The infrared optical constants of limestone particles and implications for the search of carbonates on Mars

Theoretical studies and numerical models of the atmosphere and surface of Mars need the knowledge of the optical constants of candidate materials for Martian dust. Limestone, as a carbonate bearing material, is commonly considered a likely component particularly important for its links with the climate evolution and water resources on Mars. s work we present the complex indices of refraction of submicron particles of limestone, derived from laboratory transmission spectra in the wavelength range 3.5–20 μm by means of the dispersion theory. The results are discussed and compared with the bulk optical constants of the same material already available in the literature. Since a comparison between the infrared spectrum of our dust sample of limestone (composed of more than 98% of calcium carbonate) and that of pure calcite particles shows no differences, we can consider the optical properties presented in this paper valid also for a collection of randomly oriented submicron calcite grains. tinction and scattering efficiencies of limestone grains, calculated using our particulate optical constants, have been used as input parameters in a simple model of radiative transfer to compute synthetic spectra of the surface and the aerosol of Mars. The results clearly show that, for quantitative analyses concerning the abundance and composition of the different solid materials present on the surface and atmosphere, the physical status of the various dust components has to be taken into account. This means that any extrapolation in the use of the bulk optical constants to describe the optical behaviour of small particles and vice versa has to be checked a priori.