MARTIAN ATMOSPHERE STUDIES FROM THE ISM EXPERIMENT

Observations with the infrared spectrometer ISM on board the Phobos 2 spacecraft have provided spectra of Mars in the range 0.7-3.2-mu-m, with a spatial resolution of the order of 20 km. Spectra between 1.6 and 3.2-mu-m show evidence for scattering (probably by dust particles) and for minor gaseous constituents (H2O and CO). The imaging capabilities of the ISM spectrometer allow the study of spatial variability of these atmospheric constituents. The observations of the Martian volcanoes whose vertical extension is higher than the atmospheric scale height give spectra corresponding to different altitudes of the Martian surface, which are used to study the vertical mixing ratio of H2O and CO. In particular, CO is found to be depleted in the Tharsis region, compared with whole-disk measurements from ground-based observations with, possibly, a vertical depletion of the CO mixing ratio above 10 km. An abundance of H2O corresponding to 4 precipitable-mu-m is found, with no evidence of vertical variation in the atmospheric layers sounded (0-27 km).