Published online by Cambridge University Press: 10 December 2009
This chapter reviews observations and interpretations since the 1990s from orbital telescopic and spacecraft observations of Mars from the extended visible to short-wave near-IR (VNIR) wavelength range. Imaging and spectroscopic measurements from the Hubble Space Telescope (HST), Mars Global Surveyor Mars Orbiter Camera Wide Angle (MGS MOC/WA) instrument, Mars Odyssey Thermal Emission Imaging System Visible Subsystem (THEMIS-VIS), and Mars Express High Resolution Stereo Camera (MEx HRSC) and Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) have been acquired at spatial scales from global-scale ∼ 1 to hundreds of kilometers resolution to regional-scale ∼ 20–100 m resolution. Most high-albedo regions are homogeneous in color and thus, likely, composition, a supposition consistent with the long-held idea of the presence of a globally homogeneous aeolian dust unit covering much of the surface. Despite the presence and ubiquity of dust, these measurements still reveal the presence of significant VNIR spectral variability at a variety of spatial scales. For example, color variations and possibly mineralogic variations have been detected among small-scale (tens of meters) exposures of light-toned outcrop and layered materials in Meridiani Planum, Valles Marineris, and other areas. Within low-albedo regions, much of the observed color variability appears simply related to different amounts of covering or coating by nanophase ferric oxide-bearing dust and/or ferrous silicate-bearing sand. Some VNIR color units, however, in regions spanning the full range of observed surface albedos, correlate with geologic, topographic, or thermal inertia boundaries, suggesting that either composition/mineralogy or variations in physical properties (e.g., grain size, roughness, packing density) influence the observed color.