The Escape of O⁺ and CO⁺ Ions from Mars and Titan Atmospheres by Coulomb Explosion of CO₂²⁺ Molecular Dications

Main properties of multiply charged molecular ions (energetics, structure, stability, lifetime and fragmentation dynamics) are relevant to know and to model the behavior of gaseous plasmas, planetary ionospheres, and astrophysical environments. Experimental measurements of the kinetic energy released of fragment ions produced by the Coulomb explosion of molecular dications (doubly charged molecular species) originating by double photoionization of CO₂ molecules, are reported and discussed. The kinetic energy released, as a function of the UV photon energy in the range of 34-50 eV, is extracted from the electron-ion-ion coincidence spectra recorded by using tunable synchrotron radiation coupled with ion imaging techniques at the GasPhase beam line of ELETTRA Synchrotron Light Laboratory Trieste, Italy. This kind of experiment allows assessing the probability of escape for simple ionic species in the ionosphere of Mars and Titan. The kinetic energy released, measured in the case of O⁺ and CO⁺ fragment ions, are ranging between 1.0-5.0 and 0.4-3.0 eV, respectively. These values are large enough to allow such ionic species in participating in the atmospheric escape from Mars (only O⁺ ions) and Titan (both O⁺ and CO⁺ ions) into space. In the case of Mars, we suggest a possible rationalization for the observed behavior of the O⁺ and CO₂²⁺ ion density profiles by Viking 1 lander and Mariner 6 spacecraft.