In this paper we study the evolutions of the interfaces between
gases and the vacuum for both inviscid and viscous one dimensional
isentropic gas motions. The local (in time) existence of solutions
for both inviscid and viscous models with initial data containing
vacuum states is proved and some singular properties on the free
surfaces separating the gas and the vacuum are obtained. It is
found that the Euler equations are better behaved near the vacuum than
the compressible Navier-Stokes equations. The Navier-Stokes
equations with viscosity depending on density are introduced,
which is shown to be well-posed (at least
locally) and yield the desired solutions near vacuum.