O2+ production coming from CO2 single-event electron impact

The inventory of molecules in 𝐶𝑂₂-rich atmospheres, such as our planetary neighbors Venus and Mars, can be significantly impacted, since they are constantly exposed to ionizing radiation, by the fragmentation processes of this particular species. However, the production of 𝑂₂⁺ as a direct result of 𝐶𝑂₂ fragmentation had never been quantified before. Since molecular oxygen is considered a potential biosignature, understanding the non-biotic pathways for its production is essential to rule out false positives in the search for extraterrestrial life.

In this work, we identified and measured 𝑂₂⁺ ions in absolute terms and ensured that their production results from the fragmentation of 𝐶𝑂₂ by electron impact. The break-up processes of 𝐶𝑂₂ after ionization was studied by means of a pulsed electron gun in the 30 to 800 eV energy range, a gas cell with monitored pressure and a time-of-flight mass spectrometer. The DETOF technique was used to determine the kinetic energy distributions for each produced fragment, via the collection of the recoil ion of interest for different delay times between the projectile-target interaction and the extraction of the ions from the interaction region. This methodology unequivocally ensures that the detected molecular oxygen originates from the fragmentation of the 𝐶𝑂₂ molecule, with an average kinetic energy of 1.4 eV, and not from a possible air contamination.

This non-biotic pathway for oxygen production opens new perspectives for the artificial generation of 𝑂₂. Furthermore, it may provide the answer to the high concentration of 𝑂₂⁺ in the upper layers of Mars’ atmosphere, which remains unexplained by current physico-chemical models and their computational simulations. Finally, as mentioned earlier, this pathway could be of great importance in the investigation of the possibility of extraterrestrial life, as it shows that the mere detection of 𝑂₂⁺ does not necessarily imply the presence of biological organisms.