Definitive evidence of Hesperian basalt in Acidalia and Chryse planitiae

[1] The surface materials of Acidalia and Chryse planitiae, Mars, have been variably interpreted morphologically as basalt and compositionally as andesite or altered basalt. A thorough survey using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) near‐infrared spectrometer reveals olivine and high‐calcium pyroxene signatures in the shallow subsurface exposed by impact craters. These signatures indicate that the near surface of these regions are primarily basaltic in nature and are similar to other regions of Hesperian volcanism across Mars (e.g., Syrtis Major). Previous spectroscopic studies using coarser‐resolution data sets have failed to identify these mafic signatures due to obscuration of the underlying material in addition to the relatively small‐scale exposures in impact craters. The nature of the surficial obscuration material is likely multifaceted and includes the deposition of latitude‐dependent mantling material and the presence of alteration rinds. This observation shows that the northern plains of Mars are basaltic and must be factored into the thermal and surficial evolution of Mars.