  Data assimilation is an increasingly popular technique in Mars atmospheric
science, but its effect on the mean states of the underlying atmosphere models
has not been thoroughly examined. The robustness of results to the choice of
model and assimilation algorithm also warrants further study. We investigate
these issues using two Mars general circulation models (MGCMs), with particular
emphasis on zonal wind and temperature fields. When temperature retrievals from
the Mars Global Surveyor Thermal Emission Spectrometer (TES) are assimilated
into the U.K.-Laboratoire de M\'et\'eorologie Dynamique (UK-LMD) MGCM to create
the Mars Analysis Correction Data Assimilation (MACDA) reanalysis, low-level
zonal jets in the winter northern hemisphere shift equatorward and weaken
relative to a free-running control simulation from the same MGCM. The Ensemble
Mars Atmosphere Reanalysis System (EMARS) reanalysis, which is also based on
TES temperature retrievals, also shows jet weakening (but less if any shifting)
relative to a control simulation performed with the underlying Geophysical
Fluid Dynamics Laboratory (GFDL) MGCM. Examining higher levels of the
atmosphere, monthly mean three-dimensional temperature and zonal wind fields
are in generally better agreement between the two reanalyses than between the
two control simulations. In conjunction with information about the MGCMs'
physical parametrizations, intercomparisons between the various reanalyses and
control simulations suggest that overall the EMARS control run is plausibly
less biased (relative to the true state of the Martian atmosphere) than the
MACDA control run. Implications for future observational studies are discussed.
