference in the position of the H₂O overtones and combination bands in hydrated sulfates enables clear discrimination of monohydrated versus polyhydrated sulfates on Mars⁶. Additional bands are observed for szomolnokite at 2.40 and
<figure><img src="image_6.png" /><figcaption>**Fig. 3 | Properties of ferric hydroxysulfate.** a VNIR reflectance spectra of products formed through heating rozenite, including Fe³⁺SO₄OH as well as mixtures of Fe³⁺SO₄OH with szomolnokite and mixtures of szomolnokite and rozenite. b VNIR reflectance spectra of products formed through heating szomolnokite, including</figcaption></figure>

Fe³⁺SO₄OH and mixtures of Fe³⁺SO₄OH with szomolnokite. c Comparison of lab spectra of Fe³⁺SO₄OH and Mg²⁺Fe³⁺SO₄OH to CRISM spectra from Aram Chaos (Supplementary Fig. S3b); note that dots in the CRISM spectra represent individual CRISM datapoints. d Crystal structures of szomolnokite and ferric hydroxysulfate¹⁷,²¹.

2.62 µm and for rozenite near 2.42 and 2.53 µm. The bands near 2.4 and 2.5 µm are often broadened in the spectra of polyhydrated sulfates such that only a drop in reflectance near 2.4 µm can be detected in remote sensing data.

Reflectance spectra of Fe³⁺SO₄OH were also measured under vacuum (Supplementary Fig. S4) and low-temperature (Supplementary Fig. S5) conditions to investigate changes in this potential new mineral under different environments more relevant to Mars. Overall, Fe³⁺SO₄OH appears stable under these conditions and only small shifts in a couple of bands were observed (Table 2). Additionally, the spectra measured under vacuum are very similar to those measured under controlled, dry conditions, thus providing confidence that purging the sample of H₂O overnight in the chamber before measurement adequately removes adsorbed water molecules.

A synthetic monohydrated sulfate with 50% Mg and 50% Fe²⁺ cations¹⁷ was also heated for comparison with the Fe³⁺SO₄OH produced from heating szomolnokite and rozenite. This dehydrated MgO·₅Fe²⁺·₀.₅·MHS sample contains many bands similar to those of Fe³⁺SO₄OH (Table 2), but shifted slightly, as well as a band near 2.09 µm due to the unreacted MgO·₅Fe²⁺·₀.₅·MHS. Importantly, the OH

combination band observed at 2.236 µm in spectra of Fe³⁺SO₄OH, is shifted to 2.226 µm for Fe³⁺O₅·Mg²⁺O·₅SO₄OH/H₂O. Because the presence of Mg in this sample appears to have disrupted the deprotonation process, we characterized the reaction of szomolnokite to Fe³⁺SO₄OH further.

The Fe³⁺SO₄OH crystal structure includes octahedrally coordinated Fe³⁺O₆ units with tetrahedrally coordinated SO₄ groups on opposite O atoms (-180 degrees apart) and bridging OH groups connecting the Fe³⁺ octahedra to form a linear configuration (Fig. 3c). This is similar to the structure of szomolnokite that has octahedrally coordinated Fe²⁺ instead of Fe³⁺ and two protons on bridging octahedral O atoms to form H₂O molecules instead of OH. Vibrations of these H₂O and OH groups in the structure of szomolnokite and Fe³⁺SO₄OH, respectively, are responsible for the broad H₂O combination band near 2.1 µm and the narrow OH combination band at 2.236 µm. The structure of Fe³⁺SO₄OH and the strength of the bond connecting the OH group to the Fe³⁺ cation govern the vibrational energy of the OH group. The OH fundamental stretching vibration occurs at 3460 cm⁻¹ or 2.89 µm (Table 2, Supplementary Fig S4). Jarosite also has OH groups bound to Fe³⁺ cations in an octahedral configuration, but the OH
<table><caption>Table 1 | Sample identification and descriptions</caption><thead><tr><th>Sample ID</th><th>Name</th><th>Conditions</th><th>XRD</th></tr></thead><tbody><tr><td>JB0626</td><td>Rozenite</td><td>From Iron Mountain<sup>48</sup></td><td>Rozenite</td></tr><tr><td>JB0787</td><td>Copiapite</td><td>From RioTinto<sup>6</sup></td><td>Copiapite</td></tr><tr><td>JB0788</td><td>Mixture copiapite, FeSO<sub>4</sub>OH</td><td>Heated copiapite JB787 from 100 to 300 °C, unstable</td><td>mixture</td></tr><tr><td>JB1771</td><td>MHS with 50% Mg, 50% Fe<sup>2+</sup></td><td>Talla and Wildner<sup>17</sup></td><td>Not measured; assumed similar to JB1874</td></tr><tr><td>JB1781</td><td>Szomolnokite</td><td>Talla and Wildner<sup>17</sup></td><td>Not measured; assumed similar to JB1884</td></tr><tr><td>JB1782</td><td>FeSO<sub>4</sub>OH</td><td>Heated rozenite JB626 from 50 to 300 °C</td><td></td></tr><tr><td>JB1784</td><td>Mixture MgFeSO<sub>4</sub>OH/H<sub>2</sub>O, FeSO<sub>4</sub>OH, and hydrated FeMg sulfate phases</td><td>Heated MHS 50%Fe/50%Mg JB1771 from 50 to 300 °C</td><td>Complex mixture; good fit obtained with 2x hydroxide, 2x mixed (Mg, Fe) phases and refining the lattice parameters.</td></tr><tr><td>JB1855</td><td>Szomolnokite</td><td>Synthesized at 60 °C</td><td>100% Szomolnokite</td></tr><tr><td>JB1856</td><td>FeSO<sub>4</sub>OH</td><td>Heated JB1855 szomolnokite at 250 °C for 22 h</td><td>100% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1857</td><td>FeSO<sub>4</sub>OH</td><td>Heated JB1855 szomolnokite at 200 °C for 26 h</td><td>100% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1858</td><td>Rozenite</td><td>Synthesized at 20 °C</td><td>100% rozenite</td></tr><tr><td>JB1859</td><td>FeSO<sub>4</sub>OH</td><td>Heated JB1858 rozenite at 200 °C for 26 h</td><td>100% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1860</td><td>Fe sulfate mixture</td><td>Heated JB1858 rozenite at 100 °C for 5 days, 19 h</td><td>81% FeSO<sub>4</sub>OH, 19% szomolnokite</td></tr><tr><td>JB1861</td><td>Szomolnokite</td><td>Synthesized at 60 °C</td><td>100% szomolnokite</td></tr><tr><td>JB1863</td><td>FeSO<sub>4</sub>OH</td><td>Szomolnokite JB1861, heated 200 °C at DLR, 3.5 hrs</td><td>95% FeSO<sub>4</sub>OH, 5% szomolnokite</td></tr><tr><td>JB1871</td><td>Mixture rozenite, szomolnokite</td><td>Rozenite heated at 50 °C for 7 days</td><td>90% szomolnokite, 10% rozenite</td></tr><tr><td>JB1874</td><td>MHS with 50% Mg, 50% Fe<sup>2+</sup></td><td>Talla and Wildner</td><td>100% mixed Mg-Fe<sup>2+</sup></td></tr><tr><td>JB1884</td><td>Szomolnokite</td><td>Talla and Wildner</td><td>93% szomolnokite, 7% rhomboclase</td></tr><tr><td>JB1916</td><td>Mixed szomolnokite - FeSO<sub>4</sub>OH</td><td>Heated szomolnokite at 150 °C, 56 h</td><td>7% szomolnokite, 93% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1917</td><td>Mixed szomolnokite - FeSO<sub>4</sub>OH</td><td>Heated szomolnokite at 100 °C, 8 days</td><td>73% szomolnokite, 27% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1918</td><td>Mixed rozenite - szomolnokite</td><td>Burned JB1858 rozenite sample surface with FTIR beam</td><td>5.5% szomolnokite, 94.5% rozenite</td></tr><tr><td>JB1919</td><td>Mixed szomolnokite - FeSO<sub>4</sub>OH</td><td>Heated szomolnokite at 100 °C, 19 days</td><td>62% szomolnokite, 38% FeSO<sub>4</sub>OH</td></tr><tr><td>JB1920</td><td>Butlerite</td><td>FeSO<sub>4</sub>OH rehydration for 1 week at 70% RH and room temperature</td><td>Nearly phase pure butlerite; two unindexed impurity peaks with &lt;1% of maximum intensity.</td></tr></tbody></table>

