Clay-Carbonate Co-Occurrence on Mars Reveals More Mature, Chemically Evolved, and Spatially Pervasive Aqueous Systems

Carbonates on Mars record interactions between CO₂-rich fluids and the crust, yet their apparent scarcity from orbit has long challenged models of a thicker early atmosphere. Here we show that carbonates are in fact widespread but largely concealed within clay-bearing terrains. We analyze over 500 near-infrared hyperspectral cubes from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), targeting Fe,Mg-rich clay outcrops and systematically searching for associated carbonate signatures. A weak but diagnostic 2.5 µm absorption is identified in ~77% of cases, frequently accompanied by features near 3.5 µm. These signatures demonstrate that carbonates are pervasive across clay-rich regions, yet are routinely underestimated due to spectral masking and low abundances at orbital scale. Band positions indicate Fe–Mg solid solutions spanning the siderite–magnesite series, consistent with formation in basaltic environments under reducing to mildly oxidizing conditions.
The systematic co-occurrence of clays and carbonates across diverse terrains points to extensive, chemically evolved aqueous systems rather than localized or incomplete alteration. At orbital resolution, the absence of clear serpentine signatures further suggests that alteration pathways commonly progressed beyond early-stage reactions toward mature clay–carbonate assemblages. Together, these observations indicate that aqueous alteration on early Mars was more widespread and advanced than previously inferred.
Our results also imply that current orbital inventories represent a lower bound on carbonate abundance. In situ rover measurements reporting substantial carbonate contents in spectrally ambiguous terrains support the presence of large, previously unrecognized crustal reservoirs. This hidden carbonate inventory provides a plausible sink for early atmospheric CO₂ and helps reconcile the discrepancy between predicted and observed carbonate distributions.
The widespread association of clays and carbonates highlights environments that were both water-rich and geochemically stable—conditions favorable for habitability and long-term preservation of biosignatures. By revealing a planet-scale, cryptic carbonate reservoir, this study reframes the Martian carbon cycle and identifies high-priority targets for future exploration and sample return.
This work is funded by the Italian Space Agency [ASI-INAF n.23-3-HH.0].