AbstractOne of the most important modes of planet/solar wind interaction are “foreshock transients” such as hot flow anomalies (HFAs). Here we present early observations by the NASA Mars Atmosphere and Volatile EvolutioN spacecraft, confirming their presence at Mars and for the first time at an unmagnetized planet revealing the underlying ion perturbations that drive the phenomenon, finding them to be weaker than at magnetized planets. Analysis revealed the HFA to be virtually microscopic: the smallest on record at ∼2200 km across and commensurate with the local proton gyroradius, resulting in a much stronger perturbation in solar wind protons than alpha particles. As at Venus, despite being physically diminutive, the HFA is still large (0.66 RM) when compared to the relative size of the induced magnetosphere. Given the associated order of magnitude decrease in solar wind dynamic pressure (411 pPa  70 pPa), we find that HFAs at Mars have the potential to directly impact the topside ionosphere. We thus hypothesize that the loss of a planetary magnetic dynamo left Mars far more vulnerable to the pressure pulses resulting from HFAs and related foreshock transients.