All mineral abundances in weight %; all ferric hydroxysulfate assumed to contain Fe³⁺ cations; all rozenite and szomolnokite assumed to have Fe²⁺ cations.

stretching vibrations occur near 3385 cm⁻¹ or 2.95 µm for K⁺-jarosite and 3360 cm⁻¹ or 2.98 µm for Na⁺-jarosite¹⁸. Presumably, the additional Na⁺ or K⁺ cations in the jarosite structure are pulling some of the electron density away from the OH bond, thus reducing the vibrational energy and shifting the bands towards longer wavelengths compared to the OH vibrations in ferric hydroxysulfate. In contrast, the rozenite crystal structure¹⁹ (Supplementary Fig. S6) forms isolated clusters of two Fe²⁺O₆ octahedra linked to two SO₄ tetrahedra via O atoms that are ~90 degrees apart in the Fe²⁺ octahedron. For rozenite, the O atoms not connected to sulfate groups are each bound to two protons to form four H₂O molecules per Fe²⁺O₆ octahedron. The colors of the <125 µm grain size powders of these iron sulfates are very pale green for rozenite, white for szomolnokite, and orange for Fe³⁺SO₄OH (Supplementary Fig. S7). Sample JB1917 contains 27 wt.% Fe³⁺SO₄OH and 73 wt.% szomolnokite and is a light tan color (Supplementary Fig. S7). Samples with larger grain sizes have deeper colors (not shown).

Experiments heating szomolnokite, rozenite, melanterite (Fe²⁺SO₄·7H₂O), and copiapite (Fe²⁺Fe³⁺(SO₄)₆(OH)₂·20H₂O) in ambient conditions in the lab resulted in production of Fe³⁺SO₄OH at elevated temperatures⁶⁻⁸,¹³,²⁰. Fe³⁺SO₄OH was produced by heating rozenite or szomolnokite at 200 °C for 26 h (Table 1) or melanterite to 240 °C for 18 h⁷. Rozenite heated at 30 min intervals at 50 °C, 100 °C, then 150 °C produced some Fe³⁺SO₄OH and continued heating at 50 °C intervals to 300 °C resulted in strong Fe³⁺SO₄OH spectral signatures (Supplementary Fig. S8). In contrast, interval heating of copiapite

produced some Fe³⁺SO₄OH by 200 °C and more Fe³⁺SO₄OH by 300 °C, and interval heating of szomolnokite did not result in formation of Fe³⁺SO₄OH until 300 °C (Supplementary Fig. S8). Reaction of polyhydrated sulfates at lower temperatures than szomolnokite suggests that H₂O facilitates the transformation to Fe³⁺SO₄OH.

Slower reaction kinetics were evident at lower temperatures in a series of experiments with rozenite and szomolnokite (see Table 1 for a summary of all Fe sulfate reactions). Szomolnokite heated at 150 °C for 56 hours produced 93 wt.% Fe³⁺SO₄OH. Rozenite heated at 100 °C for 6 days resulted in 81 wt.% Fe³⁺SO₄OH, but only 27 wt.% Fe³⁺SO₄OH resulted after 8 days starting from szomolnokite. Heating of rozenite at 50 °C resulted in dehydration of ~90% of the sample to szomolnokite after 7 days and heating of the rozenite sample by the FTIR beam in wide aperture mode resulted in partial formation of szomolnokite at the surface. No evidence of Fe³⁺SO₄OH was observed for these lower temperature reactions.

