Spectrum of hot O at the exobases of the terrestrial planets

The distribution of energetic O produced in dissociative recombination of O 2 + at the exobases of the terrestrial planets is important in determining the structure of the outer hot O coronas and for the escape flux of O from Mars. Using recently measured values for the branching ratios of the energetically allowed channels in O 2 + dissociative recombination, along with models of the vibrational distribution of O 2 + , we compute the velocity distribution of hot O atoms produced at the exobases of Mars, Venus, and Earth. We take into account the effects of the ion and electron temperatures, the rotational energy of the ions, and the decrease of the dissociative recombination cross section with relative velocity of the ion and electron. We find that distinct peaks appear at the energies associated with the various channels in the Martian spectrum but that for Venus, the features are less distinct, and for Earth, the distribution appears as a single, but broad and asymmetrical peak. For Mars, we compute the fractions of atoms produced with speeds greater than the escape velocity for 16 O, 17 O, and 18 O of 0.68, 0.64, and 0.58, respectively. Thus there is a small isotope effect inherent in the dissociative recombination mechanism that has not been taken into account in models of Martian atmospheric evolution or in estimates of the total inventory of oxygen that is implied by the observed lack of enhancement of the heavier isotopes relative to the terrestrial values.