Recent observations of plasma waves, electron fluxes, and ion fluxes in Mars' ionosphere indicate that ion heating may have had a significant impact on Mars' atmospheric loss. We discuss two energy sources of plasma waves: the solar wind interaction with Mars and field-aligned currents in regions of crustal magnetic fields. These plasma waves can damp through cyclotron resonance with the O+ population in the ionosphere leading to heating and subsequent O+ escape supporting the ~1025 atoms s-1 (~0.4 kg/s) O+ outflow indicated by present-day observations. A stronger solar wind and O+ source of ~4 Gyr ago could support losses of ~100 kg/s, enough to strip Mars' atmosphere or 10 m of water in a ~0.3 Gyr period. The observational evidence for ion heating is, with current data sets, largely circumstantial so we suggest needed observations.