Additional heating experiments of szomolnokite in sealed vials containing nitrogen gas or air demonstrated that oxygen is required for reaction of these hydrated ferrous sulfates to form ferric hydroxysulfate¹³ (Supplementary S1/Supplementary Table 1). Gas pressures were recorded at the beginning and end of each experiment. Initial gas pressures ranged from 1.29 to 1.36 bars and were decreased in the reactions where O₂ was consumed and Fe³⁺SO₄OH was formed. Characterization of the solid phase by X-ray diffraction and the gas phase by gas chromatography before and after the heating experiment
<table><caption>Table 2 | Ferric hydroxysulfate spectral bands</caption><thead><tr><th>Sample ID</th><th>Sample description</th><th colspan="10">Wavelengths of spectral bands in µm</th></tr></thead><tbody><tr><td colspan="11">Measured at RELAB, Brown University, under controlled dry conditions</td></tr><tr><td>JB1856</td><td>Heated szomolnokite 250 °C</td><td>0.430</td><td>0.955</td><td>1.488</td><td>1.827</td><td>1.966</td><td>2.191</td><td>2.236</td><td>2.369</td><td>2.609</td><td>2.887</td></tr><tr><td>JB1857</td><td>Heated szomolnokite 200 °C</td><td>0.430</td><td>0.955</td><td>1.486</td><td>1.827</td><td>1.966</td><td>2.191</td><td>2.236</td><td>2.369</td><td>2.608</td><td>2.884</td></tr><tr><td>JB1859</td><td>Heated rozenite 200 °C</td><td>0.430</td><td>0.940</td><td>1.486</td><td>1.827</td><td>1.962</td><td>2.191</td><td>2.236</td><td>2.369</td><td>2.607</td><td>2.885</td></tr><tr><td>JB1784</td><td>Heated MgO.5FeO.5-MHS 300 °C</td><td>0.425</td><td>0.935</td><td>1.526</td><td>1.822</td><td>2.087</td><td>2.090</td><td>2.228</td><td>2.399</td><td>2.609</td><td>2.882</td></tr><tr><td colspan="11">Measured under vacuum at PSL, DLR-Berlin</td></tr><tr><td>JB1856</td><td>initial</td><td></td><td></td><td>1.487</td><td>1.827</td><td>1.969</td><td>1.987</td><td>2.190</td><td>2.236</td><td>2.367</td><td>2.609</td><td>2.884</td></tr><tr><td>JB1856</td><td>after 15 minutes</td><td></td><td></td><td>1.486</td><td>1.827</td><td>1.971</td><td>1.987</td><td>2.190</td><td>2.236</td><td>2.367</td><td>2.609</td><td>2.884</td></tr><tr><td>JB1856</td><td>after 30 minutes</td><td></td><td></td><td>1.486</td><td>1.827</td><td>1.974</td><td>1.987</td><td>2.190</td><td>2.236</td><td>2.367</td><td>2.609</td><td>2.884</td></tr><tr><td colspan="11">Measured under vacuum and decreasing temperatures at IPAG, University of Grenoble</td></tr><tr><td>JB1856</td><td>293 K</td><td>0.43</td><td>0.95</td><td>1.49</td><td>1.825</td><td>1.96</td><td>2.19</td><td>2.235</td><td>2.37</td><td>2.605</td><td>2.89</td></tr><tr><td>JB1856</td><td>273 K</td><td>0.43</td><td>0.96</td><td>1.49</td><td>1.825</td><td>1.97</td><td>2.19</td><td>2.235</td><td>2.37</td><td>2.605</td><td>2.89</td></tr><tr><td>JB1856</td><td>243 K</td><td>0.43</td><td>0.96</td><td>1.49</td><td>1.825</td><td>1.97</td><td>2.19</td><td>2.235</td><td>2.37</td><td>2.605</td><td>2.89</td></tr><tr><td>JB1856</td><td>213 K</td><td>0.43</td><td>0.97</td><td>1.49</td><td>1.825</td><td>1.97</td><td>2.19</td><td>2.235</td><td>2.37</td><td>2.605</td><td>2.89</td></tr><tr><td>JB1856</td><td>189 K</td><td>0.43</td><td>0.97</td><td>1.49</td><td>1.825</td><td>1.97</td><td>2.19</td><td>2.235</td><td>2.37</td><td>2.605</td><td>2.89</td></tr></tbody></table>

indicates that the formation of Fe³⁺SO₄OH proceeds via the reaction in Eq. (1).

$$4 \text{Fe}^{2+} \text{SO}_4 \cdot \text{H}_2\text{O} + \text{O}_2 \rightarrow 4 \text{Fe}^{3+} \text{SO}_4\text{OH} + 2 \text{H}_2\text{O} \quad (1)$$

Rietveld refinement supports formation of C2/c monoclinic Fe³⁺SO₄OH (Fig. 4, Supplementary Fig. S9). Comparison of the crystal structures of Fe³⁺SO₄OH and szomolnokite¹⁷,²¹ suggests that the transformation is associated with a loss of a hydrogen atom bonded to the bridging oxygen likely occurring coincidently with the oxidation of Fe²⁺ to Fe³⁺. The structure remains largely intact throughout the transformation; however, the polyhedral units rotate and distort to accommodate the changes in charge distribution (Fig. 3c), where the increased charge on the Fe atom is compensated by the negative charge on the hydroxyl group. Heating experiments with kieserite were not successful in creating a stable hydroxide phase, likely because Mg does not exist in trivalent form. This is also reflected in the mixed cation monohydrated sulfate samples heated at 300 °C that only showed incomplete transformation to the hydroxide phase.

