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

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‐grained (“gray”) 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 × 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 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.