LOW-TEMPERATURE REFLECTIVITY SPECTRA OF RED HEMATITE AND THE COLOR OFMARS

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 pi gment, three samples of palagonitic tephra from the same location that contain nanophase ferric oxide as their dominant pigment, and two mix tures of the two types of pigmenting phases. Relative proportions of r ed hematite and nanophase ferric oxide were determined by Mossbauer sp ectroscopy. For samples containing red hematite as the dominant pigmen t, the positions of the ferric electronic transitions near 430, 500, 6 30, and 860 nm are essentially independent of temperature, but their w idths decrease with decreasing temperature. This decrease results in a well-defined minimum for the band at 630 nm at low temperatures and i n significant increases in reflectivity in spectral regions near 1050 and 600 nm. For example, the reflectivity ratios R-600/R-530 and R-600 /R-860 both increase by a factor as large as similar to 1.4 between 30 9 and 140 K. The spectral features from nanophase ferric oxide in samp les of palagonitic tephra are nearly independent of temperature. Spect ral data of Martian bright regions that are characterized by a shallow band minimum near 860 nm, a reflectivity maximum near 740 nm, a disti nct bend near 600 nm, and a shallow absorption edge from similar to 40 0 to 740 nm are attributed to the presence of nanophase ferric oxide p lus subordinate amounts of red hematite. The 600-, 740-, and 860-nm fe atures are associated with red hematite. Because the reflectivity of r ed 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 c onservative upper limit for the red hematite content of the optical su rface of Mars is 5%.