A different perspective for the Mars rover “Opportunity” site: Fine-grained, consolidated hematite and hematite coatings

Since 2001, there have been two, parallel interpretations of Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of Sinus Meridiani, which are: (1) coarse-grained ("gray") hematite is the only spectral match; and (2) fine-grained hematite with particles closer than similar towavelength ("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 (similar to750 km x 350 km) of coarse hematite (>5-10 mm 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 can better match TES spectra. A thin coating (similar to5-10 mm 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.