Measurements in the Mars plasma environment indicate that a friction layer develops between the solar wind and that planet's ionosphere. The observed features include a velocity boundary layer in the solar wind that streams around the flanks of the Mars ionosphere and enhanced planetary particle fluxes detected when the solar wind speed decreases to low values within the boundary layer. It is suggested that a cold plasma flow is present in the tenuous Mars upper ionosphere and that its momentum is provided by the solar wind within the adjacent velocity boundary layer. Calculations are presented to estimate the momentum flux that is delivered to the Mars upper ionosphere. It is argued that an ionospheric flow can be driven by solar wind momentum transferred from a velocity boundary layer whose thickness is 100--200 km around the terminator. By comparison a dense early Mars ionosphere could have been eroded by the momentum flux delivered from a ~1000 km thick boundary layer which would be comparable to that present around the Venus ionosphere. The total mass loss eroded by this process through Martian history should have not been larger than that of a few meters-deep global layer of water. It is further suggested that the erosion of the Mars ionosphere is not axially symmetric around the terminator but should preferably occur over the magnetic polar regions of the ionosphere. ¿ 1998 American Geophysical Union