Spectral Survey on Clay-rich Outcrops in Northern Xanthe Terra, Reference Sites for Oxia Planum

Introduction. ExoMars rover mission is planned to deliver the ‘Rosalind Franklin’ rover to explore Oxia Planum, a region that straddles between Arabia Terra and Chryse Planitia (335.5E, 18.2N). Oxia Planum bears several evidences of aqueous activity [1-4], where infrared data reveal extensive outcrops of recently exhumed clay-rich rocks. These outcrops are perfect “windows” to search for signs of past or present life on Mars, as biosignatures might be preserved therein [5]. Clays detected at Oxia Planum are among those found all over circum-Chryse Planitia [6], a flat lowland region and bottom end of many outflow channels.

Here, we focus on possible clay-bearing outcrops detected in northern Xanthe Terra (313E, 13N) (Figure 1), a region that has already been suggested for several rover missions. Like in Oxia Planum, several morphological features advocate for a fluvio-deltaic and lacustrine history in the region [7-9], with fluvial channels and fan deltas (e.g., Sabrina and Hypanis valley systems). Despite a possible detection of Fe,Mg-rich clays at the Sabrina delta [10,11], no detailed spectral survey have been done in the entire area. A detailed analysis of these outcrops is crucial to better determine possible mineral phase(s) and search for changes in clay mineralogies associated with differences in formation and weathering conditions. We examine infrared data, particularly the absorptions centered in the 1.0–2.6 μm spectral range, in order to better constrain the nature and composition of the clays.  

Figure 1 – Locations of Oxia Planum (OP) and northern Xanthe Terra (XT) at the margins of circum-Chryse Planitia.

Data & Methods. Spectral signatures of the clay outcrops are obtained from data gathered by the CRISM instrument, with spatial resolutions of 20–40 m.px-1 and a spectral resolution of 6.6 nm [12]. For this study we use several CRISM cubes acquired in the near-infrared spectral range (1–4 µm), targeting the northern Xanthe Terra. They are first pre-processed through CAT ENVI for atmospheric and photometric corrections. Corrected cubes are then denoised (column-by-column ratio) to reduce noise and residual atmospheric contributions, and to finally emphasize mineralogical absorptions in the ratioed spectrum. Once the cubes are corrected and denoised, we define our regions of interest (ROIs) to outline clays. We calculate band depths at 1.9 and 2.3 µm [13] to select pixels with strong absorptions and map the ROIs for each cube (Figure 2).

Figure 2 – Two sites of interest exposing clay-bearing outcrops: near the Sabrina fan delta at the Magong crater (313.4E, 12.1N) and an isolated outcrop in eastern Xanthe Terra (317.5E, 10N). We also include the possible clay outcrops identified at the delta [10,11].

Results. For each cube, we retrieve the band centers for all pixel of the ROIs within the three spectral ranges of interest: 1.4 µm (1.37–1.45), 1.9 µm (1.88–1.96), and 2.3 µm (2.26–2.34) (after continuum removal to emphasize the absorptions). The band centers do not strongly vary for the three narrow absorptions, with average values being 1.408 ± 0.009 µm, 1.923 ± 0.008 µm, and 2.305 ± 0.009 µm. FRT0A605 is the only cube displaying a clear overtone near 2.4 µm and a shallow absorption near 2.5 µm. We therefore retrieve the band centers near 2.4 µm (2.36–2.44) and 2.5 µm (2.49–2.57), where the cube shows average values of 2.397 ± 0.009 µm, and 2.530 ± 0.010 µm. Overall, these values are identical to those recently obtained at Oxia Planum [4] (Figure 3).

Figure 3 – (left) Spectra of CRISM denoised reflectance (normalized and offset for clarity) obtained in northern Xanthe Terra. (top right) We also include three spectra from Oxia Planum [4]. (bottom right) Lab spectra of main candidate Fe,Mg-rich clays (RELAB).

Discussion. CRISM cubes reveal several absorptions in the 1.1–2.6 µm range. Absorptions near 1.4 and 1.9 µm are common to hydrated minerals (OH/H₂O stretching), while an absorption near 2.3 µm indicates a (Fe,Mg)-OH vibration. Xanthe’s clays are consistent with Fe,Mg-rich clays, combining absorptions at 1.41, 1.92, 2.30–2.31 µm and weaker overtones near 2.39–2.40 µm. Martian Fe,Mg-rich clays generally show spectral variability in the three windows. Xanthe’s clays intermediate band centers are consistent with either vermiculites and Fe-rich saponite (Figure 3), like in Oxia [4]. Band centers within these windows vary very little throughout the region. Exact positions therein depend on the relative abundance of iron and magnesium in the clay structure, and also oxidation state of iron.

Conclusions. We provide in-depth information on the clay-bearing outcrops found in northern Xanthe Terra, to compare with recent results obtained in Oxia Planum [2,4]: (1) report exact positions of the 1.4, 1.9, 2.3, 2.4 and 2.5 µm absorption bands, and (2) map the “purest” clays in the targeted areas, in context with the morphology and topography. This allows for further investigations in context with the ExoMars rover mission or other future rover missions. 

Acknowledgments. This work is fully funded and supported by the Italian Space Agency (ASI) [Grant ASI-INAF n. 2017-48-H.0]. We are greatly thankful to the CRISM team for the CAT tool, and European Space Agency (ESA) and Russian Space Agency ROSCOSMOS for the ExoMars project.

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