## Morphologies of ferric hydroxysulfate and associated units

Evaluating the morphologies and stratigraphies of the sulfate outcrops at the Juventae Plateau and Aram Chaos enables the determination of relationships among the different sulfate units. Spectra at the Juventae Plateau were collected from -30 to 35 m thick light-toned layered deposits that exhibit spectral signatures consistent with polyhydrated sulfates and Fe³⁺SO₄OH, plus pyroxene-bearing units (Figs. 1 and 5, Supplementary Fig. S1). Thin units containing spectral features consistent with polyhydrated sulfates and Fe³⁺SO₄OH are mapped in blue and red, respectively (Figs. 1 and 5). The stratigraphy of the outcrops shows a pyroxene-bearing substrate below the light-toned layered materials termed basalt-1 (dark cyan) and a different pyroxene-bearing caprock unit covering the Fe sulfates termed basalt-2 (medium green) (Supplementary Fig. S1). CRISM spectra of the basalt-1 material include spectral properties most similar to augite (high-Ca pyroxene) with a reflectance maximum near 1.65 µm and a band centered near 2.35 µm, while the basalt-2 spectra likely contain some pigeonite (pyroxene enriched in Mg and Fe) due to shifts in the reflectance maximum and the ~2 µm band towards shorter wavelengths. Morphologies of these primary four units indicate that the pyroxene bearing substrate (dark cyan in both CRISM and HiRISE color) is flatter and appears clean (i.e., minimal dust and sand) with extensive polygonal fracturing, whereas the pyroxene-bearing caprock (medium green in CRISM and brown in HiRISE color) is partially composed of linear aeolian ripples and

appears hilly and uneven in topography due to differential erosion. The textures of the polyhydrated sulfate and Fe³⁺SO₄OH/polyhydrated sulfate units are distinct from those of the pyroxene-bearing units, but appear similar to each other with fine-scale layering that varies in brightness, color, and fracturing. The Fe³⁺SO₄OH unit is observed overlying the polyhydrated sulfate units in the stratigraphy and in some cases it is also present along the base of the light-toned materials as well. Thus, there may have been two different times when the Fe³⁺SO₄OH unit formed.

At Aram Chaos, monohydrated sulfates are the most abundant sulfate in the two CRISM images where Fe³⁺SO₄OH is present and are mapped in green (Figs. 2 and 6). The monohydrated sulfate units also exhibit variability in the band near 2.1 µm and are consistent with mixtures of szomolnokite and kieserite in some locations and of pure kieserite in other locations (Fig. 6, Supplementary Figs. S2, S3). Several patches of Fe³⁺SO₄OH are observed adjacent to and beneath the monohydrated sulfate outcrops, with some closer to kieserite-type outcrops and others next to mixed monohydrated sulfate outcrops. Variations are also observed in the polyhydrated sulfate units that are mapped in blue, with some outcrops exhibiting stronger hydration signatures. The stratigraphy indicates that polyhydrated sulfate units are observed at higher elevations than the monohydrated sulfates and Fe³⁺SO₄OH units, and in some cases pyroxene-bearing basalt is observed below the sulfates within the chaos blocks, although it is much rarer than the basalt units at the Juventae Plateau.

The Fe³⁺SO₄OH unit occurs along the floor of Aram Chaos and is stratigraphically the lowest sulfate unit relative to the monohydrated and polyhydrated sulfates. The Fe³⁺SO₄OH unit is medium-toned in brightness and exhibits polygonal fracturing and ridges, which can be either circular or linear, compared to the much brighter and scalloped fracturing observed in the monohydrated sulfate units. The monohydrated sulfate units can also be more jagged and hilly compared to the flatter and subdued morphology of the Fe³⁺SO₄OH unit. The only difference in morphology between the kieserite-type outcrops and the szomolnokite-kieserite outcrops is that the kieserite-type outcrops are slightly darker and appear mantled by loose debris and aeolian ripples. The polyhydrated sulfate unit is generally medium-toned in brightness relative to the other sulfates, which may be due to the widespread ripples and debris covering its surface. The unit is relatively flat, forming broad plateaus, but steeper cliffs along specific layers within the polyhydrated sulfate unit correspond to bright-toned, heavily fractured, and jagged outcrops. The morphology of the Fe³⁺SO₄OH and polyhydrated sulfate units are much easier to distinguish at Aram Chaos relative to the same units at Juventae Chasma.
<figure><img src="image_7.png" /><figcaption>**Fig. 4** | Rietveld refinements of the heating products of ferrous sulfates. **a** Rozenite. **b** Szomolnokite. Black lines with markers are the measured data, red solid lines are the overall fit, and dashed lines are contributions of each of the individual mineral phases to the overall fit.</figcaption></figure>

Discussion

Exposures of this unusual iron sulfate phase with spectral features at 2.236 µm at Aram Chaos closely resemble pure Fe³⁺SO₄OH formed in the lab, whereas the thinner units on the Juventae Plateau are either mixed with other components or represent incompletely formed Fe³⁺SO₄OH phases. This ferric hydroxysulfate is currently associated with monohydrated sulfate outcrops at Aram Chaos, although polyhydrated sulfate outcrops are also present nearby. In contrast, primarily polyhydrated sulfate outcrops are currently observed on the Juventae Plateau.

Small variations in the ~2.23 µm band are attributed to changes in the Fe-Mg chemistry. Although the majority of spectra have a band centered at 2.236 µm, some exhibit a band centered at 2.225 µm instead

(Supplementary Figs. S2, S3, Supplementary data 1, 2). Pure Fe³⁺SO₄OH has a band at 2.236 µm, while heated FeMg- monohydrated sulfate has a band at 2.226 µm. The Fe³⁺SO₄OH units at Aram Chaos are typically found adjacent to and beneath monohydrated sulfate outcrops (Fig. 2), including both Fe-rich monohydrated sulfate (similar to szomolnokite) with bands near 2.11–2.12 and 2.40 µm and kieserite (MgSO₄·H₂O) with spectral bands near 2.13 and 2.41 µm. However, spectra of szomolnokite and kieserite measured at colder, Mars-like temperatures have bands at ~2.11 and 2.14 µm, respectively²², similar to these observations. The monohydrated sulfate spectral units more consistent with kieserite are darker than the szomolnokite-like monohydrated sulfate units and are covered by debris and ripples (Figs. 2c and 5d, e). Some of the monohydrated sulfate units at Aram
<figure><img src="image_8.png" /><figcaption>**Fig. 5** | Morphologies of Geologic Units on the Juventae Plateau. **a** Mineral map from CRISM image FRT00005814 overlain on HiRISE DTM with red assigned to the Fe³⁺SO₄OH-bearing phase, blue to polyhydrated sulfate, dark cyan to pyroxene-bearing basalt-1, and medium green to pyroxene-bearing basalt-2. **b** View of</figcaption></figure>

zoomed in region from white box in (a). **c** View of enhanced color HiRISE image for same region shown in (b). **d-g** HiRISE enhanced color blowups. **d** Morphology of Fe³⁺SO₄OH-bearing unit. **e** Morphology of polyhydrated sulfate unit. **f** Morphology of pyroxene/basalt-1 unit. **g** Morphology of pyroxene/basalt-2 unit.

