Mars: Clearing of the 1971 dust storm

A semiquantitative analysis of clearing in the 1971 great dust storm on Mars is presented as a function of time and altitude, using Mariner 9 orange- and visual-light photos. Steady settling of dust approximately accounts for the decline of the storm after December 22, 1971. Continuous settling cannot be invoked prior to that date; injection of dust into the atmosphere, i.e., a storm resurgence, occurred in mid-December 1971. Theoretical models of optical depth versus time are developed for various distributions of particles in the atmosphere. By intespreting settling in terms of Stokes' law, estimates of the maximum radii of dust particles throughout the atmosphere have been obtained. Models which best account for the dust-storm decline indicate particles ? 5μm in diameter high in the atmosphere, with a concentration of larger particles (? 10μm) near the ground in the lowest parts of Mars. Long-term thin high hazes should persist through much of the Martian year, perhaps clearing before perihelion.