  The linear stability of viscous Keplerian flow around a gravitating center is
studied using the rheological granular fluid model. The linear rheological
instability triggered by the interplay of the shear rheology and Keplerian
differential rotation of incompressible dense granular fluids is found.
Instability sets in granular fluids, where the viscosity parameter grows faster
than the square of the local shear rate (strain rate) at constant pressure.
Found instability can play a crucial role in the dynamics of dense planetary
rings and granular flows in protoplanetary disks.