Chaos may have formerly been mixtures of szomolnokite and kieserite, where the szomolnokite transformed to FeSO₄OH and the kieserite remained. Alternatively, polyhydrated Fe and Mg sulfates may have been present that were altered to form Fe³⁺SO₄OH, szomolnokite, and kieserite, depending on variations in the geochemistry and temperatures.

The presence of pure Fe³⁺SO₄OH outcrops adjacent to mono-hydrated sulfate at Aram Chaos and less pure outcrops of Fe³⁺SO₄OH next to and mixed with polyhydrated sulfate on the plateau NW of Juventae Chasma indicate an active geochemical history in Mars' past (Fig. 7). The hydrated sulfates may have formed in sulfate brine environments and could have formed in cold or moderate waters, but the Fe³⁺SO₄OH materials would have required elevated temperatures to form. The Fe³⁺SO₄OH-bearing units at the Juventae Plateau are mixed with polyhydrated sulfate at the scale of CRISM measurements (18 m/pixel) and could also be mixed with spectrally neutral components (altered ash) that dilute the Fe³⁺SO₄OH spectral features. Interestingly, light-toned layered deposits, sinuous landforms, and inverted channels exist across a wider area on the Juventae Plateau²³,²⁴. However, the sulfates are only observed in the smaller region of the plateau at a slightly lower elevation, located between the two chasma walls that drop steeply in elevation (Supplementary Fig. S10). Sulfates may exist in other regions of the plateau below the detection limits of CRISM or below the caprock. The inverted channels and drainage features indicate fluvial activity on the Juventae Plateau that could have been associated with the sulfates. Darker regions in THEMIS nighttime images (Supplementary Fig. S10b) represent lower thermal inertia, indicating highly porous, fine-grained materials²⁵. These features are consistent with deposition of altered material at the lowest elevation where the sulfates are located. Additional blended CRISM-HiRISE views

illustrate the stratigraphy of the sulfate units sandwiched in between the upper basalt-2 and lower basalt-1 units (Fig. 8).

Models of formation mechanisms of the two different forms of the Fe³⁺SO₄OH outcrops observed at the two different locations studied here (Fig. 7) illustrate potential pathways that could have taken place on Mars. At the Juventae Plateau, formation or deposition of polyhydrated sulfates occurred on top of basaltic material that was likely derived from volcanic ash or lava. The sulfates could have formed through alteration of volcanic ash in S-rich brines. Later emplacement of a different basaltic material (basalt-2) on top of the sulfates likely covered the area. Heat from this upper basalt-2 unit, assuming it was a hot volcanic flow or ash, could then have induced reaction of the hydrous Fe²⁺ sulfates to partially alter to Fe³⁺SO₄OH at spatial scales of -20–50 m observed in the upper Fe³⁺SO₄OH unit. The majority of the Fe³⁺SO₄OH/polyhydrated sulfate unit lies stratigraphically above the polyhydrated sulfate unit, indicating the hot upper basalt-2 unit could have been the heat source for reaction of the polyhydrated sulfate. The basal Fe³⁺SO₄OH unit observed in some locations may have formed through geothermal heating or still warm lavas of the basalt-1 unit. Because only weak traces of monohydrated sulfate features are observed, temperatures were likely higher, perhaps over 200 °C. Wind erosion removed portions of the upper basalt-2 unit over time exposing the Fe³⁺SO₄OH/polyhydrated sulfate, polyhydrated sulfate, and basalt-1 units below. Small amounts of the FeSO₄OH/polyhydrated sulfate material may have mass wasted down to borders of the polyhydrated sulfate and basalt-1 units in certain areas, or some of the polyhydrated sulfate unit may have been altered by a geothermal source below this site. Light-toned layered deposits are associated with inverted channels, valley formations, and sulfate-bearing outcrops in or near many of the chasma surrounding Valles
<figure><img src="image_9.png" /><figcaption>**Fig. 6** | Morphologies of sulfate units at Aram Chaos. a Mineral map from CRISM image FRT000098B2 over MOLA with red assigned to the ~2.23 µm phase, blue to PHS-1, and green to MHS-1. b Mineral map from CRISM image FRT000098B2 with red assigned to the ~2.23 µm phase, blue to PHS-2, and green to MHS-2 (kieserite). c CRISM mineral map from (a) overlain on two HiRISE images with yellow boxes marking locations of views in (d, e). d Morphology of Fe³⁺SO₄OH and MHS-1 units. e Morphology of Fe³⁺SO₄OH and MHS-2 (kieserite) units. f Mineral map from CRISM image HRL0000646A with red assigned to the ~2.23 µm phase, blue to PHS-1, and green to MHS-1. g Mineral map from CRISM image HRL0000646A with red assigned to the ~2.23 µm phase, blue to PHS-2, and green to MHS-2 (kieserite). h Morphologies of Fe³⁺SO₄OH and MHS-1 units. i Morphologies of Fe³⁺SO₄OH, MHS-2 (kieserite), and PHS units.</figcaption></figure>

Marineris²⁶ and could include additional locations where ferric hydroxysulfate might be present.

At Aram Chaos the Fe³⁺SO₄OH unit is observed at the base of the hydrated sulfates, just above the bedrock (Fig. 6) as noted previously⁷. The chaos bedrock materials in this region likely formed through melting of subsurface ice or drainage of subsurface waters that produced catastrophic groundwater outflow and associated surface collapse e.g. ref. 27, about 3 Ga²⁸. Subsequent aqueous activity produced the layered sedimentary material²⁹, later identified as sulfates⁷. These hydrated sulfates are proposed to have formed through multiple wetting events including groundwater recharge and evaporation⁷. Finally, hydrothermal activity at this site could have been responsible for heating of the polyhydrated sulfate materials to form the monohydrated sulfate and Fe³⁺SO₄OH units (Fig. 7). The observation of ferric hydroxysulfate in some regions of Aram Chaos, primarily surrounded by and beneath monohydrated sulfate, that are partially covered by polyhydrated sulfates is consistent with heating from below, and a geothermal hot spot at Aram Chaos could have been responsible for formation of the sulfate outcrops observed today. Geothermal activity in the chaos region may have also been responsible for formation of its morphology, e.g. ref. 30.

