3:30 PM - 5:00 PM
[PCG19-P03] Mesospheric water ice clouds on Mars observed by TGO/NOMAD solar occultation
Keywords:Mars, mesospheric cloud, TGO/NOMAD, water ice, nucleation, solar occultation
In the mesosphere on Earth, noctilucent clouds are frequently observed in the low-temperature region (below -130degC) at altitudes 80-90 km in the mesospheric polar region. Two mechanisms have been proposed to explain the formation of this ice particles. One is homogeneous nucleation, in which condensation nuclei are formed from water vapor. The other is heterogeneous nucleation, in which substrate 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 can be applicable to cloud formation in other planetary atmospheres. The purpose of this study is to clarify the nucleation mechanism of the Martian mesospheric clouds by comparing the observation results by Mars orbiter with the theoretical results of the method of Tanaka et al. (2022). We applied 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). The observational data period used in this study is 4595 profiles in Martian Year 35 from 2019 March 23 to 2021 February 6. In this study we derive:
1. The distributions of the occurrence rate of the Martian mesospheric water ice clouds
2. The atmospheric background conditions (temperature, water vapor abundance, water vapor saturation, and dust density) with and without the mesospheric water ice clouds
From these results, we will attempt to elucidate the nucleation mechanism of mesospheric clouds on Mars, where atmospheric conditions are colder and thinner than those on Earth and can therefore change significantly.
We proceeded with the analysis using similar methods (Tsuda et al., 2021; https://doi.org/10.1029/2021JD035081) in order to compare our mesospheric cloud work on Mars with those on Earth. First, we derive a frequency distribution of Martian mesospheric cloud occurrence at altitudes of 40-100 km. Following the previous Mars study by Streeter et al. (2022, https://doi.org/10.1029/2021JE007065), spectral transmission at wavelengths of 320 nm and 600 nm were used to detect water ice and dust, respectively. The detection method of Martian mesospheric clouds in this study is described below. The slant opacity in the observed line-of-sight direction was derived from the transmission spectrum. The following two definitions were used for the detection:
1. the total optical depth along the line of sight (slant opacity) of water ice (320 nm) is greater than 0.01
2. the ratio of the slant opacity of water ice (320 nm) to that of dust (600 nm) is greater than 1.5
As a result, mesospheric clouds are detected in 1075 of the 4595 orbits. Unlike the Earth, especially in latitude and altitude, mesospheric clouds were commonly found at low to mid latitudes during the period around perihelion (Ls = 240-300), and their frequency of occurrence was about 45% on average. It was also found that mesospheric clouds were concentrated at altitudes of 40-60 km.
In the future, we will analyze the atmospheric conditions with and without mesospheric clouds and compare them with theoretical studies of nucleation in Martian mesospheric clouds to clarify whether homogeneous or heterogeneous nucleation is dominant.
This theory can be applicable to cloud formation in other planetary atmospheres. The purpose of this study is to clarify the nucleation mechanism of the Martian mesospheric clouds by comparing the observation results by Mars orbiter with the theoretical results of the method of Tanaka et al. (2022). We applied 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). The observational data period used in this study is 4595 profiles in Martian Year 35 from 2019 March 23 to 2021 February 6. In this study we derive:
1. The distributions of the occurrence rate of the Martian mesospheric water ice clouds
2. The atmospheric background conditions (temperature, water vapor abundance, water vapor saturation, and dust density) with and without the mesospheric water ice clouds
From these results, we will attempt to elucidate the nucleation mechanism of mesospheric clouds on Mars, where atmospheric conditions are colder and thinner than those on Earth and can therefore change significantly.
We proceeded with the analysis using similar methods (Tsuda et al., 2021; https://doi.org/10.1029/2021JD035081) in order to compare our mesospheric cloud work on Mars with those on Earth. First, we derive a frequency distribution of Martian mesospheric cloud occurrence at altitudes of 40-100 km. Following the previous Mars study by Streeter et al. (2022, https://doi.org/10.1029/2021JE007065), spectral transmission at wavelengths of 320 nm and 600 nm were used to detect water ice and dust, respectively. The detection method of Martian mesospheric clouds in this study is described below. The slant opacity in the observed line-of-sight direction was derived from the transmission spectrum. The following two definitions were used for the detection:
1. the total optical depth along the line of sight (slant opacity) of water ice (320 nm) is greater than 0.01
2. the ratio of the slant opacity of water ice (320 nm) to that of dust (600 nm) is greater than 1.5
As a result, mesospheric clouds are detected in 1075 of the 4595 orbits. Unlike the Earth, especially in latitude and altitude, mesospheric clouds were commonly found at low to mid latitudes during the period around perihelion (Ls = 240-300), and their frequency of occurrence was about 45% on average. It was also found that mesospheric clouds were concentrated at altitudes of 40-60 km.
In the future, we will analyze the atmospheric conditions with and without mesospheric clouds and compare them with theoretical studies of nucleation in Martian mesospheric clouds to clarify whether homogeneous or heterogeneous nucleation is dominant.