Quantum scattering of hot H/D on CO2: Cross sections and rate coefficients for planetary atmospheres and their evolution

Abstract Collisions between hot hydrogen atoms and CO2 play a central role in energy transfer and atmospheric escape in CO2-rich planetary atmospheres. We present quantum mechanical jz-conserving coupled-states calculations of state-resolved cross sections for H/D–CO2 collisions at energies up to 5 eV, benchmarked to within 7 % of close-coupling results. Scattering is strongly forward-peaked, yielding momentum-transfer cross sections substantially smaller than commonly assumed: mass-scaling from O/C–CO2 systems overestimates H–CO2 total cross sections by factors of 30–45, while existing scaled differential cross section fits underestimate the low-energy regime by up to ~45 %. Isotopic substitution (H/D) produces energy-dependent differences of up to 35 % at E < 0.1 eV, invalidating uniform scaling approaches for D/H fractionation. Maxwellian-averaged rate coefficients derived from our cross sections are significantly smaller than mass-scaled values, implying reduced H–CO2 energy transfer efficiency. In atmospheric escape modelling, these revisions can shift Martian exobase altitudes by 10–20 km, leading to order-unity changes in thermal escape rates, and have implications for hydrogen loss in early CO2-dominated planetary atmospheres. Our results provide essential quantum-mechanical inputs for revisiting atmospheric evolution scenarios on Mars, early Earth, and CO2-rich exoplanets.