A Two‐Martian Years Survey of the Water Vapor Saturation State on Mars Based on ACS NIR/TGO Occultations

Abstract

On Mars, condensation 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 the 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 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 Ls = 170° of MY36. We found that water is often supersaturated above aerosol layers. In the aphelion season, the water mixing ratio above 40 km in the mid‐to‐high latitudes was below 3 ppmv and yet is found to be supersaturated. Around the 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 the perihelion. Saturation varied between the 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 data set 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.

Thermal Structure of the Middle and Upper Atmosphere of Mars from ACS/TGO CO2 Spectroscopy

“…10k). An asymmetry between the hemispheres is revealed during the solstice seasons, as it was also observed by MCS and ACS NIR below 100 km (McCleese et al, 2010;Fedorova et al, 2022). In the thermosphere around 140-180 km, the morning summer at middle-high latitudes of both hemispheres is warmer than the evening one: 220-260 K versus 190-200 K, respectively (Fig.…”

“…We observe peak temperatures in the middle atmosphere (40-80 km) during the GDS of MY 34, and a few smaller peaks at the regional B and C storms, regardless of their locations in high southern latitudes and close to the polar night time (Figures 9c and 9d). Apart from the distributions observed during the stormy periods, the middle atmosphere at perihelion occultations when studying H 2 O saturation conditions (Fedorova et al, 2022). The SPICAM stellar occultations demonstrated a polar warming on the night side at altitudes below the mesopause during the period L S = 120°-150° (Forget et al, 2009).…”

“…The applied retrieval algorithm was performed for orders #222 and #223 separately since they require different calibration parameters with unique ILSs. Vertical profiles from order #223 were derived by Belyaev et al (2021) and validated with analogous schemes, such as the one adopted by Alday et al (2021a) using the NEMESIS algorithm and the one from ACS NIR (Fedorova et al, 2022) for simultaneous occultations at altitudes below 100 km. Comparison between the present MIR and the NIR temperature profiles is shown in Supplemental Figure S1, where a good consistency within ~10 K is seen in frames of the average measurement errors about 5-10 K (see also Fig.…”

“…We compare all individual profiles point-by-point, derived from simultaneous and collocated MIR and NIR observations. The ACS NIR data are taken from Fedorova et al (2022). Figure S2 shows the six temperature profiles containing CO 2 subfreezing points.…”

Global Vertical Distribution of Water Vapor on Mars: Results From 3.5 Years of ExoMars‐TGO/NOMAD Science Operations

“…In total, 2307 retrieved water vapor abundances in 77 occultations are available for the comparison in this period. We have used the most recent version of the ACS NIR retrievals (Fedova et al., 2022 ). The procedure of the ACS NIR retrievals can be found in Fedorova et al.…”