Formation of Martian Crystalline Hematite: New Information From Comparisons of Laboratory and TES Infrared Spectra

The Thermal Emission Spectrometer (TES) instrument aboard Mars Global Surveyor discovered several isolated deposits of gray, crystalline hematite in Sinus Meridiani, Aram Chaos, and Valles Marineris. Comparison of the average Sinus Meridiani hematite spectrum measured by TES to laboratory emissivity spectra for a variety of naturally occurring hematites shows small but potentially important differences. In particular, the emissivity minimum at 300 cm^-1 in the Sinus Meridiani spectrum is displaced 10-25 cm^-1 to lower wavenumbers compared to pure hematite samples. In addition, the shape of the band in the TES data is thinner than the broad band seen in natural hematites. These differences may imply that the natural variability of hematite spectra has not been fully characterized, especially with respect to the reaction pathway and crystal morphology. Here, we describe the thermal infrared spectral characteristics of several series of synthetic hematite samples derived by direct precipitation, dehydroxylation of fine-grained goethite and the oxidation of magnetite. The temperature of hematite formation, as well as the nature of the hematite precursor has distinct effects on the shape and position of the major absorption bands in the hematite infrared spectrum. Therefore, the shapes and positions of these features may be diagnostic of the hematite formation process. Comparison with TES spectra show that the best match to martian coarse-grained hematite is a synthetic sample derived by the dehydroxylation of goethite at 300°C. Spectra of goethite-precursor samples dehydroxylated at higher temperatures provide increasingly poor fits with increasing dehydroxylation temperature. In addition, spectra of samples derived by direct precipitation and high-temperature thermal oxidation of magnetite are poor fits to the martian coarse-grained hematite. One of the main sites under consideration for NASA's 2003 MER missions is the hematite-rich region of Sinus Meridiani. The presence of the Mini-TES instrument on the rover will provide a unique opportunity to build upon TES measurements made from orbit around Mars. Of great importance will be the ability of Mini-TES to record surface spectral features in the 525-875 cm^-1 region which cannot be done with TES due to the 667 cm^-1 CO2 fundamental absorption. The shape and position of the martian hematite absorption at ~540 cm^-1 will be visible for the first time, providing additional spectral information for comparison with laboratory data.