[P-CG21] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG21-07] Nucleation mechanism of mesospheric water ice clouds on Mars observed by TGO/NOMAD

*Kaito Koizumi¹, Hiromu Nakagawa¹, Naoki Terada¹, Hitoshi Fujiwara², Takuo Tsuda³, Kyoko Tanaka⁴, Yuki Kimura⁵, Yasumasa Kasaba¹, Shohei Aoki⁶, Ann Carine Vandaele⁷, Ian Thomas⁷, Bojan Ristic⁷, Frank Daerden⁷, Zachary Flimon⁷, Yannick Willame⁷, Jonathon P Mason⁸, Manish Patel⁸, Giuliano Liuzzi⁹, Giancarlo Bellucci¹⁰, José Juan López-Moreno¹¹ (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.Education and Research Center for Sustainable Development/Faculty of Science and Technology, Seikei University, 3.Department of Computer and Network Engineering, Graduate School of Informatics and Engineering, The University of Electro-Communications, 4.Department of Astronomy, Graduate School of Science, Tohoku University, 5.Institute of Low Temperature Science, Hokkaido University, 6.Graduate School of Frontier Sciences, The University of Tokyo, 7.Royal Belgian Institute for Space Aeronomy (BIRA-IASB), 8.School of Physical Sciences, The Open University, 9.School of Engineering, University of Basilicata, 10.Institute di Astrofisica e Planetologia Spaziali (IAPS/INAF), 11.Instituto de Astrofisica de Andalucia (IAA/CSIC))

Keywords:Mars, mesospheric cloud, TGO/NOMAD, water ice, nucleation

In the mesosphere on Earth, mesospheric clouds are frequently observed in the low-temperature region (below -130℃) at altitudes 80-90 km in the polar region. Two mechanisms have been proposed to explain the formation of these ice particles. One is homogeneous nucleation, in which condensation nuclei are formed from water vapour. The other is heterogeneous nucleation, in which substrates such as aerosol in the atmosphere undergo a phase change as nuclei. The latest theoretical study shows that heterogeneous nucleation is predominant in the nucleation of mesospheric clouds on Earth and that homogeneous nucleation is unlikely to occur, when compared to observed conditions (Tanaka et al., 2022; https://doi.org/10.5194/acp-22-5639-2022). This theory may apply to cloud formation in other planetary atmospheres. On Mars, mesospheric clouds like those on Earth have been observed, but there are still many unresolved issues. In particular, nucleation has only been studied theoretically by applying a classical theory for the lower atmospheric clouds (Määttänen et al., 2005), but a detailed understanding of this process is required especially for the mesospheric clouds. The purpose of this study is to clarify the nucleation mechanism of the Martian mesospheric clouds by comparing the mesospheric cloud observations obtained at Mars with theoretical results calculated by applying the method of Tanaka et al. (2022) to Mars.
We used the solar occultation spectral data obtained by the ultraviolet (UV) to visible (VIS) channel UVIS of the Nadir and Occultation for MArs Discovery (NOMAD) spectrometer on board the ExoMars Trace Gas Orbiter (TGO) to clarify the purpose. The observational data considered this study consist in 9249 transmission profiles covering the period from Ls (areocentric longitude of the sun) = 163°in MY(Martian Year) 34 to Ls = 218°in MY 36 (2018/4/22-2022/4/30). In this study, we derive the total optical depth along the line of sight (slant opacity) from the transmittance spectra (Streeter et al., 2021). We attempt to distinguish between water ice clouds and dust by comparing the slant opacity at 320 nm, where we assume a large contribution from water ice clouds, with the slant opacity of all aerosols, including dust, at 600 nm. The existence of water ice clouds is determined under the conditions that the optical thickness at 320 nm is larger than 0.01 at altitudes of 40-100 km and the slant opacity ratio (320 nm / 600 nm) is larger than 1.5. The atmospheric density, dust density, atmospheric temperature and cooling rate, dust particle size, and water vapour pressure of the atmospheric conditions are derived from the Mars Climate Database (MCD), a numerical atmospheric general circulation model, and applied to the theory of Tanaka et al. (2022) to investigate the possibility of homogeneous and heterogeneous nucleation.
Following the thresholds described above, Martian mesospheric water ice clouds were detected in 966 out of 9249 altitude distributions (152 in MY 34, 615 in MY 35, and 199 in MY 36). Out of the 966 data, data were selected by saturated vapour pressure corresponding to 140 K and 150 K, leading to a subset of respectively, 9 data near 140 K (1 at MY34, 1 at MY35, and 7 at MY36) and 49 data near 150 K (0 at MY 34, 29 at MY 35, and 20 at MY 36). These were compared with the theoretical results. The results suggested that when the background atmospheric temperature at the time of the cloud formation is around 150 K, heterogeneous nucleation is dominant and homogeneous nucleation is unlikely to occur, as on Earth. On the other hand, when the background temperature is 140 K, homogeneous nucleation can occur at altitudes above 70 km. The latter is unexpected. Detailed analysis of the dust density and dust particle size using the results derived from the instrument's observation data is needed in the future, to clarify the atmospheric conditions under which homogeneous nucleation can occur, which is not seen on Earth but is suggested on Mars.