A possible surprise for the Mars rover "Opportunity": The inferred coarse hematite may instead be fine-grained, consolidated hematite

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 par- ticles closer than ~wavelength ,(“fine-intimate hema- tite”, e.g., coating, ferricrete) is a better match, but coarse hematite is also ,viable. The TES team inter- preted the spectra as consistent ,only with a large ,de- posit (~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 ob- served signatures. The distinction is important ,be- cause: (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