The 2.4- 45 μm spectrum of Mars observed with the infrared space observatory

The spectrum of Mars at 2.4- 45 μm has been observed on July 31, 1997 ( L_s=157°) by the short-wavelength spectrometer of the Infrared Space Observatory. The data consist of a high signal to noise, complete grating spectrum (resolving power R∼1500-2500) and portions of the 20- 40 μm spectrum observed in Fabry-Perot mode ( R∼31000). The data show the infrared bands of known martian atmospheric species (CO ₂,H ₂O, and CO) with an unprecedented amount of details. The vertical distribution of H ₂O is determined, showing saturation near 10 km. Evidence for scattering in the saturated CO ₂ band at 2.7 μm and for fluorescence emission in the CO₂ 4.3 μm band is obtained. No detection of new atmospheric species is achieved, but upper limits are obtained for CH ₄ and H ₂CO. In the solar reflected part of the spectrum, which dominates at λ⩽4.2 μm, the surface reflectance clearly shows the hydration band with maximum absorption at 2.9 μm, from which a 2.0-2.7% (by weight) water content in the martian uppermost layer is estimated. A decrease of reflectance from 3.8 to 5 μm is also seen. This behaviour is consistent with basalts and palagonite, but not hematite. In the thermal part, mineralogic signatures at 5- 12 μm are globally consistent with a basaltic composition. Specific minima are also detected at 5.7, 6.3 (tentative), 7.2 and 11.1 μm. Reexamination of earlier datasets indicates that the latter two have been observed before, although generally not discussed. The presence of additional absorptions at 26.5, 31 and 43.5 μm is also indirectly suggested. Carbonate minerals are tentatively detected from this ensemble of features, though no single carbonate species can be unambiguously identified.