Velocity Distributions of C Atoms in CO+ Dissociative Recombination: Implications for Photochemical Escape of C from Mars

摘要

We have carried out Monte Carlo calculations to determine the velocity distributions of C atoms produced by dissociative recombination of CO+ using recent data for the branching ratios of various allowed channels and ion and electron temperatures appropriate to the Martian thermosphere. We find that the fractions of 12C atoms with velocities greater than the escape velocity are ∼ 0.66 and ∼ 0.62, and those for 13C are ∼ 0.47 and ∼ 0.48 at the plasma temperatures characteristic of the exobases at low and high solar activities, respectively. The ratio of the escape fractions of 13C and 12C, which is a measure of the isotope effect inherent in the CO+ dissociative recombination mechanism, is thus in the range 0.72–0.77. Using model thermospheres and ionospheres for low solar activity from Viking, and for high solar activity from the Mars Thermospheric General Circulation Model, we have computed the global escape flux of C due to dissociative recombination of CO+ as (3 − 5) × 105 cm−2 s−1. This value is of the same order as current estimates for the escape flux of C in all forms due to sputtering by O+ pickup ions at the current epoch but may be much less than that due to sputtering at earlier times in the history of the planet.