DUST IN THE MARTIAN ATMOSPHERE - POLARIMETRIC SENSING

Several thousands of determinations of the degree of linear polarizati on have been recorded over the disk of Planet Mars, at Tokyo Observato ry station, from 1977 to 1991. In addition to the polarization produce d by reflectance over the Martian soil surface, there is some perturba tions by local veils of crystal or by dust storms which have been alre ady analyzed. But a new type of unusual event has been discovered, dis playing temporary anomalies in the degree of polarization. Their analy sis characterizes faint and transparent veils, localized over specific areas, and at certain times of the Martian year corresponding to the maximum solar heating. The scattering theory of Mie indicates solid, a bsorbing sub-micron size grains, reminiscent of small dust particles r aised from the soil surface. Dust is observed in the Martian atmospher e at three levels: (1) a permanent faint haze made of very small dust particles. (2) Temporary local transparent veils made of submicron siz e grains, preferentially during seasons and latitudes of maximum solar heating. (3) Occasionally, an evolution of these veils to produce ext ensive, dense, dust storms made of micron size grains, which may sprea d all over the planet for weeks.