Lunar and Planetary Institute

Abstract

Since 2001, there have been two, parallel interpretations of Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of Sinus Meridiani, which are: (1) coarse hematite is the only spectral match; and (2) fine-grained hematite with particles closer than ~wavelength ("fine-intimate hematite", e.g., coating, ferricrete) is a better match, but coarse hematite is also viable.The TES team interpreted the spectra as consistent only with a large deposit (~750 km x 350 km) of coarse hematite (>5-10 µm grain size).Coarse hematite is considered strong evidence for longstanding water, which led to the decision to land the rover Opportunity there.On the other hand, the Aerospace/LPI remote sensing team argued that fine-intimate hematite (e.g., a coating) can better match TES spectra.A thin coating (~5-10 µm thick) and a low exposure (<5%) could cause the observed signatures.The distinction is important because: (1) It is unknown whether fine-grained hematite implies abundant water; (2) Fine-intimate hematite may explain the non-detection of coexisting aqueous alteration minerals and the lack of hematite wind streaks; (3) Current "hematite abundance maps" may instead map the surface texture; (4) Coatings may be of astrobiology interest; (5) Studies are needed to determine whether visible-infrared spectra can definitively distinguish fine-intimate from coarse hematite.