  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.
