Using Spectroscopy to Measure Geochemical Transformations of Gypsum for Ca-Sulfate Detection on Mars

Research was conducted exploring the dehydration pathways of gypsum (CaSO₄·2H₂O) and its interactions with chloride (Cl) salts under a range of thermal and environmental conditions relevant to Earth and Mars. Calcium sulfate and Cl salts, including NaCl and CaCl₂, are widespread on both planets, making their interactions particularly significant for understanding planetary geochemistry. The study emphasized that a dehydration environment, not just temperature or technique, profoundly influences gypsum transformation. The presence of Cl salts like CaCl₂ under hydrated and elevated temperature conditions promotes the formation of transitional Ca-sulfate phases, including bassanite and anhydrite. This information should expand our understanding of gypsum dehydration under variable planetary conditions, aiding the interpretation of orbital and in situ data from Mars. The findings are relevant for assessing the Martian geochemical record, including water activity, habitability potential, and broader planetary processes. Spectroscopy spoke to Merve Yeşilbaş, corresponding author of a paper based on this work (1), about the research.

How do the dehydration pathways of gypsum under various experimental conditions inform our broader understanding of geochemical processes on Mars and other planetary bodies?
Our study indicates that gypsum dehydration pathways vary significantly with environmental factors such as temperature, presence of chloride salts (NaCl, CaCl₂), and water availability. This helps us understand how similar environmental conditions on Mars and the other planetary bodies could lead to formation of diverse mineral phases. Our results also support the interpretations of orbital (for example, Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)) and rover (CheMin, SuperCam) data. Furthermore, the temperature-dependent dehydration pathways observed (from gypsum → bassanite → anhydrite) allow us to interpret the past aquatic and climate activity on Mars as well as habitability and preservation potential for biosignatures. Hydrated sulfate minerals, including gypsum, have been identified as potential preservers of microbial life signatures. Identifying the dehydration pathways, therefore, contributes to astrobiological exploration and strategy planning for Mars missions.

Your work demonstrates that both the presence of chloride (Cl)-salts and environmental conditions (like in vacuo vs. in situ) significantly affect gypsum transformations. How do you ensure experimental reproducibility given these variables?
To ensure experimental reproducibility, we used multiple complementary analytical methods, such as Raman, visible and near-infrared (VNIR) spectroscopy, mid-infrared (MIR) spectroscopy, and X-ray diffraction (XRD) for cross-validation of results. We carefully prepared experimental protocols for preparing samples, managing the experimental protocols and conditions (for example, heating rate, water availability). We conducted multiple experimental trials and presented all experimental procedures transparently in our publication, enabling other researchers to verify our findings.

What challenges have you encountered when using Raman, visible-near-infrared (VNIR, vis-NIR), and mid-IR spectroscopy to distinguish between gypsum, bassanite, and anhydrite, especially under overlapping thermal and compositional conditions?
During our investigation using Raman, VNIR, and mid-IR spectroscopy, we encountered several notable challenges in distinguishing gypsum, bassanite, and anhydrite under overlapping thermal and compositional conditions:

Ca-sulfate mineral phases (gypsum, bassanite, anhydrite) share sulfate (SO₄²⁻) vibrational modes at close wavenumbers, especially in the SO₄ spectral band region (~ 1000 cm^-1) as detected by Raman and mid-IR spectroscopy techniques. This complicates the precise identification and differentiation of these phases, especially in mixed or intermediate dehydration phases.

The low abundance or transient existence, and the similarities between crystal structures of certain phases, especially intermediate dehydration products like bassanite, pose challenges for detection and identifying the phases. For instance, previous studies have reported the challenges to distinguish of the bassanite and soluble-anhydrite phases by using the CheMin XRD instrument (sensitivity, 2) that was deployed on the Curiosity rover. In our XRD experiments, we used further modeling using Rietveld refinement analyses, and found out the weight percentages of gypsum and the other dehydrated phases’ in the mixtures.

How do you account for or correct the masking effects of ice and frost on spectral features when conducting low-temperature spectroscopy studies?
To account for and correct the masking effects of ice and frost on spectral features during our low-temperature spectroscopy studies, we used the baseline correction techniques to subtract the spectral contributions from ice and frost. By collecting spectra of pure ice or frost on an empty sample stage under identical temperatures, we generated precise reference spectra, enabling us to effectively remove their contributions from gypsum-related features under a computational environment (MATLAB).

