First ENA Observations at Mars: Charge Exchange ENAs Produced in the

2Measurements of energetic neutral atoms (ENA) generated in the magnetosheath at Mars are reported. These ENAs are the result of charge exchange collisions between solar wind protons and neutral oxygen and hydrogen in the exosphere of Mars. The peak of the observed ENA flux is 1.3 × 1011m−2sr−1s−1. For the case studied here, i.e., the passage of Mars Express through the Martian magnetosheath around 20:15 UT on 3 May 2004, th