Climatology of the CO Vertical Distribution on Mars Based on ACS TGO Measurements

Abstract

Carbon monoxide is a non‐condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long‐term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls = 163° of MY34 to the end of MY35. We obtained a mean CO mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls = 100–200° in high southern latitudes with a layer of 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid‐latitudes southern summer atmosphere amounts to 700–750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.

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

“…In the first step, we retrieve the temperature and pressure profiles from orders 49 and 54. The spectral resolution of NIR allows resolving the rotational structure of the CO 2 bands and disentangling weak CO features within (the CO retrievals from order 49 are described in the companion paper, Fedorova et al, 2022). The retrieval employs an algorithm proposed by Ceccherini (2005) and Ceccherini et al (2007), which combines the Levenberg-Marquardt iterative scheme with Tikhonov regularization.…”

“…For this study, we used measurements of diffraction order 49 (6,318-6,387 cm −1 ), order 54 (6,960-7,040 cm −1 ), and order 56 (7,217-7,300 cm −1 ) (Figures 1a-1d). Order 49 was initially used to retrieve the temperature and CO 2 density using the 1.57 μm CO 2 band and CO mixing ratio using a weaker overlapping band (Fedorova et al, 2020(Fedorova et al, , 2022 (Figure 1b). Order 56 contains the strongest lines of the 1.38 μm water vapor band and was also used by Fedorova et al (2020) (Figure 1d).…”

“…It covers the 0.7–1.7 μm spectral range using diffraction orders 101 through 49 with a high resolving power of about 28,000 (Korablev et al., 2018; Trokhimovskiy, Korablev, et al., 2015). During an occultation, ACS NIR measures ten pre‐selected diffraction orders every 2 s. Extremely high spectral resolution allows ACS NIR to sound different atmospheric constituents such as H 2 O, CO, CO 2 , and weak band of O 2 (Fedorova et al., 2020, 2022; Korablev et al., 2018). The duration of occultation, from 100 km tangent altitude down to the surface, varies from 1 to 4 min depending on the angle between the orbit plane and the spacecraft–Sun vector (the beta angle), which gradually changes throughout the year (Korablev et al., 2018).…”

Photochemistry on Mars reduces the abundance of heavy isotopes in atmospheric CO and their escape efficiency to space

“…ACS observations have been previously used to measure for the first time the vertical distribution of CO and its seasonal variations in the atmosphere of Mars (Fedorova et al, 2022;Olsen et al, 2021). Here, we use the MIR data employed by Fedorova et al, but studying the relative abundances of 12 C 16 O, 13 C 16 O and 12 C 18 O.…”

“…The retrieval scheme from the NEMESIS algorithm works using the optimal estimation framework, which aims to find an optimal solution that is consistent with the observed spectra subject to a minimal departure from the a priori. Given that the absorption features of CO2 measured in ACS MIR secondary grating position 6 typically disappear at lower tangent altitudes than the CO ones, we use temperature profiles from simultaneous observations by ACS NIR (Fedorova et al, 2022) as the a priori in our retrievals, which ensures an accurate description of the temperature field at all altitudes. The use of accurate measurements of temperature is crucial to derive precise measurements of the CO isotopic ratios, which are highly sensitive to this parameter (see Supplementary Material).…”