Can you elaborate on how dehydration environment constraints (for example, sealed capillary vs. dry N₂ flow) influence the detectability of intermediate sulfate phases, and what this implies for remote sensing missions?
Sealed capillary conditions (like in our XRD experiments) restrict water vapor escape, favoring a gradual dehydration pathway and stabilizing intermediate phases such as bassanite. On the other hand, under dry N₂ flow conditions, water is quickly removed, promoting rapid dehydration to anhydrite, which often minimizes the stable presence of intermediate phases.

Intermediate phase (like bassanite) become more detectable in sealed environments, as these conditions extend their existence, allowing clearer and more distinct spectroscopic identification. In contrast, rapid dehydration (for example, under dry N₂ flow) shortens their lifespan, reducing their spectral detectability.

Remote sensing observations, such as those from the Martian spacecrafts (CRISM, OMEGA, SuperCam, CheMin), often capture snapshots from surface and near-surface minerals. Understanding how environmental dehydration conditions affect the duration and detectability of intermediate phases provides critical insights into Martian geologic history, water availability, and habitability potential. Longer-lived intermediate phases’ formation could indicate prolonged aqueous or humid environments, whereas rapid dehydration suggests brief water availability under certain time periods.

How do your spectroscopic findings on calcium sulfate hydrates enhance the interpretation of orbital or surface spectral data collected from Martian missions?
Our results may provide a set of reference spectra, enabling clearer identification of these minerals from the spacecraft data by precisely characterizing distinct spectral features of gypsum, bassanite, and anhydrite. Especially, the collected low-temperature spectral dataset is important to mimic the Martian spacecraft data. Mars is like a cold and frozen desert today. The temperature varies through day and night and between seasons. For instance, temperature could drop to -71°C during the night, and then increase to 20 °C at daytime in the early springs and summers. This ensures more accurate mineralogical mapping and reduces ambiguity in spectral interpretations from mission data. Providing exact spectral signatures under Mars-like environmental conditions also could support for mission planning, instrument design, and optimizing the exploration strategies for future Mars missions.

Given the different transformation temperatures observed in Raman and X-ray diffraction (XRD) due to varying dehydration environments, how can future planetary missions be designed to resolve such discrepancies in mineral detection?
To resolve discrepancies in mineral detection caused by varying dehydration environments and transformation temperatures, future planetary missions should integrate multiple instruments such as Raman spectroscopy, XRD, and IR as well as mass spectrometers, where we can perform the measurements at same conditions. We should continue in situ environmental monitoring of temperature, pressure, and humidity that is required for accurately interpreting mineral transformations. The further development of Mars-relevant spectral libraries based on laboratory analogs will provide reliable references for data interpretation. Additionally, advanced data integration and modeling tools, and even implementing artificial intelligence (AI) would be critical for distinguishing the detection and interpretation varied mineralogical phases. Finally, our study revealed the importance of environmental conditions (heating rate, dehydration environment) to detect these phases. In this context, I think the future planned Mars Sample Return missions would benefit the scientific community with analyzing Martian samples in a carefully controlled laboratory conditions by highly-sensitive analytical instruments for better interpretation of the spacecraft data and understand bio(geo)chemical features on Mars.

In your view, what are the most promising spectral indicators for differentiating between hydrated and anhydrous calcium (Ca)-sulfate phases in cold planetary environments?
In cold planetary environments, the most promising spectral indicators to differentiate between hydrated (gypsum, bassanite) and anhydrous (anhydrite) calcium sulfate phases include:

OH-stretching (around 3400–3600 cm⁻¹) and H₂O bending modes (around 1600 cm⁻¹) observed in Raman and mid-IR spectra are robust indicators of hydration states of the Ca-sulfates.

These features disappear or significantly diminish when transitioning from gypsum and bassanite to anhydrite, clearly distinguishing hydrated from anhydrous phases. For instance, while gypsum has doublet bands in the OH-stretching and H₂O bending modes, bassanite only has one spectral band in these spectral regions.

Hydrated sulfates show distinct shifts and splitting in SO₄²⁻ stretching vibrational peaks compared to anhydrous forms. For instance, gypsum and bassanite exhibit characteristic sulfate vibrational features (around 1000–1150 cm⁻¹ region), differing significantly from the anhydrite.

The hydration bands appear around 1.4 µm and 1.9 µm due to structurally bound water in gypsum and bassanite. The absence of these features in anhydrite provides a clear spectral marker for hydration state differentiation using VNIR spectroscopy.

