Tracking Down Carbonates Lurking in Martian Clay-Rich Rocks

Carbonates on Mars provide key insights into the planet’s past environmental conditions, as their formation typically results from interaction between CO2-bearing alkaline waters and ultramafic rocks commonly associated with a dense, CO2-rich atmosphere. While ferromagnesian (Fe,Mg-rich) clays are particularly widespread across the Martian surface [1,2], carbonates remain comparatively rare in orbital observations. This scarcity suggests that carbonates may be buried, altered, or spectrally obscured within clay-bearing rocks [3]. Here, we examine the presence of possible carbonates, along with clays, by analyzing approximately 517 near-infrared (1–4 µm) spectral cubes acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Our results reveal new carbonate-rich deposits and confirm earlier detections. A detailed investigation of the absorption bands near 2.3, 2.5 µm and around 3.4–3.5 µm indicates that carbonates on Mars are best represented as Fe–Mg solid solutions spanning the siderite–magnesite series, rather than pure endmembers [4]. Such compositions are geochemically plausible on Mars; they likely formed under reducing conditions and may have persisted despite later exposure to the more acidic, oxidizing surface environment [5]. Spectral mixing models better clarify the influence of clays on carbonate signatures and provide important constraints for further laboratory analog studies [6,7]. The recurring spatial asso-ciation of carbonates and clays across multiple outcrops implies either coprecipitation or closely related formation pathways within neutral to alkaline aqueous environments during the Noachian (3.7–4.0 Gyr ago), offering strong evidence for sustained liquid water and conditions potentially favorable to microbial life. Our results expand the known distribution of carbonates on Mars, emphasize their astrobiological relevance, and provide strategic guidance for future rover operations and sample-return site selection targeting preserved biomarkers (organic compounds). Overall, this work advances our understanding of early Martian habitability and the role of carbonates in recording ancient CO2-water interactions.

This study closely aligns with the objectives of ESA’s “Rosalind Franklin” mission [8], whose rover will explore Oxia Planum and investigate clay-bearing terrains and possible carbonates in the search for well-preserved biosignatures throughout subsurface rocks and soils [9-11]. This work is thereby financially supported by the Italian Space Agency (ASI) [Grant ASI-INAF n. 2023–3–HH.0].

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