Abstract
On Mars, saturation is the major factor constraining the vertical
distribution of water vapor. Recent measurements of water and
temperature profiles showed that water can be strongly supersaturated at
and above the level where clouds form during aphelion and perihelion
seasons. Since 2018, the near-infrared spectrometer (NIR) of the
Atmospheric Chemistry Suite onboard the Trace Gas Orbiter has measured
H2O and temperature profiles using solar occultation in
the infrared from below 10 km to 100 km of altitude. Here we provide the
first long-term monitoring of the water saturation state. The survey
spans 2 Martian years from Ls=163° of MY34 to the Ls=180° of MY36. We
found that water is often supersaturated above aerosol layers. In the
aphelion season, water mixing ratio above 40 km in the mid-to-high
latitudes was below 3 ppmv and yet is found to be supersaturated. Around
perihelion, water is also supersaturated above 60 km with a mixing ratio
of 30-50 ppmv. Stronger saturation is observed during the dusty season
in MY35 compared to what was observed in MY34 during the Global Dust
Storm and around perihelion. Saturation varied between evening and
morning terminators in response to temperature modulation imparted by
thermal tides. Although water vapor is more abundant in the evening,
colder morning temperatures induce a daily peak of saturation. This
dataset establishes a new paradigm for water vapor on Mars, revealing
that supersaturation is nearly ubiquitous, particularly during the dust
season, thereby promoting water escape on an annual average.