Reflectance Spectra of Mars: Laboratory Experiments Concerning the LIR-slope and Macroscopic Mixtures.

Abstract

The martian surface has a bimodal brightness distribution and can be roughly subdivided into dark and bright areas. Dark region spectra exhibit a negative slope in the near infrared, indicating an Fe3+- rich coating on basaltic or andesitic rocks and a low porosity. This interpretation coincides with data for the photometric properties and thermal inertias. Spectrophotometric laboratory measurements which were performed at 0.5-2.5 ym of rock powders and of hand specimens support the assumption of a low porosity and of a compact surface for dark regions on Mars. The most striking difference between the reflectance spectra is the negative slope in the NIR for broken surfaces. At 0.5-0.0 ym martian spectra are influenced by absorptions of Fe3+. The terrestrially weathered susurface of the eucrite Millbillillie exhibits a similar spectral signature following the trend for hand specimens in the NIR. Reflectance spectra, taken e.g. by an orbiter imaging experiment of areas of bright and dark martian materials having dimensions below the spatial resolution are considered to represent a linear mixture of endmember spectra. As a simulation we performed spectrophotometric measurements of a cut monzodiorite surface covering an increasing area with an optically thick layer of Fe2O3. The negative slope vanishes when the FeeO3-surface cober exceeds 3 % of the FOV. This corresponds to an area with a diameter of 17.3 m assuming a resolution of 100 m.