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Observations show that the lower thermosphere of Mars ($\sim$100--140 km) isup to 40 K colder than the current general circulation models (GCMs) canreproduce. Possible candidates for physical processes missing in the models arelarger abundances of atomic oxygen facilitating stronger CO$_2$ radiativecooling, and thermal effects of gravity waves. Using two state-of-the-artMartian GCMs, the Laboratoire de M\'et\'eorologie Dynamique and Max PlanckInstitute models that self-consistently cover the atmosphere from the surfaceto the thermosphere, these physical mechanisms are investigated. Simulationsdemonstrate that the CO$_2$ radiative cooling with a sufficiently large atomicoxygen abundance, and the gravity wave-induced cooling can alone result in upto 40 K colder temperature in the lower thermosphere. Accounting for bothmechanisms produce stronger cooling at high latitudes. However, radiativecooling effects peak above the mesopause, while gravity wave cooling ratescontinuously increase with height. Although both mechanisms act simultaneously,these peculiarities could help to further quantify their relative contributionsfrom future observations.

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