  Previous experiments have revealed that shock waves in thermally relaxing
gases, such as ionizing, dissociating and vibrationally excited gases, can
become unstable. To date, the mechanism controlling this instability has not
been resolved. Previous accounts of the D'yakov-Kontorovich instability, and
Bethe-Zel'dovich-Thompson behaviour could not predict the experimentally
observed instability. To address the mechanism controlling the instability, we
study the propagation of shock waves in a simple two-dimensional dissipative
hard disk molecular model. To account for the energy relaxation from
translational degrees of freedom to higher modes within the shock wave
structure, we allow inelastic collisions above an activation threshold. When
the medium allows finite dissipation, we find that the shock waves are unstable
and form distinctive high density non-uniformities and convective rolls on
their surface. Using analytical and numerical results for the shock Hugoniot,
we show that both DK and BZT instabilities can be ruled out. Instead, the
results suggest that the clustering instability of Goldhirsch and Zanetti in
dissipative gases is the dominant mechanism.
