Modeling Atmospheric Alteration on Titan: Hydrodynamics and Shock-Induced Chemistry of Meteoroid Entry

Meteoroid entry into planetary atmospheres generates bow shocks, resulting in high-temperature gas conditions. In shocked gas, high temperatures accelerate chemical reactions, leading to significant compositional changes. However, as the gas expands and cools, the reaction rate decreases (cf. Arrhenius's law) and eventually becomes slower than the cooling timescale, causing chemical reactions to freeze out. Thus, the final chemical composition is governed by two key fliud dynamical processes: shock heating and subsequent cooling. 

However, many previous studies have estimated the final chemical products under the assumption of equilibrium neglecting fluid dynamics. In this paper, we perform three-dimensional hydrodynamic simulations of meteoroid entry using the Athena++ code, coupled with chemical kinetics calculations via Cantera to model the non-equilibrium chemistry triggered by atmospheric entry. Our aerodynamical simulations reveal the formation of complex shock structures, including secondary shock waves, which influence the thermodynamic evolution of the gas medium. By tracking thermodynamic parameters along streamlines, we analyze the effects of shock heating and subsequent expansion cooling on chemical reaction pathways.

Our results demonstrate that chemical quenching occurs when the cooling timescale surpasses reaction rates, leading to the formation of distinct chemical products that deviate from equilibrium predictions. We show that the efficiency of molecular synthesis depends on the object’s size and velocity, influencing the composition of the post-entry gas mixture. Applying our model to Titan, we demonstrate that organic matter can be synthesized in the present environment of Titan. Also, we find that nitrogen, the dominant atmospheric component, remains stable, while water vapor is efficiently removed, a result inconsistent with equilibrium chemistry assumptions. Moreover, we compare our simulation results with laser experiments and find good agreement in chemical yields. Subsequent impact on the ground surface generates vaporized gas, which can also contribute to atmospheric alteration. Finally, we assess the relative contributions of atmospheric entry heating and impact-induced vaporization in driving atmospheric evolution.