Cooling of the Martian thermosphere by CO$_2$ radiation and gravity waves: An intercomparison study with two general circulation models

TL;DR

This study compares two Martian general circulation models to evaluate how CO₂ radiative cooling and gravity waves contribute to the observed colder temperatures in Mars's lower thermosphere, highlighting their combined effects and spatial variations.

Contribution

It provides a direct intercomparison of two advanced GCMs to quantify the roles of CO₂ radiative cooling and gravity waves in Martian thermospheric temperature regulation.

Findings

Both mechanisms can cause up to 40 K cooling in the thermosphere.

Cooling effects peak at different altitudes, with radiative cooling peaking above the mesopause.

Combined effects lead to stronger cooling at high latitudes.

Abstract

Observations show that the lower thermosphere of Mars ($\sim$100--140 km) is up to 40 K colder than the current general circulation models (GCMs) can reproduce. Possible candidates for physical processes missing in the models are larger abundances of atomic oxygen facilitating stronger CO$_2$ radiative cooling, and thermal effects of gravity waves. Using two state-of-the-art Martian GCMs, the Laboratoire de M\'et\'eorologie Dynamique and Max Planck Institute models that self-consistently cover the atmosphere from the surface to the thermosphere, these physical mechanisms are investigated. Simulations demonstrate that the CO$_2$ radiative cooling with a sufficiently large atomic oxygen abundance, and the gravity wave-induced cooling can alone result in up to 40 K colder temperature in the lower thermosphere. Accounting for both mechanisms produce stronger cooling at high latitudes. However, radiative cooling effects peak above the mesopause, while gravity wave cooling rates continuously increase with height. Although both mechanisms act simultaneously, these peculiarities could help to further quantify their relative contributions from future observations.