Quantum-mechanical H/D-CO₂ collisions and their impact on atmospheric escape and evolution of CO₂-rich planets

Collisions between suprathermal hydrogen atoms and CO₂ are a controlling microphysical process in the upper atmospheres of CO₂-dominated planets, governing energy transfer, momentum loss, and ultimately atmospheric escape and isotopic evolution. Despite their importance, H/D-CO₂ collisional parameters used in planetary atmosphere models are still largely based on reduced-mass scaling, surrogate collision systems, or classical approximations developed decades ago.

We present new quantum-mechanical state-resolved, total, and momentum-transfer cross sections for H-CO₂ and D-CO₂ collisions at collision energies up to 5 eV, computed using coupled-states scattering calculations on a high-level ab initio potential energy surface. The results reveal strongly forward-peaked scattering, leading to momentum-transfer cross sections and rate coefficients that are an order of magnitude smaller than values commonly adopted in planetary escape models. Mass-scaling from heavier projectiles (O-CO₂, C-CO₂) is shown to overestimate H-CO₂ cross sections by factors of 30-45. Isotopic substitution (H/D) introduces energy-dependent differences of up to ~35% at low energies, invalidating uniform scaling approaches used in D/H fractionation studies.

Maxwellian-averaged momentum-transfer rate coefficients derived from the new cross sections imply significantly reduced collisional thermalization efficiency for hot hydrogen in CO₂-rich thermospheres. In simple escape formulations, these revisions correspond to shifts in the exobase altitude of order 10–20 km and order-unity changes in thermal escape rates. For non-thermal escape, where suprathermal atoms experience only a few collisions, the impact on escape probabilities and isotopic fractionation is expected to be even more direct.

We will demonstrate the implications of these new cross sections using basic photochemical and escape calculations for Mars, and discuss their relevance for Venus, early Earth, and other CO₂-dominated planetary and exoplanetary atmospheres. These results provide long-missing quantum-mechanical inputs for revisiting atmospheric evolution scenarios where hydrogen escape plays a central role.