Vertical profiles of dust and ozone in the Martian atmosphere deduced from solar occultation measurements

Abstract

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 ≅ 8 km below 20 km 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 ≅ 0.05 to ≅ 2 and a vertical extent in the range from 5 to 15 km. Indications on the particles size ( r ≫ 0.01 μm) 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 cm 2 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 cm 2 s −1 seems to be required.