Determining a temperature for these hydrothermal sulfate reactions is difficult given the potential for long geologic timescales. Lab heating experiments at 100 °C observed slow formation of Fe³⁺SO₄OH from powders of both rozenite and szomolnokite, although the reactions proceeded faster from rozenite. This is presumably because of the additional H₂O molecules present in the rozenite structure that facilitate the reaction. Lab experiments heating rozenite at 50 °C did

not produce Fe³⁺SO₄OH after several days. Additionally, szomolnokite crystals synthesized over 35 years ago³¹ and stored at room temperature in Earth's oxygen-rich atmosphere remain well preserved, suggesting that the transformation to Fe³⁺SO₄OH does not occur under present-day martian atmospheric conditions, which are both oxygen-poor and low in temperature. Precipitation of Fe³⁺SO₄OH was found to occur via slow formation from a saturated aqueous iron sulfate solution at 120 °C after several weeks³², but was not observed to form in highly concentrated sulfuric acid solutions at temperatures up to 210 °C¹⁷. The reason for this is likely that Fe³⁺SO₄OH is not a dehydrated form of szomolnokite, but rather it is formed via coordinated deprotonation and oxidation reactions. Thus, aqueous precipitation of Fe³⁺SO₄OH is not expected at low temperatures and is only sluggish at elevated temperatures. Further, it would be difficult to explain the observed stratigraphy of sulfates at Aram Chaos if Fe³⁺SO₄OH had formed via aqueous precipitation. Together, this reasoning suggests that the hydrothermal event at Aram Chaos that formed Fe³⁺SO₄OH reached temperatures above 50 °C. The largest support for our formation model at Aram Chaos, based on the burial and subsequent exhumation of Fe²⁺ sulfate hydrates, is the stratigraphic context of the Fe³⁺SO₄OH phase observed from orbit. Potentially, long-term heating of rozenite on Mars could have occurred at lower temperatures to form the szomolnokite and then shorter-term hydrothermal events produced the Fe³⁺SO₄OH phase. Wind erosion at Aram Chaos produced wide corridors where the bright sulfate-rich materials were removed³³. The ~2.23 µm spectral feature is only observed in sites where erosion was sufficiently significant to expose the lowermost chaos floor⁷.

Sulfate outcrops including associated polyhydrated sulfate and monohydrated sulfate units are common throughout the greater Valles Marineris region¹ and the ~3 Ga chaotic terrains associated with outflows from Chryse Basin³⁴,³⁵. The hydrated sulfates at Aram Chaos and the Juventae Plateau must have formed after these chaotic terrains and the ferric hydroxysulfate would have formed even more recently, perhaps during the Amazonian period (<3 Ga). Other sulfate-bearing regions in the chaotic terrains may also contain small outcrops of ferric hydroxysulfate that have not yet been well characterized. Sowe et al.³⁶ observed a narrow band near 2.23 µm in Aureum Chaos (west of Aram Chaos) that they interpreted as a jarosite-like ferric sulfate phase and may actually be Fe³⁺SO₄OH. This current study highlights the active nature of the martian surface, where geothermal processes altered hydrated materials more recently than formerly realized.

## Methods

### CRISM image analysis

CRISM images can be found at the Planetary Data System (PDS) Geosciences server under Mars Orbital Data Explorer: https://ode.rsl.wustl.edu/mars/. We used CRISM images FRT00005814, FRT000098B2, and HRL0000646A for this study. Mineral outcrops were investigated using advanced calibration versions⁹ of these CRISM images that provide improved resolution of small outcrops and offer cleaner spectra that facilitate identifying specific minerals. This algorithm includes simultaneous atmospheric correction and denoising of CRISM images in the 1.0–2.6 µm spectral range that removes most of the residual atmospheric bands and spurious noise⁹.

We applied a recently developed algorithm that leverages the features learned by Generative Adversarial Networks (GAN) trained on hyperspectral data from CRISM¹⁰. This feature set has shown the ability to discriminate among different spectral shapes. CRISM maps of pyroxene, polyhydrated sulfates, monohydrated sulfates, Fe³⁺SO₄OH, and a mixed phase containing a ~2.23 µm band and polyhydrated sulfate-type features were used to guide acquisition of spectra for minerals in the scene. Spectra were collected without ratioing the spectra of different spot sizes. For the thin polyhydrated sulfate and mixed
a Juventae Plateau

<figure><img src="image_10.png" /><figcaption>2. Pyroxene-bearing basalt-2 emplaced on top of sulfate unit</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>3. Sulfate unit altered by hot volcanics or other heat sources</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>4. Erosion reveals sulfates and basalt-1 below surface</figcaption></figure>

<figure><img src="image_13.png" /></figure>

**Fig. 7** | Diagrams illustrating potential formation models for Fe³⁺SO₄OH. **a** This model outlines formation or deposition of polyhydrated sulfates on top of basaltic ash or lava, then emplacement of a different basaltic material on top of the sulfates. Heat from this upper basalt-2 unit then induces reaction of hydrous Fe²⁺ sulfates to partially alter to Fe³⁺SO₄OH at spatial scales of 20–50 m at the Juventae Plateau. Heat from the lower basalt-1 unit or another source heated the

b Aram Chaos

<figure><img src="image_14.png" /><figcaption>2. Geothermal heat alters polyhydrated sulfates, forming dehydrated sulfates</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>3. Erosion reshapes surface units</figcaption></figure>

<figure><img src="image_16.png" /></figure>

lower portion of the hydrous Fe²⁺ sulfates as well to form Fe³⁺SO₄OH in some places. **b** The second model begins similarly with formation or deposition of polyhydrated sulfates on top of a mafic unit at Aram Chaos, followed by geo-thermal heating of the region to form a combination of monohydrated sulfates and Fe³⁺SO₄OH, where Fe³⁺SO₄OH forms at higher temperatures, closer to the heat source.

