The main uncertainties between minimum and maximum atmospheric loss are related to present uncertainties of the efficiency of loss rates triggered by plasma instabilities and momentum transfer effects between the solar wind and the ionosphere. We investigated the growth rate for the Kelvin Helmholtz (KH) instability at the Martian ionopause and found that this instability may evolve into a non-linear stage at the whole terminator plane, but preferably at the equatorial flanks (Fig.). Escape rates of O⁺ ions (~10²⁴ s^-1) due to detached plasma clouds implicate that loss by instabilities should be comparable with other loss processes studied by the ASPERA-3 instrument on board of Mars Express.