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

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. Background TES measures spectra over the ~6–50 μm spectral range. TES spectra of Sinus Meridiani exhibit ~18, 23, and 33 μm bands that are broadly consistent with hematite [Christensen et al., 2000a; Christensen et al., 2001; Estep-Barnes, 1977; Onari et al., 1977]. The 23 μm band has an observed spectral contrast of ~2%. Fig. 1 illustrates the definition of band contrast. Strong bands such as hematite may exhibit at 18, 23, and 33 μm are called “reststrahlen bands.” When reststrahlen bands troughs exhibit spectral contrast greater than ~1–5%, that indicates the presence of some smooth-surfaced material in one of two broad forms: (1) fine-intimate material or (2) coarse particles. “Fine-intimate material” is material composed of fine particles or grains that are closer than ~wavelength together. Closely-spaced particles or grains may occur because the material formed that way (e.g., precipitate coating) or was consolidated or cemented (e.g., closely packed fines, duricrust, ferricrete, desert varnish). Spectrally, “coarse particles” (e.g., coarse hematite) means large enough to be optically thick (high opacity) [Salisbury et al., 1987], which for hematite is greater than ~1–5 μm over the ~15–50 μm range [EstepBarnes, 1972]. The size at which a material becomes optically thick is tied to the absorptivity. Hematite absorptivity ranges from ~5000–10000 per centimeter for the 18, 23, and 33 μm bands [Estep-Barnes, 1972]. The penetration depth is given by absorbance/(weight percent of the material*absorptivity). For pure hematite, the weight percent is 100. For absorbance=1 (i.e., transmission=10%), and absorptivity of 10000 and 5000 per centimeter, the penetration depth is 1 and 5 μm, respectively. Scattering effects explain why both fine-intimate and coarse hematite are consistent with the TES data. Material can scatter light through surface or volume scattering [Vincent and Hunt, 1968; Hapke, 1993]. A strong absorption causes high opacity, and the resulting mirror-like surface reflectance (“surface scattering”) causes a clear emissivity trough (a spectral band) because radiance exiting the material is reflected inward (Figure 1, “solid surface”) [Planck, 1914]. On the other hand, when unconsolidated particles are small enough for light to survive passage through the material, volume absorption (volume scattering) occurs. Volume scattering causes a reststrahlen band emission peak, (Figure 1, lower trace), which counters the surface scattering trough, and thus causes low spectral contrast (Figure 1, upper trace) [Vincent and Hunt, 1968; Salisbury et al., 1991; Salisbury and Wald, 1992]. Preprint: Accepted Geophysical Research Letters, Jan, 2004