Infrared spectra obtained from the Mariner 9 spacecraft during the 1971–1972 dust storm are used to derive information on the composition and particle size distribution of the dust and to study the time evolution of the storm. The dust is not composed of pure granite, basalt, basaltic glass, obsidian, quartz, andesite, or montmorillonite. The infrared spectra suggest that the dust is a mixture of materials, dominated by igneous silicates with >62;60% SiO₂, or weathering products such as clay minerals, but the dust could possibly have a significant component of lower SiO₂ materials such as basalt. Substantial quantities of carbonates, nitrates, or carbon suboxide are excluded from the mixture. All infrared, visible, and ultraviolet data on the Martian surface composition seem consistent with a mixture of basalt and clay minerals or high SiO₂ igneous rocks, with a surface patina of oxides of iron. For all candidate compositions, the data are best matched with a size distribution that approximates a differential power law function of slope −4. This size distribution is quite similar to terrestial size distributions in regions remote from sources of dust. The relative abundance of particles between 1- and 10-μm radius did not change during the Mariner 9 mission; thus suspended particles did not experience Stokes-Cunningham fallout but instead were supported by turbulence with an eddy diffusion coefficient, K _e ⋍ 7 × 10 ⁶cm ²sec ⁻¹. The aerosol optical depth, standardized to 0.3-μm wavelength, varied from about 1.5 early in the mission to about 0.2 at Orbit 200.