A simple scale analysis demonstrates that rapidly-expanding Martian dust storms are driven by extremely large density differences. Thermal gradients appear to be inadequate to produce these density contrasts. When dust produces the densities required, the storms become compressible because of the slowing of the speed of sound due to the addition of particles. Though compressible, particle-laden flows have never been analyzed due to their highly complex nature, pressure waves produced by these flows could be hypothesized to provide a positive feedback capable of producing rapid growth. ¿ 2000 American Geophysical Union