Negative ions in the ionospheres of planetary bodies without atmospheres

Negative ions may be formed in the ionospheres of Mercury, the Moon and Jupiter's satellites with densities about a few % of the ionospheric electron density. The negative ions are produced by three mechanisms at the planetary surface: charge inversion during energetic proton scattering, with simultaneous secondary negative ion emission, and micrometeorite impacts. The density and distribution of negative ions around planetary bodies depends primarily upon the negative ion life-times determined by photodetachment by solar radiation.