ORBITAL EVIDENCE FOR MORE WIDESPREAD FELDSPATHIC ROCKS ON MARS

Decades of studying Mars with orbiting and landed spacecraft and through laboratory analyses of Martian meteorites have pointed consistently toward a crust dominated by basaltic compositions. Yet in the past few years, all three types of studies have revealed occurrences of material with substantially lower mafic and higher feldspar abundances. Wray et al. (2013) interpreted these "feldspathic" rocks to have crystallized from silicic melts formed through several cycles of magma evolution (partial melting or fractional crystallization), while Carter and Poulet (2013) interpreted spectrally similar materials as anorthosites. Rogers and Nekvasil (2015) later suggested that porphyritic rocks with feldspar phenocrysts in a relatively Fe-poor matrix could produce the same spectral signature. In any case, these feldspathic materials are spectrally distinct from typical Martian basalts, and record distinct magmatic processes.

Here we use data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board NASA's Mars Reconnaissance Orbiter to further characterize the spatial distribution of feldspathic materials. Feldspars with minor Fe substitution can be identified through a diagnostic crystal field absorption in the shortwave-infrared range sensed by CRISM, but only where more Fe-rich phases (olivine, pyroxene) are relatively scarce. More complete mapping of the Nili Patera caldera now allows us to estimate the true extent of evolved igneous compositions in this portion of the Syrtis Major volcanic construct. In the southern highlands, Wray et al. (2013) reported feldspathic outcrops across an area of ~150,000 km²; through continued mapping of highland crater floors, we are now finding similar materials across a swath of terrain >10 times as wide. Applying similar spectral analyses globally will provide a much-improved perspective on what this surprisingly widespread rock type implies about the evolution of Mars.