Clay Minerals Distribution across the Martian Crustal Dichotomy: Insights into Aqueous Environments on Early Mars

Over nearly two decades numerous studies have revealed evidence for widespread aqueous activities on Mars’ surface, likely occurring from Noachian to Hesperian epochs [1,2]. Clay-bearing deposits are valuable targets to constrain the water history on early Mars and consequently search for signs of life [3]. The possible co-occurrence of carbonates throughout these deposits may also suggest a high potential for biosignature preservation. Nevertheless, the definitive characterization of these deposits remains a challenging task. This study builds upon our previous works [4–6] by focusing on hyperspectral data gathered by the CRISM instrument onboard NASA’s Mars Reconnaissance Orbiter mission [7] to analyze the spectral signatures of clay deposits in the 1–4 µm range, and their potential mixtures with carbonates. We collected data along the Martian crustal dichotomy (Figure 1) – the boundary between the ancient, cratered highlands and the northern lowlands – where extensive clay deposits are exposed, selecting several regions of interest (ROIs). For all ROIs we conducted a spectral survey to determine the exact positions of the band centers for key clay absorptions around 1.4, 2.3, and 2.4 µm, and after applying a continuum removal to emphasize the absorptions. Band centers observed for the clay deposits show some variations from one region to another, as expected from ferromagnesian (Fe,Mg-rich) clays on Mars.

Figure 1 – Global map of Mars with colorized topography (MOLA). We investigated more than 600 CRISM cubes along the crustal dichotomy (color-coded circles). We found ferromagnesian clays in around 400 cubes, mostly concentrated in the vicinity of Mawrth Vallis, Nili Fossae, Libya Montes. Other mineral phases are also detected, such as sulfates in Aram Chaos and Meridiani Planum, or even chlorites in Lyot crater.   

Nontronites (Fe-rich) are extensively detected around Mawrth Vallis, whereas saponites (Mg-rich) are more locally distributed across Nili Fossae and Lybia Montes. Oxia Planum – ESA’s ExoMars rover landing site – exhibits more “intermediate” clays such as vermiculites or even ferrosaponites. These variations may depend on the relative abundance of iron and magnesium in the clay structure, or even the oxidation state of iron [8]. Additionally, clay deposits often display a shallow absorption near 2.50–2.53 µm, suggesting the presence of carbonates intermixed with the clays. Overall, the carbonates possibly found in these deposits appear to have variable compositions, spanning within the siderite (Fe-rich) and magnesite (Mg-rich) series. This may indicate the presence of carbonates as siderite/magnesite solid solutions, likely reflecting multiple alteration events [9].

This work follows previous in-situ and orbital measurements and prepares for upcoming missions and campaigns on Mars (e.g., ExoMars rover mission, Mars Sample Return, etc.), by offering a more comprehensive understanding of clay mineralogies to inform the search for biomarkers.

Fundings: This work is supported by the Italian Space Agency (ASI) [ASI-INAF n.23-3-HH.0]

References: [1] Carter et al. (2013) JGR Planets 118: 831–858. [2] Ehlmann and Edwards (2014) Ann. Rev. Earth Planet. Sci. 42:291–315. [3] Vago et al. (2017) Astrobiology 17: 471–510. [4] Brossier et al. (2022) Icarus 386: 115114. [5] Brossier et al. (2023) PSJ 4:27. [6] Brossier et al. (2024) PSS 247: 115924. [7] Murchie et al. (2007) JGR Planets 112: E05S03. [8] Michalski et al. (2015) EPSL 427: 215–225. [9] Beck et al. (2024) Earth and Space Science 11: e2024EA003666