5:15 PM - 7:15 PM
[PCG19-P07] Nucleation mechanism of mesospheric water ice clouds on Mars observed by TGO/NOMAD
Keywords:Mars, Nucleation, Cloud, Mesosphere
In the mesosphere on Earth, water ice 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 vapor. The other is heterogeneous nucleation, in which aerosols such as dust 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 (Tanaka et al., 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, the nucleation has only been studied theoretically by applying a classical theory for the lower atmospheric clouds (Määttänen et al., 2005). 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 theory of Tanaka et al. (2022).
We used the solar occultation spectral data obtained by the UVIS of the NOMAD spectrometer on board the ExoMars TGO. The observational data considered this study consist in 9249 atmospheric transmission profiles covering the period from Ls (solar longitude) = 163°in Martian Year (MY) 34 to Ls = 218°in MY 36 (April 22 in 2018 to April 30 in 2022). In this study, we derive the total optical depth along the line of sight (slant opacity) at 320 nm and 600 nm from the transmittance spectra to identify water ice clouds. We attempt to distinguish between water ice clouds and dust by the slant opacity ratio between 320 nm and 600 nm. We assume the criteria to identify water ice clouds as follows: the slant opacity at 320 nm is larger than 0.01 at altitudes of 40-100 km; the slant opacity ratio (320 nm / 600 nm) is larger than 1.5; the signal-to-noise ratio (S/N) at 320nm is greater than 30. The atmospheric density, dust density, atmospheric temperature and cooling rate, dust particle size, and water vapor pressure of the atmospheric conditions are inferred from the Mars Climate Database (MCD) (version 6.1; Millour et al., 2015), a numerical atmospheric general circulation model, to apply the theory of Tanaka et al. (2022) to clarify the nucleation mechanism.
Martian mesospheric water ice clouds were detected in 965 profiles out of 9249 profiles. The water ice clouds when the water vapor pressures were above 10-5 Pa and the temperatures were below 200 K. The heterogeneous nucleation was predominant in the mesospheric water ice clouds under the Martian atmospheric conditions when the background atmospheric water vapor pressures were above 1.56×10-5 Pa which corresponds the saturated water vapor pressure at 150 K. Furthermore, focusing exclusively on the data where clouds were detected, almost all cases exhibited atmospheric water vapor pressures above 1.56×10-5 Pa, indicating heterogeneous nucleation. This fact suggests that the formation of water ice clouds in the Martian mesosphere requires a sufficient amount of water vapor and that heterogeneous nucleation is the dominant process. On the other hand, the homogeneous nucleation could be possible when the water vapor pressure was less than 9.40×10-7 Pa which corresponds the saturated water vapor pressure at 140 K. In fact, clouds were detected in 9 profiles at altitudes above 70 km below the saturated water vapor pressure of 140 K, which suggested the possibilities of homogeneous nucleation. The possibility to occur homogeneous nucleation on Mars is surprising, that cannot occur on Earth. Furthermore, the dust number density, dust radius, water vapor pressure, and temperature were replaced with values obtained from observations instead of those estimated by the MCD, and further analysis of the nucleation process was conducted. As a result, contrary to the results based on the MCD, atmospheric conditions which homogeneous nucleation predominate were rarely observed. We concluded that the nucleation of water ice clouds in the Martian mesosphere was predominantly by heterogeneous nucleation. The relationships between supersaturation and nucleation process were also investigated.
We used the solar occultation spectral data obtained by the UVIS of the NOMAD spectrometer on board the ExoMars TGO. The observational data considered this study consist in 9249 atmospheric transmission profiles covering the period from Ls (solar longitude) = 163°in Martian Year (MY) 34 to Ls = 218°in MY 36 (April 22 in 2018 to April 30 in 2022). In this study, we derive the total optical depth along the line of sight (slant opacity) at 320 nm and 600 nm from the transmittance spectra to identify water ice clouds. We attempt to distinguish between water ice clouds and dust by the slant opacity ratio between 320 nm and 600 nm. We assume the criteria to identify water ice clouds as follows: the slant opacity at 320 nm is larger than 0.01 at altitudes of 40-100 km; the slant opacity ratio (320 nm / 600 nm) is larger than 1.5; the signal-to-noise ratio (S/N) at 320nm is greater than 30. The atmospheric density, dust density, atmospheric temperature and cooling rate, dust particle size, and water vapor pressure of the atmospheric conditions are inferred from the Mars Climate Database (MCD) (version 6.1; Millour et al., 2015), a numerical atmospheric general circulation model, to apply the theory of Tanaka et al. (2022) to clarify the nucleation mechanism.
Martian mesospheric water ice clouds were detected in 965 profiles out of 9249 profiles. The water ice clouds when the water vapor pressures were above 10-5 Pa and the temperatures were below 200 K. The heterogeneous nucleation was predominant in the mesospheric water ice clouds under the Martian atmospheric conditions when the background atmospheric water vapor pressures were above 1.56×10-5 Pa which corresponds the saturated water vapor pressure at 150 K. Furthermore, focusing exclusively on the data where clouds were detected, almost all cases exhibited atmospheric water vapor pressures above 1.56×10-5 Pa, indicating heterogeneous nucleation. This fact suggests that the formation of water ice clouds in the Martian mesosphere requires a sufficient amount of water vapor and that heterogeneous nucleation is the dominant process. On the other hand, the homogeneous nucleation could be possible when the water vapor pressure was less than 9.40×10-7 Pa which corresponds the saturated water vapor pressure at 140 K. In fact, clouds were detected in 9 profiles at altitudes above 70 km below the saturated water vapor pressure of 140 K, which suggested the possibilities of homogeneous nucleation. The possibility to occur homogeneous nucleation on Mars is surprising, that cannot occur on Earth. Furthermore, the dust number density, dust radius, water vapor pressure, and temperature were replaced with values obtained from observations instead of those estimated by the MCD, and further analysis of the nucleation process was conducted. As a result, contrary to the results based on the MCD, atmospheric conditions which homogeneous nucleation predominate were rarely observed. We concluded that the nucleation of water ice clouds in the Martian mesosphere was predominantly by heterogeneous nucleation. The relationships between supersaturation and nucleation process were also investigated.