The near-infrared continuum slope of Martian dark region reflectance spectra

The planet Mars has a bimodal albedo distribution with bright and dark regions. Earth-bound reflectance spectra of bright Martian regions are generally characterized by a flat to positive continuum slope in the near-infrared (NIR), while those of dark regions tend to have a negative slope. Spectra obtained with the instrument ISM (Imaging Spectrometer for Mars) onboard the Phobos 2 spacecraft usually show the same relationship between brightness and NIR continuum slope. Nevertheless, some ISM spectra show deviations from this relationship, the most conspicuous ones being observed in the Syrtis Major and Valles Marineris regions, where some dark areas are characterized by a flat continuum slope. So far, this was interpreted to indicate a basaltic material covered with a weathering coating or a thin layer of dust. The results of a new spectroscopic investigation of analog materials for the Martian crust revealed that reflectance spectra of bulk samples generally show a preference for a negative NIR continuum slope, in contrast to spectra of powder samples. This suggests a different interpretation of the ISM spectra, namely that the unusual dark regions, which are characterized by a flat NIR continuum slope, are covered by sand-sized basaltic particles. In contrast, the common dark regions, which are characterized by a blue continuum slope, are dominated by bedrock or blocky basalt rocks. This interpretation is in agreement with the findings of other methods of remote sensing.