An Ancient Evolved Crust in Early Mars Deciphered by Remote Sensing

The Terra Cimmeria-Sirenum (TSC) region has been shown to host one of the most ancient crustal components with a composition that differs from any other region on Mars [1]. K-Th-rich and Fe- depleted compared to other ancient regions, TSC hides an early crustal component that might be more evolved than a basalt [1]. Combining visible/near-infrared spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument and thermal infrared spectra from the Thermal Emission Spectrometer (TES) and the Thermal Emission Imaging System (THEMIS), we explored the presence of feldspar-rich terrains in TSC that would potentially reveal an evolved crust as its chemical composition suggests. Based on the diagnostic absorption bands in reflectance and emission spectra, we found 10 feldspar-rich locations in crater rims, crater peaks, along the edge of eroded blocks, and in deep fractures. High Resolution Imaging Science Experiment (HiRISE) images suggest light-toned coherent materials that do not present any of the cracking textures previously observed in a few other feldspar-rich regions [2]. Relative age dating indicates that these exposures are > 4.0 Ga, and thermal emission signatures suggest an intermediate composition (andesite/diorite). These ancient intermediate terrains scattered in TSC echo the Noachian intermediate rocks and fragments from Gale crater located at the boarder of TSC, and the martian meteorite Northwest Africa 7034 that originated from the north of TSC [3] (up to 80 wt.% feldspar [4-5]). All of these feldspar-rich crustal materials thus suggest an intermediate crustal component that formed early in Mars' history and that represents the remnant of an evolved primordial or early secondary crust on Mars.

[1] Bouley et al., 2020, Nat. Geosc. 13,2; [2] Carter et al., 2013, Nat. Geosc. 6,12; [3] Lagain et al., 2022, Nat. Comm. 13,1-8; [4] Humayun et al., 2013, Nat. Geosc. 503,7477, [5] Sautter et al., 2015, Nat. Geosc. 8,605-609.