Martian surface mineralogy from 0.8 to 1.05 μm TIGER spectro-imagery measurements in Terra Sirenum and Tharsis Montes formation

Abstract

New 0.8–1.05 μm reflectance spectra of Mars are derived from TIGER spectro-imaging measurements obtained at Mauna Kea CFH observatory during the 1990 opposition, from a systematic survey of the Tharsis volcanic region with spatial resolution of 200–250 km. Primary goals of this investigation are to identify the physico-chemical origins of the observed spectral variations, to map the surface units based on their spectroscopic properties, and to draw inferences in terms of compositional heterogeneities of bright and dark materials. Detailed information regarding the ferric and ferrous mineralogy of undocumented or poorly documented soils is presented, based on a reliable set of about 1500 highly-resolved spectra (R = λΔλ∼ 600) and with application of mixture modelling and spectral variable analyses to the dataset. The spectral signatures exhibit absorption features that can be distributed in three major groups: (1) the 0.85–0.89 μm bands, (2) the 0.90–0.94 μm bands, and (3) the 0.95–1.0 μm bands. Results of the mixture modelling reveal that at our spatial resolution, nearly 90% of the investigated terrains may be explained by various combinations of clinopyroxene-dominated with hematite-dominated minerals. The remaining 10% present subtle spectral variations that might result from slight differences in composition or from physical effects. Based on these combinations, the spectroscopic information shows that a linear mixture of low- and high-albedo soils mentioned in previous martian observations is not systematic to explain intermediate-albedo soils. Both ferric and ferrous characteristics are found to be present in the spectra of most dark and bright regions, for surface units of about 250 km × 250 km in areal coverage. In addition, spectral analyses show the occurrence of unusual associations of spectral properties that provide strong arguments to demonstrate the compositional heterogeneity of the soils. These heterogeneities are thought to be driven by variable mixtures of hematite-like minerals, hematite-pyroxene assemblages, and/or varied pyroxene phases. Relationships between spectral and thermal properties seem to be more complex than the classical bimodal distribution. Ferric and ferrous characteristics appear to be related to thermal inertias, though this is not systematic, with a few regions escaping from the general observed trend. The northern bright region of Tharsis, including Amazonis Planitia, is interpreted as spectrally uniform at a large spatial scale and covered by ferric-dominated materials, as demonstrated by the frequent occurrence of a hematite-related absorption centred at 0.86 μm. A surface alteration at large spatial scale is inferred from our spectral observations of this region. The southern dark terrains of Terra Cimmeria, Terra Sirenum, and south Daedalia Planum are spectrally documented and their spectral properties indicate a greater inherent variability with respect to the bright areas. The analysis of these areas suggests a composition of clinopyroxene-like minerals. Combined spectral and thermal properties also support the presence of bonded ferrous materials and crusts that remain partially exposed. Highly fractured bright areas of Noctis Labyrinthus, Noctis Fossae, northern Claritas Fossae, Ceraunius Fossae, and the erosional remnants of the plateau west of Amazonis are proposed to constitute isolated deposits of heavily altered fine-grained dust. The transition between the bright northern terrains of Tharsis and the southern cratered plateau is interpreted as variable in composition, according to the importance and spatial distribution of the alteration of the basaltic substrate. The proportion of exposed surface may also vary over the dark basaltic areas, and probably becomes less important towards north Tharsis.