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 Météorologie 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 .