While the Mars upper atmosphere is continuously bombarded by charged particles of solar and planetary origins, the energy flux carried is often not sufficient to significantly affect the neutral atmosphere.However, we show that this is not the case during major space weather events.By applying two Mars global models-a Monte Carlo model for simulating pickup O+ precipitation at the exobase and a thermosphere-ionosphere model for assessing its global impact, we find that the thermospheric effects of reentering ions can change from negligible to very important when upstream solar w ind conditi ons vary from normal to extreme.The atmospheric response under the most extreme conditions includes dramatic neutral temperature enhancement, significant neutral composition and wind changes, and increased importance of sputtering loss and possibly even thermal escape of heavy species.