VERTICAL PROFILES OF DUST AND OZONE IN THE MARTIAN ATMOSPHERE DEDUCED FROM SOLAR OCCULTATION MEASUREMENTS

The high atmosphere of Mars (z > 30 km) has been observed from Phobos 2, using the solar occultation technique. Due to a major error in the pointing system, resulting from a wrong orientation of the spacecraft in the pointing software, the lower atmosphere is never observed in ultraviolet (220-330 nm) and high resolution near-infrared (760 nm, 936 nm) channels. Pointing data sometimes provide useful information on the visible opacity below 30 km, showing that the scale height of dust decreases from congruent-to 8 km below 20 km to congruent-to 4 km above. Among 32 occultations, four show the presence of a water ice cloud high in the atmosphere (z = 50 km), with a maximum tangential optical thickness varying from congruent-to 0.05 to congruent-to 2 and a vertical extent in the range from 5 to 15 km. Indications on the particles size (r >> 0.01-mu-m) are obtained using spectral information. From a simple cloud model, where eddy diffusion and sedimentation processes are taken into account, an upper limit of 10(7) cm2 s-1 is derived for the eddy diffusion coefficient. Similarly, the absence of ozone in detectable amounts above 30 km is interpreted as the signature of a rather weak value of K. Using a simple stationary photochemical model, a value of K as low as 10(6) cm2 s-1 seems to be required.