Carbon Dioxide Retrievals From NOMAD‐SO on ESA's ExoMars Trace Gas Orbiter and Temperature Profiles Retrievals With the Hydrostatic Equilibrium Equation: 1. Description of the Method

Abstract

The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument has been scanning the Martian atmosphere for almost 2 Martian years. In this work, we present a subset of the NOMAD SO data measured at the mesosphere at the terminator. From the data set, we investigated 968 vertical profiles of carbon dioxide density and temperature covering the Martian Year (MY) 35 as well as MY 36 up to a solar longitude (Ls) of 135° and altitudes around 60–100 km. While carbon dioxide density profiles are directly retrieved from the spectral signature in the spectra, temperature profiles are more challenging to retrieve as unlike density profiles, temperature profiles can present some spurious features if the regularization is not correctly managed. Comparing seven regularization methods, we found that the expected error estimation method provides the best regularization parameters. The vertical resolution of the profiles is on average 1.6 km. Numerous warm layers and cold pockets appear in this data set. The warm layers are found in the Northern hemisphere at dawn and dusk as well as in the Southern hemisphere at dawn. Strong warm layers are present in more than 13.5% of the profiles. The Southern hemisphere at dusk does not present any warm layer between Ls 50° and 150°. The height and latitudinal distribution of those warm layers were similar in MY 35 and MY 36 during the first half of the year (Ls = 0°–135°).

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

“…This analysis is complementary and agrees with a similar analysis by Trompet et al. (2023) in a companion paper in this special issue, who studied the variation of the atmospheric density in NOMAD retrievals for MY35. In the absence of GDS, the MY35 presents a maximum around Ls 270° in their work.…”

“…In addition to this work, other two manuscripts studying the thermal structure in SO data from the TGO/NOMAD and ACS instruments have been submitted to this issue. In the first one, Trompet et al (2023) analyzed the NOMAD/SO channel's diffraction order 148 during Mars Years 35 and 36, excluding MY34 and focusing on mesospheric altitudes. The second presents profiles up to thermospheric altitudes combining two diffraction orders of the ACS/MIR channel, in a study covering two full Martian years which is an extension of Belyaev et al (2021).…”

Martian Ozone Observed by TGO/NOMAD‐UVIS Solar Occultation: An Inter‐Comparison of Three Retrieval Methods

“…The transmittance calibration is based on Trompet et al (2016) and further improved in Trompet et al (2023) for NOMAD occultation spectra. The spectra recorded as an occultation are split in three main regions: The Sun region (S) when UVIS points to the Sun and no atmospheric absorption is present, the atmospheric region (A) where UVIS line of sight crosses the atmosphere, and the Umbra region (U) when UVIS line of sight crosses the planet.…”

Depletion of 13C in CO in the Atmosphere of Mars Suggested by ExoMars-TGO/NOMAD Observations

“…This division removes systematic instrumental effects. The reference solar spectrum is created by extrapolation of those recorded above the atmosphere (see Trompet et al 2022a for details). The calculated transmittances are compared with synthetic spectra computed by a radiative transfer model to retrieve abundances of trace gases and aerosols along the line of sight (see below for further details).…”