Low-temperature reflectivity spectra of red hematite and the color of Mars

Reflectivity spectra (visible and near IR) were measured near 141, 210, and 300 K for four red and well-crystalline powders of hematite (red hematite) used as commercial pigments, two samples of volcanic tephra from Mauna Kea volcano that contain red hematite as their dominant pigment, three samples of palagonitic tephra from the same location that contain nanophase ferric oxide as their dominant pigment, and two mixtures of the two types of pigmenting phases. Relative proportions of red hematite and nanophase ferric oxide were determined by Mössbauer spectroscopy. For samples containing red hematite as the dominant pigment, the positions of the ferric electronic transitions near 430, 500, 630, and 860 nm are essentially independent of temperature, but their widths decrease with decreasing temperature. This decrease results in a well-defined minimum for the band at 630 nm at low temperatures and in significant increases in reflectivity in spectral regions near 1050 and 600 nm. For example, the reflectivity ratios A₆₀₀/R₅₃₀ and A₆₀₀/R₈₆₀ both increase by a factor as large as ∼1.4 between 300 and 140 K. The spectral features from nanophase ferric oxide in samples of palagonitic tephra are nearly independent of temperature. Spectral data of Martian bright regions that are characterized by a shallow band minimum near 860 nm, a reflectivity maximum near 740 nm, a distinct bend near 600 nm, and a shallow absorption edge from ∼400 to 740 nm are attributed to the presence of nanophase ferric oxide plus subordinate amounts of red hematite. The 600-, 740-, and 860-nm features are associated with red hematite. Because the reflectivity of red hematite at 600 nm is strongly dependent on temperature and because this wavelength is in the red part of the visible spectrum, the color of the Martian surface may vary as a function of its temperature. A conservative upper limit for the red hematite content of the optical surface of Mars is 5%.