Martian atmospheric thermal structures and dust distributions during the global dust storm and the regional dust storm of MY 34 from ACS TIRVIM observations onboard ExoMars TGO

Abstract

Martian dust particles play an important part in radiative processes in Martian atmosphere, being involved in dust, H₂O and CO₂ cycles on Mars [1]. The most significant manifestations of such cycles are dust storms which last several weeks and strongly affect the thermal structure of Martian atmosphere [2]. In addition to regular storms, global dust storms (hereinafter GDS) occur irregularly once in several Martian Years (hereinafter MY) and cover the entire planet for months. GDS still remain unpredictable despite numerous studies [3]. The latest global dust storm occurred in MY 34 and has been simultaneously monitored by several spacecrafts including ExoMars Trace Gas Orbiter (TGO) [4].

In this paper we present results of analysis of thermal emission spectra of Mars measured with a Fourier-transform spectrometer ACS TIRVIM onboard TGO [5]. The main scientific goal of TIRVIM is long-term monitoring of temperatures and aerosols (dust and water ice) distribution in Martian atmosphere in nadir mode of observations in the 5–16 μm spectral range [6]. Here we consider two datasets obtained in MY 34: the first one covered the first month of the GDS in a period from 26^th of May 2018 till 15^th of July 2018 (Martian season L_s = 182.2°–211.7°), the second one lasted from 22^nd of December 2018 till 18^th of March 2019 (L_s = 310.9°–357.3°) and fully covered the regional dust storm (C-storm).

Retrieval of temperatures, dust and ice content from the thermal emission of Martian atmosphere in 15 μm CO₂ band has been done in a number of previous similar experiments [7, 8, 9]. We use iterative retrieval algorithm based on optimum estimation approach [10]. With TIRVIM spectral resolution Δν = 1.17 cm^-1, temperature profiles may be retrieved from the surface to 55–60 km of altitude with the vertical resolution in the order of few kilometers in the lower atmosphere. Spectral range from 780 to 1300 cm^-1 is free from CO₂ absorption lines and is suitable for retrieving the temperature of Martian surface and column abundances of aerosols. Dust and water ice contents are defined as optical depths at wavelengths of local maxima of their spectral extinction cross sections [11].

We have obtained the thermal structures of Martian atmosphere at various local times during the first month of the global dust storm and during the whole regional storm. We have compared temperature fields in the nighttime and daytime at the peaks of the two observed storms with structure of the pure from dust atmosphere. Asymmetric heating with dust particles ejected into the atmosphere is clearly seen in Fig. 1. Diurnal contrasts of temperature between nighttime and daytime are represented in Fig. 2. We notice an excellent agreement with co-located MCS/MRO measurements [12], with the account for a worse vertical resolution of nadir sounding with TIRVIM.

Along with the temperature field we also consider the variability of dust distribution during the two storms in MY 34. Spatial distributions of dust in terms of column optical depth at 1075 cm^-1 at the peaks of the global dust storm and the regional dust storm are shown in Fig. 3. Due to unavoidable degeneracy of the dust and ice retrieval problems at some temperature conditions, these maps contain also unreliable high and zero values of optical depth, which are to be filtered out.

Acknowledgements

Science support in IKI is funded by the RSF grant #20-42-09035.

References

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