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Energy relaxation in superthermal collisions of carbon with oxygen: the influence of isotopic substitution

Cheikh T. Bop, Marko Gacesa

TL;DR

The study addresses how superthermal carbon escape from Mars depends on energy relaxation through collisions with atmospheric oxygen, challenging the common proxy use of O+O data for C+O. It advances the field by computing a complete set of 18 adiabatic PECs for the C(^3P) + O(^3P) system using MRCISD+Q with AV5Z, including all spin states and accurate long-range behavior, and by deriving statistically averaged elastic and differential cross sections across all relevant isotopologues. The results show that C+O elastic cross sections are up to about 2× larger than O+O at low energies, with isotopic substitutions altering cross sections by up to 8% and simple mass-scaling being inadequate. These findings have direct implications for carbon escape and isotopic fractionation modeling on Mars and similar atmospheres, and the data provide a resource for transport coefficients in CO$_2$-rich environments and related planetary photochemistry.

Abstract

The transition from a once-dense Martian atmosphere to the thin one observed today implies a substantial loss of carbon, either through atmospheric escape or surface deposition. Accurately modeling this carbon escape necessitates accounting for collisions between energetic carbon atoms and the primary atmospheric constituents, including oxygen. To this end, we computed a highly accurate and comprehensive set of potential energy curves (PECs) for the C($^3$P) + O($^3$P) system. Based on these PECs, we derived statistically averaged total elastic and differential cross sections. Comparison with literature data for O($^3$P) + O($^3$P) collisions reveals that cross sections involving carbon can differ by up to a factor of two, indicating that oxygen is not a good proxy for modeling carbon escape. Furthermore, we evaluated the impact of all possible isotopic combinations in C($^3$P) + O($^3$P) collisions and found variations in cross sections of up to 8\%. Given the observed isotopic enrichment of carbon and oxygen in the Martian atmosphere, even such moderate differences can have a significant effect on escape models and the interpretation of planetary evolution.

Energy relaxation in superthermal collisions of carbon with oxygen: the influence of isotopic substitution

TL;DR

The study addresses how superthermal carbon escape from Mars depends on energy relaxation through collisions with atmospheric oxygen, challenging the common proxy use of O+O data for C+O. It advances the field by computing a complete set of 18 adiabatic PECs for the C(^3P) + O(^3P) system using MRCISD+Q with AV5Z, including all spin states and accurate long-range behavior, and by deriving statistically averaged elastic and differential cross sections across all relevant isotopologues. The results show that C+O elastic cross sections are up to about 2× larger than O+O at low energies, with isotopic substitutions altering cross sections by up to 8% and simple mass-scaling being inadequate. These findings have direct implications for carbon escape and isotopic fractionation modeling on Mars and similar atmospheres, and the data provide a resource for transport coefficients in CO-rich environments and related planetary photochemistry.

Abstract

The transition from a once-dense Martian atmosphere to the thin one observed today implies a substantial loss of carbon, either through atmospheric escape or surface deposition. Accurately modeling this carbon escape necessitates accounting for collisions between energetic carbon atoms and the primary atmospheric constituents, including oxygen. To this end, we computed a highly accurate and comprehensive set of potential energy curves (PECs) for the C(P) + O(P) system. Based on these PECs, we derived statistically averaged total elastic and differential cross sections. Comparison with literature data for O(P) + O(P) collisions reveals that cross sections involving carbon can differ by up to a factor of two, indicating that oxygen is not a good proxy for modeling carbon escape. Furthermore, we evaluated the impact of all possible isotopic combinations in C(P) + O(P) collisions and found variations in cross sections of up to 8\%. Given the observed isotopic enrichment of carbon and oxygen in the Martian atmosphere, even such moderate differences can have a significant effect on escape models and the interpretation of planetary evolution.

Paper Structure

This paper contains 6 sections, 6 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Computational details
  3. Adiabatic potential energy curves
  4. Cross sections
  5. Results and discussions
  6. Conclusion

Figures (5)

  • Figure 1: Potential energy curves for CO electronic states that correlate with the first asymptote.
  • Figure 2: Total elastic cross sections for the different CO electronic states, correlating with the first asymptote, as a function of kinetic energy. The labels of the cross sections are derived from the electronic states.
  • Figure 3: Dependence on kinetic energy of the statistically averaged cross sections induced by C($^3$P) + O($^3$P) collisions (this work) and O($^3$P) + O($^3$P) scattering kharchenko2000energy.
  • Figure 4: Kinetic energy dependence of the relative deviations upon isotopic substitution compared to CO statistically averaged total elastic cross sections.
  • Figure 5: Angular dependence of statistically averaged differential cross sections, for kinetic energies of 0.05, 0.5, and 5.0 eV, for the CO collision system.