**Low-Order Model of Martian General Circulation and Aperiodicity of Dust Storms.**

**A. A. Pankine, A. P. Ingersoll (Caltech)**

We use a three variable model that is analogous to the classical
Lorenz model of nonperiodic flow (Lorenz, J. Atmos. Sci., 20, 130-141, 1963)
to describe the dynamics of the Martian atmosphere with dust. The three
variables represent the cross-equatorial Hadley circulation, and asymmetries
in horizontal and vertical distributions of atmospheric dust, respectively.
Absorption of sunlight by the dust enhances circulation that changes horizontal
and vertical dust distributions. Our model resembles the model of Ingersoll
and Lyons (Ingersoll and Lyons, J. Geophys. Res., 98, 10951-10961, 1993) in
that it uses hemispheric dust distribution as a variable, but we also use
vertical dust distribution and treat circulation explicitly. It is well known
that the solutions of the Lorenz system are unstable and some of them are
nonperiodic. We suggest that aperiodicity of the Martian dust storms can be
explained in the light of the presented analogy as an intrinsic property of the
Martian global circulation.