The interaction of neutral and charged atmospheric particles with the surface of spacecraft moving at orbital velocities releases positive ions that are detected by the ion mass spectrometer and retarding potential analyzer on the Atmosphere Explorer spacecraft. the emitted particles have a characteristic energy of approximately 1 ev in the spacecraft frame of reference and lack the normal ram energy possessed by the ambient ions. Alkali ions Na+ and K+, as well as nonionospheric NO+ and O2+, are observed at low altitudes. All these except NO+ appear to be released by impacts of neural N2 or O2 with the surfaces. These ion currents decrease with a time constant of approximately 6 weeks for the alkali and 4 weeks for the O2 ions. The efficiency of the impact process is approximatey one ion released for each 106 N2 or 105O2 collisions. Alkali ions are also released by collisions of O+ ions with such surfaces, with the efficiency 3¿103 O+ impacts per ion released. The scale heights of the alkali ion currents follow that of N2 and O2 at low altitudes and that of O+ at altitudes above 300 km, where O+ becomes their dominant source, NO+ ions appear to be formed by O collisions with the surfaces. Their abundance is highly variable, indicating that the supply of surface nitrogen atoms is greatly variable.