Feldspar-Rich Outcrops on Mars Could Indicate an Early Evolved Crust

Feldspar-rich outcrops identified in this study (red stars) and previous studies (white stars). The background terrain is color-coded according to age, with brown to dark green to light green representing Early, Middle, and Late Noachian-aged terrain, and dark and light blue representing Hesperian and Amazonian-aged terrain. Credit: Payré et al., 2022.

Global spectral analyses of the surface of Mars from orbit suggest that it has a generally basaltic crust characterized by mafic minerals such as olivine and pyroxene. However, laboratory studies of clasts within some martian meteorites, as well as analyses within Gale Crater by the Curiosity Rover, have identified rocks of intermediate to even felsic composition, which are richer in silica. These findings are supported by orbital measurements by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter, which found evidence for feldspar-rich outcrops across the southern hemisphere in Noachian-aged (more than 3.7 billion years old) terrains.

A team led by Valerie Payré of the University of Iowa investigated whether such feldspar-rich outcrops, possibly consisting of evolved igneous rocks, are present in the Terra Sirenum/Cimmeria (TSC) region, which is estimated to be one of the oldest exposed crustal terrains on Mars at more than 4.2 billion years old. Using CRISM visible/near-infrared spectral data, they identified nine feldspar-rich sites within the region and confirmed an intermediate composition using thermal infrared data acquired by the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor. The outcrops all appear in excavated areas, such as in crater walls or central peaks.

These results suggest that the TSC region may contain the remnants of evolved crustal materials that formed early in martian history, either as an initial primary crust or as a secondary crust that was produced through protracted magmatism that followed initial planetary differentiation and was subsequently buried by basaltic volcanism. This hypothesis is supported by geophysical and seismic data, which indicate a less-dense subsurface than would be expected from a homogenous basaltic crust. Altogether, these results indicate that the early crusts of planetary bodies were not just basaltic but could have contained evolved rock types as well. READ MORE