Retrieval of Martian Atmospheric CO Vertical Profiles From NOMAD Observations During the First Year of TGO Operations

Abstract

We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, LS = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around LS = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed.

Martian Atmospheric Temperature and Density Profiles During the First Year of NOMAD/TGO Solar Occultation Measurements

“…(2023) and by Modak et al. (2023), respectively). At the core of the inversion scheme is a line‐by‐line radiative transfer Forward Model (KOPRA, see e.g., Stiller (2000)) and an inversion control program (or Retrieval Control Program (RCP) in short, see von Clarmann et al.…”

“…The inversion of temperature and density presented here is part of a broader effort at IAA/CSIC devoted to the scientific exploitation of NOMAD and ACS measurements with a common data analysis scheme. We followed a sequential retrieval strategy, starting with the inversion of atmospheric opacity from the continuum in the SO data, then the temperature and density, and these are used later in the inversion of trace species (see the two companion papers on H 2 O and CO by Brines et al (2023) and by Modak et al (2023), respectively). At the core of the inversion scheme is a line-by-line radiative transfer Forward Model (KOPRA, see e.g., Stiller (2000)) and an inversion control program (or Retrieval Control Program (RCP) in short, see von Clarmann et al ( 2003)) originally developed at the University of Karlsruhe, and briefly described below in Section 3.2.…”

Water Vapor Vertical Distribution on Mars During Perihelion Season of MY 34 and MY 35 With ExoMars‐TGO/NOMAD Observations

“…These values are used during gas retrievals, like H 2 O, CO 2 (Lopez‐Valverde et al., 2022), or CO Modak et al. (2023) following a sequential retrieval chain. This preprocessing works considering the measured transmittances ( T ) corrected for spectral shift (Δ λ ) as a proportional combination of the forward model ( T 0 ) scaled by a factor ( k ), the residual bending ( T b ), and the aerosol slant opacity ( τ ).…”

“…(2023) derives nature and composition of Martian aerosols, and Modak et al. (2023) retrieves CO vertical profiles. Here, we present the vertical distributions of water vapor abundances retrieved from NOMAD data during the first half of the perihelion season in two consecutive Martian years (MYs).…”

Martian Atmospheric Aerosols Composition and Distribution Retrievals During the First Martian Year of NOMAD/TGO Solar Occultation Measurements: 1. Methodology and Application to the MY 34 Global Dust Storm

“…The continuum curvature is evaluated by fitting a 4th order polynomial to the signal continuum. The orders used in this work have also been exploited by our team for deriving temperature, water vapor and carbon monoxide profiles, and a quantitative description of the correction of these features by our pre‐processing can be found in a few companion papers (Brines et al., 2022; López Valverde et al., 2022; Modak et al., 2023).…”