Widespread clay–carbonate co-occurrence reveals advanced aqueous alteration on Mars

Technical Summary: Widespread clay–carbonate co-occurrence reveals advanced aqueous alteration on Mars

Problem Statement
Carbonates on Mars are critical indicators of past habitability and atmospheric evolution, as their formation typically requires neutral-to-alkaline aqueous environments and a dense, CO₂-rich atmosphere. However, orbital observations have historically identified carbonates only sparsely (e.g., in Nili Fossae and Jezero Crater), creating a discrepancy with the expectation that a CO₂-rich early Mars should host widespread carbonate reservoirs. A primary challenge in resolving this discrepancy is the spectral difficulty of detecting carbonates when they are intimately mixed with other minerals, particularly ferromagnesian clays (smectites). In the near-infrared (1–2.6 µm) range, the diagnostic 2.3 µm absorption band of carbonates overlaps significantly with the 2.3 µm band of Fe-Mg clays. Furthermore, the 2.5 µm carbonate band is often weak and can be masked by spectral mixing, dust, or the strong water absorption feature near 3 µm. Consequently, carbonate-bearing deposits may be systematically underdetected in orbital datasets, leading to an incomplete understanding of Mars' aqueous history.

Methodology
The authors conducted a targeted survey using 540 CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) near-infrared hyperspectral cubes (1–4 µm) selected from a broader dataset of over 1,500 observations. The selection criteria focused on regions exhibiting the 2.3 µm absorption band, indicative of ferromagnesian clays. The study employed a multi-step analytical approach:

1. Data Processing: CRISM cubes were processed using the CRISM Analysis Toolkit (CAT) for atmospheric and photometric corrections, followed by a "column-by-column" ratioing technique to denoise the data and enhance diagnostic mineral absorptions.
2. Spectral Analysis: The authors analyzed band centers and depths for absorptions at 2.3 µm, 2.5 µm, 3.5 µm, and 3.9 µm. While the 2.3 µm band is shared by clays and carbonates, the 2.5 µm band serves as a more direct, albeit often weak, indicator of carbonates. The study also examined the "bulge-like" feature and specific absorptions in the 3.4–3.9 µm range, despite challenges such as thermal emission and lower signal-to-noise ratios in this region.
3. Correlation and Modeling: The study investigated the correlation between the depth of the 2.3 µm and 2.5 µm bands to distinguish carbonate-clay mixtures from pure clay assemblages. Additionally, spectral mixing models were generated using ferrosaponite (a clay) and magnesiosiderite (an Fe-Mg carbonate) to simulate the spectral behavior of these mixtures and validate the detection of carbonates in the presence of clays.
4. Exclusion of Alternatives: The authors systematically ruled out alternative mineralogies, such as serpentines and chlorites, which can also exhibit absorptions near 2.3 and 2.5 µm, by analyzing band positions and the absence of characteristic 2.1 µm features.

Key Results

• Widespread Co-occurrence: The analysis of 540 CRISM cubes revealed that 418 (approximately 77%) exhibited a measurable 2.5 µm absorption band alongside the 2.3 µm band. This indicates a widespread co-occurrence of carbonates and clays that was previously unrecognized due to spectral masking.
• Spectral Correlation: A strong positive correlation (R² = 0.83) was found between the depths of the 2.3 µm and 2.5 µm bands. The data suggests that as carbonate content increases within a clay-carbonate mixture, the 2.5 µm band deepens systematically, while the 2.3 µm band remains largely stable but deepens slightly. No spectra showed a strong 2.3 µm band without an accompanying 2.5 µm feature, suggesting that pure, abundant clays were not the dominant phase in these specific detections.
• Mineralogical Composition: The band centers for the 2.5 µm (mean 2.514 ± 0.009 µm) and 3.5 µm (mean 3.481 ± 0.020 µm) absorptions are consistent with Fe-Mg solid solutions within the siderite-magnesite series (magnesiosiderite/ferromagnesite). The data overlaps with siderite and Fe-Mg carbonates but shows limited evidence for calcite or dolomite, aligning with the planet's Fe- and Mg-rich basaltic composition.
• Spatial Distribution: Carbonate signatures were identified across various regions, including Nili Fossae, Jezero Crater, Oxia Planum, and Xanthe Terra. The study identified 342 cubes where all three diagnostic bands (2.3, 2.5, and 3.5 µm) were jointly observed, with varying degrees of robustness.
• Advanced Alteration: The absence of clear serpentine signatures and the dominance of clay-carbonate assemblages suggest that aqueous alteration on Mars progressed beyond early-stage serpentinization to more evolved mineralogical systems.

Significance and Claims
The paper claims that the widespread co-occurrence of clays and carbonates reveals a more extensive history of aqueous alteration on Mars than previously recognized. The findings suggest that carbonate deposits are likely systematically underdetected in orbital data because their spectral signatures are often masked by associated clays. This implies that the true abundance of carbonates in the Martian crust may be significantly higher than current estimates, potentially representing a major carbon reservoir.

The association of Fe-Mg carbonates with clays indicates formation in neutral-to-mildly alkaline waters, implying prolonged aqueous activity and sustained liquid water during the Noachian and early Hesperian periods. These conditions are considered potentially favorable for microbial life. Furthermore, the prevalence of Fe-Mg carbonates (siderite-magnesite series) over Ca-rich carbonates (calcite/dolomite) supports models where early Martian environments were reducing to mildly oxidizing, allowing Fe²⁺ to remain stable before later oxidation altered the deposits. The study concludes that these mineral assemblages represent advanced, mature alteration systems, highlighting promising targets for future biosignature exploration and suggesting that the early Martian carbon cycle was more complex and widespread than previously modeled.