<figure><img src="image_17.png" /></figure>

**Fig. 8** | 3D views of sulfate-bearing regions on Juventae Plateau. **a** CRISM parameters over HiRISE with expanded view compared to Fig. 1c with 3x vertical exaggeration. Basalt-1 is shown in dark green, polyhydrated sulfate in blue, the Fe³⁺SO₄OH-bearing phase in red, and basalt-2 in medium green. **b** Different view with same colors as (a).

polyhydrated sulfate/Fe³⁺SO₄OH units at the Juventae Plateau 3 × 3 pixel regions were used, while larger 5 × 5 and 10 × 10 pixel regions were used for the pyroxene outcrops at the Juventae Plateau and most regions at Aram Chaos. CRISM spectra collected in this way and used for this study are available in Supplementary data 1.

**Blended views of CRISM, HiRISE, CTX, and HRSC imagery**

CRISM mineral detections are merged with Context (CTX) and HRSC images over HRSC DTMs for regional stratigraphic views and over HiRISE images and DTMs for local stratigraphic views. ArcGIS software (ESRI) was used to overlay CRISM mineral maps over Mars
Reconnaissance Orbiter (MRO) HiRISE images at ~30 cm/pixel surface resolution³⁷ with coordinated mosaics of MRO CTX images at ~6 m/pixel surface resolution³⁸ and Mars Express HRSC images at ~10 m surface resolution³⁹,⁴⁰. The HRSC DTM has a grid size of 50 m and the HiRISE DTM has a grid size of 1 m. The HiRISE DTM was generated using methods developed by Kirk et al.⁴¹ and McEwen et al.⁴².

## Lab experiments with hydrous ferrous sulfates

Synthesis of the rozenite⁴³ and szomolnokite⁴⁴ samples were loosely based on previous methods. Rozenite was formed by the dehydration of fine-grained FeSO₄·7H₂O reagent powder at room temperature (~22 °C) in a dry (i.e., 33% relative humidity) atmosphere maintained by a saturated MgCl₂ solution⁴⁵ in air over 7 days. Szomolnokite was obtained by dehydrating FeSO₄·7H₂O reagent powder over four days at 60 °C using the same humidity-buffer conditions. These samples are described in Table 1.

The phase identity and purity of the obtained sample materials and their heating products were confirmed by X-ray diffraction (Fig. 4, Supplementary Fig. S9). X-ray diffraction patterns were acquired on a Rigaku Smartlab diffractometer in Bragg-Brentano mode using Cu-Kα₁,₂ radiation and D/teX Ultra 250 1D silicon strip detector. Diffraction patterns were acquired in the 5–65° 2θ range with a step size of 0.01° and a maximum scan speed of 1.5°/min. The instrument profile parameters were derived from a NIST 640c silicon standard and all Rietveld refinements were performed using GSAS-II⁴⁶. The Rietveld refinements for the heating products of rozenite and szomolnokite are shown in Fig. 4.

Heating experiments were performed at temperatures of 50, 100, 200 °C for rozenite, and 100, 150, 200 and 250 °C for szomolnokite. The compositions of these samples are listed in Table 1.

## Heating experiments in closed vials containing nitrogen gas and air

1.0 g of szomolnokite, synthesized and analyzed for phase purity following the workflow detailed above, were transferred into four individual vials of a volume of 155 ml. Three vials (two containing mineral, one empty control) were sparged with nitrogen gas for 2 min, whereas the other three contained ambient air as a top gas. The vials were then closed with a rubber stopper and sealed.

Using a Keller LEO2 manometer adapted to fit a 25 G disposable needle for insertion through the rubber stopper into vials, we observed a drop in pressure inside the vial during preliminary experiments, thus 50 ml of the respective top gas were injected into the vials using a 100 ml gas-tight Hamilton syringe before heating to end up with a pressure exceeding ambient atmospheric pressure. This allowed for gas sampling and analysis of O₂ composition at the end of the experiment. The vial pressures were measured before and after the heating cycle using the manometer. This was followed by the measurement of the oxygen concentration in the air vials on an SRI 8610C Gas Chromatographer (GC) equipped with a HaySepD column set to a temperature of 80 °C and a thermal conductivity detector. Nitrogen was used as the carrier gas with a flow rate of 29 ml/min. A quantification curve for oxygen was built by performing triplicate injections with 50 µl and 250 µl Hamilton gas-tight syringes fitted with removable 22 s gauge needles of 10, 25, 50, 100, and 250 µl volumes of a research-grade (99.999% purity) oxygen standard stored in a 155 ml serum bottle to emulate experimental conditions. This achieved standard concentrations of 4, 10, 20, 40 and 100% O₂ respectively. 250 µl of the experimental vial top gas was sampled in triplicate for analysis on the GC. The analytical error for this analysis is <1% and the experiment details are summarized in Supplementary S1/Supplementary Table 1.

The samples were heated on a hot plate by increasing the temperature by 50 °C every 10 min from 100 to 250 °C, followed by 20 min each at 280, 300 °C and 30 min at 320 °C. The same holding times and temperature steps were used when the hot plate temperature was decreased to 100 °C. Then the hot plate was turned off with the vials

still on the hot plate for 10 min, after which they were removed to allow them to cool down to room temperature prior to measuring the vial pressure.

This was followed by another quantification of the oxygen-concentration in the air-filled vials using the workflow stated above. Lastly, all vials were opened, and the solid heating product was analyzed by means of X-ray diffraction on the Rigaku Smartlab diffractometer using the data acquisition parameters described in detail in the previous section.

## Reflectance spectroscopy measurements of Fe³⁺SO₄OH

**SETI institute.** Spectra were collected from 0.35 to 2.5 µm relative to Spectralon under ambient conditions using an Analytical Spectral Devices (ASD Inc; now part of Malvern Panalytical) FieldSpecPro FR instrument to check progress of the reactions in our experiments. The spectral resolution is 3 nm from 0.35 to 1.0 µm and 10 nm from 1.0 to 2.5 µm. Spectra were acquired with a contact probe on particulate samples in a black Teflon dish.