Monitoring spectral changes at low temperatures (relevant to planetary surfaces) allows clear detection of dehydration-induced spectral shifts, particularly in water-related bands, helping confirm hydration states under cold planetary conditions.

How can your low-temperature spectral datasets of gypsum and gypsum-Cl mixtures be used to refine models of Martian surface mineralogy or past aqueous activity?
Our spectral data analyses with gypsum-Cl salt mixtures enable more accurate identification of these minerals from orbital and surface spectral data, reducing misinterpretation due to temperature-dependent spectral shifts. Our results also help interpretations of Ca sulfate and chloride-rich regions on Mars, offering clues into past aqueous chemistry, brine evolution, and evaporative conditions. Furthermore, our results present transitions between hydrated, intermediate, and anhydrous sulfate phases from low to high temperatures that could be used for modeling the stability and transformation kinetics of hydrated minerals on Mars with given experimental parameters (such as heating rate or temperature). As gypsum is known as a biosignature preserver, understanding how gypsum transforms under Martian conditions helps assess the potential for past habitability and target specific regions for biosignature searches.

What role do kinetics—like heating rates and water retention—play in interpreting mineral transformations in both experimental and extraterrestrial contexts?
Slow heating rates allow progressive dehydration, stabilizing intermediate phases (like bassanite), making them detectable and distinguishable by analytical instruments such as vibrational spectroscopy. In contrast, rapid heating rates can rapidly transform these intermediate phases, complicating detection.

Experimentally controlling heating rates helps interpret about how quickly environmental changes (such as water loss and evaporative rich brine deposits) occurred on planetary bodies like Mars.

Water retention (such as sealed vs. open air environments like those which are N₂-exposed)) significantly affects Ca-sulfate minerals’ transformation pathways. Constrained environments (like in our XRD experiments) prolong water interaction, favoring slower, stepwise transformations with allowing formation of intermediate mineral phases.

In summary, understanding kinetic processes—specifically heating rates and water retention—may be also essential for interpreting new Ca-sulfate mineral phase formations from experiments and planetary exploration by the spacecraft data, providing deeper insights into past environmental conditions, mineral stability, geological history, and habitability potential.

How do you approach the integration of multiple spectroscopic techniques with XRD to build a comprehensive mineralogical profile, and where do you see the potential for methodological improvements?
We combine the unique strengths of each method. XRD provides definitive structural identification and crystallinity information for mineral phases. Raman and IR spectroscopies offer molecular vibrational details sensitive to hydration states and the OH- structural variations in the mineral phases. We confirm the phase identification and mineralogical transformations in molecular level by combining these methods in same experimental conditions. We further apply spectral modeling and data processing tools, such as spectral deconvolution and multivariate analysis, to interpret complex and overlapping spectral features.

I believe that we can still potentially improve the methodology by conducting simultaneous XRD and spectroscopy measurements during Ca-sulfate phase transformations. Molecular dynamics, machine learning and AI-driven data interpretation techniques could also be used for the interpretation of complex Ca-sulfate mineral transformation processes under different environmental conditions.

Your results suggest that the released structural water from gypsum significantly affects phase formation. Have you explored real-time monitoring techniques to capture transient phases during dehydration?
In this study, we conducted stepwise mid-IR, Raman and XRD measurements at incremental temperature intervals to probe the transient phases—especially bassanite, which often exists short-time interval during dehydration. These measurements were all timed based to improve our chances of observing intermediate phases before full transformation to anhydrite. I am building up a new cryogenic-IR spectrometer system in my laboratory nowadays to monitor the time-resolved spectra for the phase transformations of hydrated mineral systems. This will provide a kinetic view of mineral dehydration under Mars-like environmental conditions. I am planning to include synchrotron-based in situ XRD techniques for ultrafast structural data collection, enabling detection of even the shortest-lived phases to distinguish complex Ca-sulfate phases (such as bassanite-anhydrite) in the next studies.

Do you see potential for developing more refined in situ instrumentation for Mars or lunar missions based on the spectral subtleties observed in your lab studies?
Absolutely—our laboratory findings, particularly the spectral subtleties tied to phase transitions in calcium sulfates, highlight several promising opportunities for advancing refined in situ instrumentation for Mars and lunar missions. Here's how:

A compact spectrometer combining Raman + IR + thermal analyses could enable simultaneous tracking of dehydration-hydration, and environmental conditions (for example, temperature, relative humidity), significantly improve the interpretation of Ca-sulfate phases and/or the mixture phases. I am particularly excited with Franklin Rosalind rover of ESA that will deploy several IR spectrometers, even inside a drill that will drill the Martian surfaces down to 2 m. This will be the first mission ever to detect the near surfaces of Mars. IR spectrometer is very sensitive technique to detect water vapor and CO₂, but this might not be a big issue when it works in the near surfaces. IR spectroscopy is also quite powerful itself to show any organics, sulfate, carbonate or water traces in minerals and rocks. Although the Moon lacks hydrated minerals to the extent of Mars, analog instrumentation adapted for sulfate detection could help study volatile trapping, space weathering products, or even water-ice interactions in permanently shadowed regions.

