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

Abstract

Since the beginning of the Trace Gas Orbiter (TGO) science operations in April 2018, its instrument “Nadir and Occultation for MArs Discovery” (NOMAD) supplies detailed observations of the IR spectrums of the Martian atmosphere. We developed a procedure that allows us to evaluate the composition and distribution's parameters of the atmospheric Martian aerosols. We use a retrieval program (RCP) in conjunction with a radiative forward model (KOPRA) to evaluate the vertical profile of aerosol extinction from NOMAD measurements. We then apply a model/data fitting strategy of the aerosol extinction. In this first article, we describe the method used to evaluate the parameters representing the Martian aerosol composition and size distribution. MY 34 GDS showed a peak intensity from LS 190° to 210°. During this period, the aerosol content rises multiple scale height, reaching altitudes up to 100 km. The lowermost altitude of aerosol's detection during NOMAD observation rises up to 30 km. Dust aerosols reff were observed to be close to 1 μm and its νeff lower than 0.2. Water ice aerosols reff were observed to be submicron with a νeff lower than 0.2. The vertical aerosol structure can be divided in two parts. The lower layers are represented by higher reff than the upper layers. The change between the lower and upper layers is very steep, taking only few kilometers. The decaying phase of the GDS, LS 210°–260°, shows a decrease in altitude of the aerosol content but no meaningful difference in the observed aerosol's size distribution parameters.

MAVEN/IUVS Observations of OH Prompt Emission: Daytime Water Vapor in the Thermosphere of Mars

“…IUVS dayglow limb observations near perihelion reveal aerosols present at 110 km (Figure 9), with temperatures below the water frost point (Figure 12). Micron size water ice particles typically prevail from 20 to 80 km (e.g., Liuzzi et al., 2020, 2021; Montmessin, Quémerais, et al., 2006; Stolzenbach et al., 2023a, 2023b), and smaller particle sizes near 0.1 microns (100 nm) have been reported from 80 to 100 km (e.g., Clancy et al., 2019; Jiang et al., 2019; Liuzzi et al., 2020; Montmessin, Bertaux, et al., 2006). Therefore, multiple studies show that the water ice particle radius decreases with increasing altitude, so that we might expect even smaller particles near 110 km during the day.…”