Effects of dissociative recombination on the composition of planetary atmospheres

Abstract

Because dissociative recombination (DR) reactions of molecular ions are often highly exothermic, in the thermospheres of the Earth and planets DR may be a source of translationally and internally excited fragments. DR is important, therefore, for thermospheric neutral heating; if the excited fragments radiate to space, however, DR may be also a source of thermospheric cooling. DR may produce metastable fragments, which may live long enough to participate in reactions that are not available to ground state species. It is rare, however, for DR to be a significant source of minor species in their ground states. An exception appears to be the DR of CO+2, which has recently been found to produce C + O2 about 9% of the time by Seiersen et al.. Because of the significant rearrangement of bonds that must take place, the branching ratio for the latter channel has been assumed to be negligible, and DR of CO+2 has been assumed to produce mainly CO + O. In order to test the effect of including the branching ratio of CO+2 DR that produces C + O2 on the ambient densities of thermal and escaping C in planetary thermospheres,we have we have constructed revised models of the thermospheres/ionospheres of Mars and Venus. Because of space limitations, we discuss here only the high solar activity models. For Mars, we find that DR of CO+2 is the most important source of thermal C, but that the production rate of escaping C is not increased. There are important differences between the thermospheres of Venus and Mars, and we find that the inclusion of the C + O2 channel in the Venus models increases the production rate of atomic carbon in the Venus thermosphere by only 10–16%. At high altitudes on Venus, C+ is mostly produced by photoionization and electron-impact ionization of C, with some contribution from the charge transfer reaction, O+ + C → C+ + O. We compare our computed C density altitude profiles to those inferred by Paxton from Pioneer Venus Orbiter Ultraviolet Spectrometer limb scans of the atomic carbon emission features at 1561 and 1657 Å. Since the most important loss process for C is reaction with O2, this allows us to to constrain the abundance of O2 in the Venus thermosphere. We then compute density profiles of C+ and compare them to those measured by the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) (e.g., Taylor et al. [3]) to determine the rate coefficient for the charge transfer reaction of O+ to C.