Isotope Effects on the Photochemical Escape of O from Mars

Dissociative recombination of O ₂ ⁺ ions is the most important photochemical escape mechanism for oxygen atoms from Mars at the current epoch. We construct here models of the Martian thermosphere/ionosphere for low and high solar activities, and determine the rate of O ₂ ⁺ dissociative recombination (DR) as a function of altitude for each model. Using a Monte Carlo method, we compute the probabilities of escape for ¹⁶ O and ¹⁸ O atoms and compare them to see if there is a significant isotope effect. The escape probability for ¹⁸ O atoms is expected to be less than that of ¹⁶ O atoms, partly because ¹⁸ O is released with less energy in DR of ³⁴ O ₂ ⁺ than that of ¹⁶ O in DR of ³² O ₂ ⁺ , and its escape energy is larger. This isotope effect, which is inherent in the mechanism, is independent of the fractionation that exists between the homopause and the exobase. The energy distribution of O produced in O ₂ ⁺ DR is determined by the values of the ion and electron temperatures at a given altitude, and by the channels by which the DR proceeds. This energy distribution will be computed using a Monte Carlo method as a function of altitude from 100 to 400 km. Because the isotopes of O are found not to be fractionated in the Martian atmosphere relative to earth, a significant reservoir of O that can exchange with the atmosphere is implied. We also compute the total rates of photochemical escape of topes of O are found not to be fractionated in the Martian atmosphere relative to earth, a significant reservoir of O that can exchange with the atmosphere is implied. We also compute the total rates of photochemical escape of ¹⁶ O from the Martian atmosphere for high and low solar activity models and compare them to the results of the many other investigators who have examined this escape mechanism.