We investigate the global seasonal variations of near-surface relative humidity and
relevant attributes, like temperature and water vapor volume mixing ratio on Mars
using calculations from modelled and measurement data. We focus on 2 AM local time
snapshots to eliminate daily effects related to differences in insolation, and to
be able to compare calculations based on modelling data from the Laboratoire de Meteorologie
Dynamique Mars General Circulation Model with the observations of Mars Global Surveyor
Thermal Emission Spectrometer. We study the seasonal effects by examining four specific
dates in the Martian year, the northern spring equinox, summer solstice, autumn equinox,
and winter solstice. We identify three specific zones, where the near-surface relative
humidity levels are systematically higher than in their vicinity regardless of season.
We find that these areas coincide with low thermal inertia features, which control
surface temperatures on the planet, and are most likely covered with unconsolidated
fine dust with grain sizes smaller than similar to 40 mu m. By comparing the data
of relative humidity, temperature and water vapor volume mixing ratio at three different
heights (near-surface, similar to 4 m and similar to 23 m above the surface), we demonstrate
that the thermal inertia could play an important role in determining near-surface
humidity levels. We also notice that during the night the water vapor levels drop
at similar to 4 m above the surface. This, together with the temperature and thermal
inertia values, shows that water vapor likely condenses in the near-surface atmosphere
and on the ground during the night at the three aforementioned regions. This condensation
may be in the form of brines, wettening of the fine grains by adsorption or deliquescence.
This study specifies areas of interest on the surface of present day Mars for the
proposed condensation, which may be examined by in-situ measurements in the future.