The CO₂–O Quenching Rate as a Controlling Parameter of the Martian Upper-Atmosphere Energy Budget and Dynamics

The Martian mesosphere and thermosphere form a highly variable transition region coupling the lower atmosphere to the space environment. The thermal structure and circulation in this region are controlled by a delicate balance between radiative heating and cooling, gravity-wave forcing, large-scale transport, and solar forcing. Among these processes, non-LTE infrared cooling in the CO₂ 15 µm bands and solar heating by near-IR CO₂ absorption bands dominate the radiative energy budget over a broad altitude range. However, substantial uncertainty remains in quantifying collisional CO₂–O vibrational quenching processes that control these mechanisms in the Martian general circulation models (MGCMs).

In this study, we investigate the sensitivity of the Martian upper-atmosphere energy budget and dynamics to the CO₂–O quenching rate coefficient using the MAOAM MGCM (Medvedev et al, 2015). The calculations employ a modified version of the terrestrial non-LTE CO₂ radiative routine developed by Kutepov & Feofilov (2024). The routine was extended to include additional CO₂ isotopes, vibrational levels, and radiative bands in the 1–15 µm spectral region, allowing simultaneous treatment of daytime and night-time non-LTE CO₂ cooling and solar heating processes under Martian conditions.

The simulations were performed using the laboratory-supported CO₂–O quenching rate coefficient k=1.5×10⁻¹² cm³ s⁻¹.

This value is half the size of the k=3.0×10⁻¹² cm³ s⁻¹ coefficient currently adopted in essentially all existing Martian GCM non-LTE parameterizations, despite the absence of compelling laboratory or theoretical evidence supporting the larger value.

Our calculations demonstrate that both CO₂ 15 µm cooling and solar heating by absorption in the 1.0–2.7 µm CO₂ bands are approximately proportional to the adopted quenching coefficient. As a consequence, replacing the commonly used value by the laboratory-supported coefficient produces a dramatic reduction of both cooling and heating rates, reaching ~100–150 K sol⁻¹ in the 110–150 km altitude region. These changes modify the mean thermal structure and circulation, with variability affected the most. The results imply that the radiative energy balance and dynamical variability predicted by present-day MGCMs may be systematically biased. The results also have important implications for interpretation of stellar and solar occultation observations and for model–observation comparisons in the Martian upper atmosphere.

References

Kutepov, A. and Feofilov, A.: New routine NLTE15µmCool-E v1.0 for calculating the non-local thermodynamic equilibrium (non-LTE) CO₂ 15 µm cooling in general circulation models (GCMs) of Earth's atmosphere, Geosci. Model Dev., 17, 5331–5347, https://doi.org/10.5194/gmd-17-5331-2024, 2024.

Medvedev, A. S., F. González-Galindo, E. Yiğit, A. G. Feofilov, F. Forget, and P. Hartogh, Cooling of the Martian thermosphere by CO₂ radiation and gravity waves: An intercomparison study with two general circulation models, Journal of Geophysical Research: Planets, 120, 913–927, doi:10.1002/ 2015JE004802, 2015.