  Previous experiments have revealed that shock waves driven through
dissipative gases may become unstable, for example, in granular gases, and in
molecular gases undergoing strong relaxation effects. The mechanisms
controlling these instabilities are not well understood. We successfully
isolated and investigated this instability in the canonical problem of piston
driven shock waves propagating into a medium characterized by inelastic
collision processes. We treat the standard model of granular gases, where
particle collisions are taken as inelastic with constant coefficient of
restitution. The inelasticity is activated for sufficiently strong collisions.
Molecular dynamic simulations were performed for 30,000 particles. We find that
all shock waves investigated become unstable, with density non-uniformities
forming in the relaxation region. The wavelength of these fingers is found
comparable to the characteristic relaxation thickness. Shock Hugoniot curves
for both elastic and inelastic collisions were obtained analytically and
numerically. Analysis of these curves indicate that the instability is not of
the Bethe-Zeldovich-Thompson or Dyakov-Kontorovich types. Analysis of the shock
relaxation rates and rates for clustering in a convected fluid element with the
same thermodynamic history outruled the clustering instability of a homogeneous
granular gas. Instead, wave reconstruction of the early transient evolution
indicates that the onset of instability occurs during the re-pressurization of
the gas following the initial relaxation of the medium behind the lead shock.
This re-pressurization gives rise to internal pressure waves in the presence of
strong density gradients. This indicates that the mechanism of instability is
more likely of the vorticity-generating Richtmyer-Meshkov type, relying on the
action of the inner pressure waves development during the transient relaxation.
