Clay - Carbonate Assemblages along the Martian Crustal Dichotomy: A Key to Assess Aqueous Conditions on Early Mars

Introduction – On Mars, the crustal dichotomy marks the transition from ancient southern highlands to northern lowlands. Infrared datasets reveal several sites therein with extensive clay-rich deposits, further testifying for widespread aqueous conditions on early Mars [1]. Here, we investigate the deposits found in key regions along this boundary (Fig. 1) [2,3]. Clay deposits are perfect “windows” to search for signs of life on the planet, as clays are known to accumulate and preserve organic compounds [4]. Infrared spectroscopy is a powerful tool to constrain the surface composition as near-infrared spectra show diagnostic bands. Knowing the exact positions of absorption band centers is therefore essential to ascertain possible species, and search for changes in clay mineralogies associated with differences in formation and alteration conditions. We examine hyperspectral data to better characterize the near-infrared signatures of clay deposits, and also verify for eventual mixing with carbonates.

Figure 1 - Mars with colorized topography (MOLA), and regions included in this survey.

Data & Methods –  Spectral signatures of clays are obtained from infrared data gathered by the CRISM instrument [5]. We used ~100 CRISM cubes acquired in the infrared range (1–4 µm), targeting regions selected along the dichotomy. They were pre-processed with the CAT ENVI toolkit for basic atmospheric and photometric corrections. Corrected cubes were also denoised to reduce noise and residual atmospheric contributions, and finally emphasize mineralogical absorptions. We calculated band depths at 1.9 and 2.3 µm [6] to select pixels with strong paired absorptions and outline the clays to define regions of interest (ROIs) for each cube.

Clay Diversity? – We retrieved the band centers for all pixels composing the ROIs within the absorptions of interest. Band centers obtained for most outcrops generally correlate with ferrosaponites or vermiculites, with average values being centered around 1.410, 2.305, and 2.397 µm, notably in Oxia Planum [2,7]. Some exceptions are observed, like in Mawrth Vallis where absorptions are slightly shifted to 1.42, 2.29 and 2.40 µm, rather consistent with nontronites. The exact position therein depends on the relative abundance of iron and magnesium in the clay structure, or even the oxidation state of iron [8]. Overall, these “intermediate” clays correspond to Fe-rich species, particularly ferrous smectites with a trioctahedral composition. Conversely, nontronites are rather ferric smectites with a dioctahedral composition. Interestingly, ferric smectites may form from oxidation of the ferrous smectites. The reducing atmosphere of early Mars would favor the formation of ferrous smectites, while their subsequent oxidation would explain the presence of ferric smectites nowadays [e.g., 9,10].

Possible Carbonates? – All CRISM cubes analyzed display an additional, shallow absorption centered near 2.5 µm, which is detected across all clay-bearing outcrops. Such an absorption could indicate the presence of carbonates intermixed with clays [e.g., 2,8,11]. Carbonates are usually best identified by paired absorptions at 2.3 and 2.5 µm, although these features are masked by the clays. An absorption near 2.53 µm would be consistent with Fe-rich carbonates (siderites), whereas an absorption at shorter wavelengths is often associated with Mg-rich carbonates (magnesites) [12]. Clay-bearing outcrops mainly present an absorption centered at 2.53 µm. Interestingly, the McLaughlin crater also bears outcrops with an absorption at 2.50 µm associated with Mg-rich clays (saponites).

We also searched for a specific pattern in the 3–4 µm range, where a broad peak is expected between deep absorptions occurring near 3.4–3.5 µm and 3.9 µm. We computed the band depth near 3.9 µm [11,12] to display the presence of carbonates intermixed with clays (Fig. 2). A clear absorption near 3.4–3.5 µm is generally missing in our spectra, likely due to deep water absorption near 3 µm induced by clays. This prevents a definitive characterization of carbonates. Nonetheless, the coprecipitation of clays and carbonates throughout the outcrops further strengthens the exobiological potential of the selected regions, where biosignatures might still be preserved.

Figure 2 - (A) False-color “RGB” composites. (B) Fe,Mg-rich clay deposits combining absorptions at 1.4, 1.9, 2.3, and 2.4 µm (also near 2.5 µm). (bottom) “BD3900” index outlines the drop of reflectance near 3.9 µm, and may indicate the presence of carbonates with the clays.

Conclusions – We follow-up recent investigations where we perform spectral surveys on clay deposits found in key regions along the Martian crustal dichotomy [2,3], and compare them with carefully selected terrestrial analogs. By doing that, we retrieved the exact positions of absorption band centers, and searched for possible variations therein. Variations observed between the targeted clay deposits suggest subtle changes in the iron and magnesium content, and also the oxidation state of iron. Mineralogical similarities observed in most regions suggest that clay deposits (ferrosaponites or vermiculites) may share a common weathering history. Conversely, nontronites extensively detected near Mawrth Vallis and surroundings indicate a different formation and alteration setting. Nonetheless, the region may have shared a common aqueous history with other deposits found elsewhere on the planet, and then diverged to form more leached or oxidized clays with Al,Fe-rich compositions [13].

The presence of additional absorptions near 2.5 µm and 3.9 µm typical of carbonates, may indicate their presence within clay deposits. This testifies for a widespread distribution of carbonates on Mars, more ubiquitous than previously thought.

Funding. This work is supported by the Italian Space Agency (ASI) [Grant ASI-INAF n. 2023-3-HH.0].

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