REMOVAL OF MARS ATMOSPHERIC GAS-ABSORPTION FROM PHOBOS-2/ISM SPECTRA

ISM is an imaging spectrometer in the range of the near infrared that hew onboard of Soviet probe Phobos 2 in 1989. Its first objective was to obtain information about the mineralogical composition of the soil of Mars and its satellite Phobos, and about the spatial and temporal variability of the Martian atmosphere. In the spectral range of the instrument (0.76 divided by 3.16 mu m) the radiation emerging from Mars' atmosphere is almost entirely due to the solar radiation reflected by the soil. Therefore, independent knowledge of the spectral transmittance of the atmosphere allows us to eliminate the atmospheric effect from the ISM data and so to obtain the spectral signature of the planet soil. In the present work the Martian atmospheric transmittance has been computed using FASCODE and the spectral line atlas HITRAN of AFGL. The atmospheric profile has been defined on the basis of the work of Moroz et al. /1/. Then, the convolution of the computed transmittance with the response functions of ISM has been carried out to obtain the atmospheric spectrum as ''seen'' by the instrument. To eliminate the atmospheric absorption from the measurements it is necessary to renormalize the transmittance computed with FASCODE so that the depth of the absorption bands is the same as that of the bands measured by ISM. Finally dividing the measured spectra by the computed ones we obtain the spectral signature of Martian soil from which it is possible to deduce the mineralogical composition of the observed zones.