A study of the velocity, temperature and density of the shocked solar wind in the vicinity of the Mars magnetospheric tail is presented. It is argued that the deficiency of momentum flux seen in that region implies the existence of an antisolar magnetospheric flow with velocities much larger than the planet's escape velocity. It is further suggested that the planetary particle fluxes may extend across most of the cross-sectional area of the magnetosphere near the terminator. The implications of the magnetospheric flow on the evolution of the Mars atmosphere are discussed. It is concluded that a dense Martian atmosphere could have been eroded by the solar wind in a time span of the order of 109 years. ¿American Geophysical Union 1987