Global seasonal variations of the near-surface relative humidity levels on present-day Mars

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 Météorologie 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 ∼ 40 μm. By comparing the data of
relative humidity, temperature and water vapor volume mixing ratio at three different heights (near-surface, ∼
4m and ∼ 23m 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 ∼
4m 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.