The planetary ionospheres around the nonmagnetic planets Mars and Venus are directly exposed to the shocked solar wind. An interaction between the solar wind protons and the ionospheric oxygen takes place in a narrow turbulent region referred to as the plasma mantle. In this letter the microphysics of the dayside mantle is investigated numerically using a one-dimensional hybrid code that retains the inertia of the electron species. It is shown that lower hybrid waves propagating perpendicular to the magnetic field are destabilized. Wave saturation is caused by electrostatic trapping of the proton species, and the saturated amplitudes are shown to be in reasonable agreement with Pioneer-Venus observations. Oxygen pick-up and acceleration is found to be dominated by wave effects, resulting in significant ion heating, consistent with Phobos observations.¿ 1997 American Geophysical Union