THE IMPLANTATION AND INTERACTIONS OF O+ IN TITAN'S ATMOSPHERE: LABORATORY MEASUREMENTS OF COLLISION-INDUCED DISSOCIATION OF N2 AND MODELING OF POSITIVE ION FORMATION

Energetic oxygen ions are an important component of the plasma incident onto Titan's atmosphere. Therefore, we report measurements of electron capture and ionization collisions of N2 with incident O+ over the energy range 10-100 keV. Using time of flight coincidence counting techniques we also measured the collision-induced dissociation of N2 following ionization and electron capture. The electron capture and ionization cross sections were found to have comparable magnitudes. Capture collisions are dominated by non-dissociative processes with the dissociative processes providing contributions that are only slightly smaller. In contrast, ionization is entirely dominated by the dissociative processes. The energy distributions of the N+ and N atom fragments ejected by 20, 50, and 100 keV incident O+ projectiles have also been determined. These fragments carry considerable amounts of energy and if produce in the exobase region can readily escape. The cross sections measured here have been used with Cassini energetic ion and atmospheric density data to determine the ionization by and neutralization of energetic O+ penetrating Titan's N2 rich atmosphere. Neutralization by charge exchange is found not to occur efficiently above Titan's exobase, so energetic particles with large gyroradii penetrate the atmosphere primarily as ions. When the energetic O+ flux is large, we also show it is an important source of ionization and heating at depth into Titan's atmosphere and the fragments contribute to the net atmospheric loss rate.