Mars: Near‐infrared spectral reflectance of surface regions and compositional implications

Abstract

Reflectance spectra (0.65–2.50 μm) are presented for 11 Martian areas. The spectral resolution is ∼1½% and the spatial resolution is 1000–2000 km. These are the first high‐quality spectrophotometric data at these wavelengths for regions on the surface. Spectral features previously observed are confirmed and better defined, and a number of important spectral properties are characterized for the first time. The spectra show water ice absorptions in the 1.5‐ and 2.0‐μm regions, which if due to surface frost, would imply the presence of 1 to 2 mg/cm2 H2O. However, other studies have shown that the presence of an unprotected surface frost in late morning is unlikely. Water ice is stable at night even at the equator and might persist until late morning when the air is well undersaturated if it is intimately dispersed in weathering products, especially if clays are present. The dark region spectra indicate about 4 times less water ice than seen in bright regions. Since some bright material is present in dark regions there may be no water ice associated with the dark materials themselves. This tends to confirm that weathering products (thought to be more abundant in bright regions) are involved in the mechanism that allows temporary persistance of unstable water ice. The presence of weak 2.3‐μm features in many of the spectra are consistent with the presence of hydroxylated magnesium‐rich minerals such as sheet silicates (serpentine, talc, and magnesian smectites) or amphiboles (anthophyllite). The apparent absence of a 2.2‐μm absorption implies that montmorillonite may not be a major component of the Martian regolith. Many of the spectra also show an apparent absorption at 1.9 μm, in the wing of the 2.0‐μm Martian atmospheric CO2 absorption, which would indicate bound molecular water. Observed dark regions have distinctive near‐infrared spectral shapes, previously not well determined, which are characteristic of thin semi‐transparent alteration coatings overlying dark unaltered rock. Previously observed ferrous‐ and ferric‐iron absorptions in the 1‐μm region are better defined by these new data. Clinopyroxene (augite) is definitely present, but olivine is not spectrally apparent.