Sources of Energetic Neutral Atoms at Mars

    ENAs are produced by a charge exchange between the Martian exosphere containing H, H₂, He, and O and the different plasma populations such as the supersonic solar wind, the shocked solar wind, and accelerated planetary ions.  The tiny Phobos atmosphere can also interact with both the supersonic and the shocked solar wind resulting in ENA generation.  The heavy planetary O⁺ ions picked up by the plasma flow can re-enter the atmosphere causing effective sputtering of oxygen.  The back-scattered atoms have energies up to a few hundred eV and form the ENA albedo.  With the ASPERA-3 experiment we are going to study all of these ENA sources and search for new ones.