  We consider the dynamics of porous icy dust aggregates in a turbulent gas
disk and investigate the stability of the disk. We evaluate the random velocity
of porous dust aggregates by considering their self-gravity, collisions,
aerodynamic drag, turbulent stirring and scattering due to gas. We extend our
previous work by introducing the anisotropic velocity dispersion and the
relaxation time of the random velocity. We find the minimum mass solar nebular
model to be gravitationally unstable if the turbulent viscosity parameter
$\alpha$ is less than about $4 \times 10^{-3}$. The upper limit of $\alpha$ for
the onset of gravitational instability is derived as a function of the disk
parameters. We discuss the implications of the gravitational instability for
planetesimal formation.
