Oxidizing Proto-atmosphere on Titan: Constraint from N₂ Formation by Impact Shock

Abstract

Titan is the only satellite that possesses a thick atmosphere, composed mainly of N₂ and CH₄. However, its origin and evolution remain largely unknown. Knowledge of the acquirement of a N₂ atmosphere on Titan would provide insights into nitrogen evolution in planetary atmospheres as well as the formation of satellite systems around gas giants. Previous studies have proposed that the atmospheric N₂ would have been converted from NH₃ via shock heating by accreting satellitesimals in the highly reducing proto-atmosphere composed of NH₃ and CH₄. Nevertheless, the validity of this mechanism strongly depends on both the composition of the proto-atmosphere and kinetics of shock chemistry. Here, we show that a CO₂-rich oxidizing proto-atmosphere is necessary to form N₂ from NH₃ efficiently by atmospheric shock heating. Efficient shock production of N₂ is inhibited in a reducing proto-atmosphere composed of NH₃ and CH₄, because CH₄ plays as the coolant gas owing to its large heat capacity. Our calculations show that the amount of N₂ produced in a CO₂-rich proto-atmosphere could have reached ~20 times that on the present Titan. Although further quantitative analysis are required (especially, the occurrence of catalytic reactions), our results imply that the chemical composition of satellitesimals that formed the Saturnian system is required to be oxidizing if the current atmospheric N₂ is derived from the shock heating in the proto-atmosphere during accretion. This supports the formation of regular satellites in an actively supplied circumplanetary disk using CO₂-rich materials originated from the solar nebula at the final stage of gas giant formation.