**Reflectance Experiment Laboratory (RELAB) at Brown University.** Spectra were measured under dry air with a diffuse gold standard from ~1 to 100 µm using a Thermo Nexus 870 FTIR instrument equipped with a tungsten-halogen lamp and a CaF₂ beam splitter (NIR), a globar source, a KBr beam splitter (MIR-FIR), and Deuterated TriGlycine-Sulfate (DTGS) detectors with spectral resolutions of 2 cm⁻¹ (NIR) and 4 cm⁻¹ (MIR-FIR) as in recent studies, e.g. ref. 47. The samples were placed in the sample chamber for ~12 h under N₂ in order to remove H₂O and CO₂ adsorbed on the surface of the grains or in the air above them, e.g. refs. 48,49. These data were scaled near 1.2 µm to bidirectional VNIR spectra recorded from 0.3 to 2.5 µm relative to Spectralon at 5 nm spectral sampling under ambient conditions as in previous studies, e.g. ref. 50. The spectrum of rozenite burnt by the FTIR beam was achieved using the globar source with an aperture size of 150 for 90 min. Only the upper few grains of the sample surface were affected. The change in spectral features of the rozenite sample including formation of bands consistent with szomolnokite occurred together with a darkening spot on the surface of the sample. Spectral measurements of rozenite samples with an aperture size of 55 or smaller did not change color or spectral properties. RELAB spectra of the samples discussed in this study are included in Supplementary data 2.

**Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) at the University of Grenoble Alpes (UGA).** Spectra were measured from ~0.4 to 4.2 µm, relative to Spectralon below 1.3 µm and Infragold above that, using the SHINE bidirectional spectro-goniometer⁵¹. All spectra have been acquired with nadir illumination and an emission angle of 20°. Spectral sampling was 10 nm with a spectral resolution varying between 5 and 38 nm. Measurements were carried out within the CARBONIR environmental chamber under 10⁻⁴ mbar vacuum at room temperature and under 12 mbar pure N₂ gas at lower temperatures down to 189 K at 30 K intervals, as in previous studies⁵²,⁵³.

**Planetary Spectroscopy Laboratory (PSL) at the German Aerospace Center (DLR) in Berlin.** Spectra were measured from ~1 to 25 µm using a Bruker FTIR instrument under 10⁻³ torr vacuum relative to a rough gold surface, as in previous studies⁵².

## Data availability

The CRISM, CTX, and HiRISE images used in this study are available on the Planetary Data System (PDS) Geosciences Node under "Mars Reconnaissance Orbiter" at https://ode.rsl.wustl.edu/mars/productsearch. The spectral data created for this study are provided as xls files as part of the supplementary information. Supplementary data 1 includes CRISM spectra of Mars shown in Supplementary Figs. S1-S3. Supplementary data 2 includes lab spectra of minerals and Fe³⁺SO₄OH
used for comparison with the CRISM spectra. These data are also inclu-
ded at https://ahed.nasa.gov/datasets/b425b97a4c6d28c04f87d5b49f36.
Additional mineral spectra are available at the RELAB spectral library
(https://speclib.rsl.wustl.edu/search.aspx?catalog=RELAB) and the USGS
spectral library (https://www.usgs.gov/labs/spectroscopy-lab/science/
spectral-library).

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## Acknowledgements

The authors are grateful to the CRISM, HiRISE, CTX, and HRSC teams for collecting and archiving the images used in this study, the Planetary Spectroscopy Laboratory (PSL) at the DLR-Berlin, the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) at UGA and to G. Alemanno, O. Poch, and K. Wilk for assistance with the spectral measurements, and to T. Hoehler and M. Kubo for providing lab space, resources, and assistance with the GC measurements. Support from NASA SSW grant #80NSSC23K0032 (J.L.B., M.R.D.G., T.F.B., B.L.), NASA MDAP grants #80NSSC21K1103 (J.L.B., C.M.W., M.P., A.M.S., Y.I., M.R.D.G.) and 80NSSC23K1074 (C.M.W., J.L.B.), Austrian Science Fund (FWF) project #P34227-N (D.T., M.W.), Swedish Research Council project (V.R.) 2021-05859 (M.Y.), NASA postdoctoral program fellowships (J.M.M., A.E.G.H.), and the EU Europlanet TA program (J.L.B., M.Y., B.S., A.M.) are greatly appreciated. Europlanet 2024 RI has received funding from the European Union's Horizon 2020 research and innovation

programme under grant agreement No 871149. RELAB is a multiuser facility supported by the NASA PSEF program (T.H.). The Open Planetary Science Initiative and AHED thank NASA's Planetary Sciences Division for its continued support (T.F.B., B.L.).

## Author contributions

Author roles: J.L.B. conceived of the study, conducted a large part of the analyses, prepared most figures, and wrote most of the text. J.M.M. prepared most samples, conducted most XRD analyses, created some figures, and wrote part of the text. D.T. prepared some samples and participated in sample analysis. A.E.G.H. led the reactions under controlled environments. J.L.B., M.Y., T.F.B. & B.L. contributed to initial heating experiments and sample analysis. M.W. assisted with sample synthesis, sample analysis, and figure preparation. T.H. measured reflectance spectra of all samples at RELAB. M.Y. & B.S. conducted the low-temperature VNIR experiments. A.M. & J.L.B. conducted the vacuum FTIR measurement. M.P., A.M.S. & Y.I. prepared the improved CRISM images and constructed the mineral maps. M.R.D.G. & J.L.B. analyzed the CRISM images, collected CRISM spectra and compared them to spectra of minerals. C.M.W. analyzed the HiRISE images, prepared blended CRISM-HiRISE views and wrote part of the text. C.G. analyzed the HRSC images and prepared the blended CRISM-HRSC views. J.L.B., M.A. & C.M.W. developed the formation models. All authors provided comments and/or revisions to the paper.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41467-025-61801-2.

**Correspondence** and requests for materials should be addressed to J. L. Bishop.

**Peer review information** *Nature Communications* thanks Kathleen Benison, Megan Elwood Madden and Clément Royer for their contribution to the peer review of this work. A peer review file is available.

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