Recognition of minor constituents in reflectance spectra of Allan Hills 84001 chips and the importance for remote sensing on Mars

Abstract— Reflectance spectra of splits 92 and 271 from the Martian meteorite Allan Hills (ALH) 84001 are presented and analyzed in this paper. Although the visible and infrared spectra of both chips show that the dominant mineralogy is low-Ca pyroxene, the focus here is on identification of the minor constituents. Infrared spectra measured at multiple spots along the surface of chips 92 and 271 show subtle spectroscopic variations due to changes in the low-Ca pyroxene texture and composition and to the presence of secondary minerals. Absorption bands observed near 0.93 and 1.95 μm are characteristic of low-Ca pyroxene. Strong mid-infrared reststrahlen bands are observed near 9 and 19.5 μm in all surface spectra, and additional bands near 7, 10.5, 11.4, 17.8, 20.5 and 23 μm are variable depending on the low-Ca pyroxene texture and the presence of secondary minerals. Selected spectra exhibit carbonate features near 4, 6.4–7.1 and 11.3 μm. Detailed analysis of these carbonate features indicates the presence of Mg-Fe carbonate, which is consistent with petrographic studies. Many of these spectra with strong carbonate features exhibit a magnetite feature near 17.9 μm and a shoulder near 20.5 μm that cannot be uniquely ascribed to one mineral. Spectroscopic identification of the minor carbonate and magnetite minerals in this probable piece of Mars indicates that detection of small amounts of these minerals of possible biological significance will be possible using infrared hyperspectral analyses of the Martian surface. Also of importance for remote sensing on Mars is the result that Mg, Fe and Mg-Fe carbonates in a low-Ca pyroxene matrix should be distinguishable from one another in the spectral region measured by the thermal emmitance spectrometer (TES).