  Hydrodynamical instabilities are believed to power some of the small scale
(0.1-10 pc) turbulence and chemical mixing in the interstellar medium.
Identifying such instabilities has always been difficult but recent
observations of a wavelike structure (the Ripples) in the Orion nebula have
been interpreted as a signature of the Kelvin-Helmholtz instability (KHI),
occurring at the interface between the HII region and the molecular cloud.
However, this has not been verified theoretically. In this letter, we
investigate theoretically the stability of this interface using observational
constraints for the local physical conditions. A linear analysis shows that the
HII/molecular cloud interface is indeed KH unstable for a certain range of
magnetic field orientation. We find that the maximal growth-rates correspond to
typical timescales of a few 1e4 years and instability wavelengths of 0.06 to
0.6 pc. We predict that after 2e5 years the KHI saturates and forms a turbulent
layer of thickness ~0.5 pc. The KHI can remain in linear phase over a maximum
distance of 0.75 pc. These spatial and time scales are compatible with the
Ripples representing the linear phase of the KHI. These results suggest that
the KHI may be crucial to generate turbulence and to bring heavy elements
injected by the winds of massive stars in HII regions to colder regions where
planetary systems around low mass stars are being formed. This could apply to
the transport of 26Al injected by a massive star in an HII region to the
nascent solar-system.
