Infrared reflectance spectroscopy of Martian analogues

One of the main goals of Mars exploration is to determine the characteristi cs of surface materials to shed light on its geological evolution. To this end, infrared spectra, acquired by ground-based observations and by space-b orne remote sensing, represent one of the most diagnostic tools. In order t o best interpret these data a careful comparison with laboratory spectra of Martian surface analogues is needed. In the present work we report the res ults of transmittance, specular, and diffuse biconical reflectance measurem ents of three materials potentially representative of bright regions on the surface of Mars: andesite, palagonite, and montmorillonite. Few spectrosco pic data are available in the literature for andesite; the palagonite from Etna volcano exhibits important differences from Hawaiian palagonite. Altho ugh montmorillonite has been already studied by various authors, it was inc luded here in order to build a self-consistent set of data. The infrared sp ectral range covered by our measurements (400-6000 cm(-1)) matches most of the Mars radiance spectrum, and the spectral resolution of 2 cm(-1) has all owed us to perform an accurate band identification. Using the measurements, we have computed the optical constants of the samples. The chemical and mo rphological characterization of the analyzed samples has allowed us to perf orm a careful interpretation of the observed spectral features. The depende nce of the band intensities, observed in diffuse reflectance, on the partic le size was used to identify fundamental and combination/overtone bands. Th is complex of laboratory data obtained by morphological, chemical, and spec troscopic analyses provides a reference for the Martian mineral identificat ion through interpretation of remote observations.