A two-Martian year survey of the water vapor saturation state on Mars
based on ACS NIR/TGO occultations
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.