Our findings support the case for designing next-generation, high-fidelity, multi-spectral, and environmentally robust instruments. These tools would not only advance mineralogical precision but also help reconstruct planetary environmental histories and assess potential habitability with unmatched clarity.

Are there any unexpected findings or anomalies in your spectral data that might point to currently unknown dehydration mechanisms or mineral interactions on Mars?

One of the most surprising observations was that 1 wt.% of CaCl₂ brine triggered dehydration of gypsum more rapidly than expected in low temperature cycles. We noted unique band shifts and distortions in mid-IR spectra of gypsum–Cl-salt mixtures, distinct from pure gypsum or bassanite. This suggests chloride-induced dehydration, potentially indicating ion exchange processes that could occur under cold, briny Martian conditions—mechanisms not widely studied in existing mineralogical models. We need to further investigate this issue by using different Cl-salts and concentrations to make a comprehensive framework to explain this phenomenon.

These anomalies open the door to previously unrecognized dehydration pathways and mineral-fluid interactions on Mars, especially in chloride-bearing terrains, evaporite deposits, or ancient hydrothermal systems. In fact, these findings remind us that Mars may still hold subtle but scientifically rich mineralogical clues that we are only beginning to uncover.

Looking ahead, what do you see as the most critical unanswered question in the study of sulfate mineralogy in planetary environments—and how might spectroscopy help address it?

How stable are hydrated sulfate minerals under dynamic Martian surface and subsurface conditions, especially mixed with other types of salts (such as chlorides or perchlorates)? Mars is a salty planet, and we still need to investigate their interactions with other types of salts and stability under Mars-like conditions. Furthermore, we should investigate if they can record and preserve signatures of past aqueous environments or potential biosignatures?

Spectroscopy is a technique to show “fingerprints” of the materials, such as minerals, water, ice, and cryosalts as well as organics.Spectroscopy provides the sensitivity to detect even subtle differences in hydration levels—crucial for evaluating whether a mineral has been partially dehydrated, hydrated, or remained stable. Spectroscopic techniques can reveal phase transition pathways and transient phase behavior, highlighting the stability of minerals. Spectroscopic techniques, especially Raman and IR, can potentially detect organic inclusions or biosignatures trapped within sulfate matrices—if those minerals have preserved them.

Overall, spectroscopy is far more than a diagnostic tool—it opens a window into the deep history of planetary surfaces. It reveals not only what phases are present but also how they formed, what environmental conditions they’ve experienced, and whether they preserve evidence of past water or habitability by probing the vibrational signatures. This analytical precision enables us to reconstruct complex geochemical histories and identify sites of high astrobiological interest on Mars. In fact, minerals act as a time capsule, and spectroscopic techniques helping scientists interpret the dynamic processes that have shaped other worlds over billions of years. I believe that the development and integration of spectroscopic techniques will be at the heart of the next generation of planetary exploration.

References

1. Yeşilbaş, M.; Vu, T. H.; Hodyss, R. et al. Geochemical Transformations of Gypsum Under Multiple Environmental Settings and Implications for Ca-Sulfate Detection on Mars. ACS Earth Space Chem.2025, 9 (3), 433–444. DOI: 10.1021/acsearthspacechem.4c00137

Merve Yeşilbaş is an Assistant Professor of Chemistry at Umeå University and the Group Leader of the Yeşilbaş Lab, where she leads interdisciplinary research at the intersection of geochemistry, spectroscopy, and astrobiology. Merve completed her PhD studies at Umeå University, and then received several postdoctoral grants from Swedish Research Council and NASA to study Mars (geo)chemistry in the SETI Institute and the NASA Astrobiology Institute.Merve studies the geochemical changes on planet Mars focusing on extreme environments on Earth from the coldest tundras of Arctic to the hottest volcanic environments. She has a wide expertise studying martian analogue mineral-water and ice interfacial chemistry and cryomineral formation using cryogenic vibrational spectroscopy techniques.