Perseverance at Krokodillen: first in situ observations of the clay-bearing Noachian basement of the Nili Fossae region, Mars.

Multiple orbital studies have highlighted the importance of Fe/Mg phyllosilicate minerals on Mars, especially in Noachian terrains (e.g. Poulet et al., 2006; Mangold et al., 2007;), even showing that they are the dominant hydrous mineral family on Mars (Carter et al., 2013). Although widespread in exposures of the oldest terrains on Mars, it is hard to constrain either their composition  or formation process based on orbital data only (Ehlmann et al., 2011; Carter et al., 2015). The best matches for the orbital spectra were proposed to be smectite (nontronite, saponite) and/or vermiculite (Carter et al., 2013). In the Nili Fossae region specifically, the Noachian basement was shown to be bear widespread signatures of Fe/Mg smectites (Goudge et al., 2015).

After exploring diverse geological units inside Jezero Crater (Nili Fossae, Mars), and going over the rim of the crater, the Perseverance rover has reached a unit informally called Krokodillen, at the base of the outer part of the rim. It is thought to be part of the Noachian crust that was locally uplifted by the emplacement of Jezero Crater (Sun & Stack et al., 2020). Dark looking from orbit, it is surrounded on the North, West and South by ridges and an exposure of the regional olivine-rich unit, understood to be younger.

We will present the data acquired on rocks of the Krokodillen area with the SuperCam instrument (ref Maurice et al., SSR 2021; Wiens et al., SSR 2021). Overall structureless, the rocks of Krokodillen are generally fine grained, with locally some millimetric granules. The chemical composition characterized with LIBS shows a relatively homogeneous composition intermediate between the average basaltic crust of Mars and orthopyroxene. This is likely an average, close to the bulk composition, due to the mixing of multiple fine-grain mineral phases within the footprint of the LIBS analysis. Visible and near infrared (VISIR) reflectance spectroscopy data show strong and ubiquitous signatures of Fe-Mg phyllosilicates, closely matching those observed from orbit in the broader Nili Fossae region.

We propose that the rocks of Krokodillen are representative of the Noachian clay-bearing rocks characterized from orbit, specifically the (ridged) Altered Basement mapped by Goudge et al., (2015) in the Jezero watershed. In that case, the in situ measurements from the Mars 2020 mission provide the first in situ constraints on the composition, aqueous alteration and emplacement mechanism of these rocks.