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. Issue 3 (28th February 2023) - Trompet, L.
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 ( L s ) 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 L s 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 ( L s = 0°–135°). Plain Language Summary: Martian mesosphere (the region of the atmosphere between 50 and 100 km in altitude) is the place where many large and small‐scale thermal features occur, which necessitates observations made at a fine spatial resolution. The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument is an infrared spectrometer sensitive in a spectral range where CO2, H2 O, CO, and CH4 have spectral signatures. NOMAD‐SO probes the Martian atmosphere with a high signal‐to‐noise ratio and a fine vertical sampling using the solar occultation technique. We implemented a scheme to fine‐tune the regularization of the vertical profiles leading to the best possible resolution of the retrieved profiles. We show here almost a year and a half of the measurements of carbon dioxide density and temperature of the Mars mesosphere at the terminator, that is, the separation between the day side and the night side. On average, the mesosphere is characterized by a decrease in the temperature with height. At the high resolution achieved here, we highlight many warmer layers in the Northern hemisphere and also in the Southern hemisphere at dawn but not at dusk. Stronger warm layers are found around the perihelion. Key Points: We show global vertical distributions of CO2 and temperature with high resolution in the Mars mesosphere at the terminator Many mesospheric temperature profiles at the terminator present an inversion layer with a warm layer around 0.1 Pa The distribution of warm layers is similar in Martian years 35 and 36. Warm layers are absent at dusk in the South hemisphere at L s 50°–150°